Throughout the day, we listen the various types of sounds like our father's voice, our mother's voice, our teacher's voice, chirping of birds, ringing of a school bell, a telephone ringing, a guitar being played, a siren, a jet engine roaring in the sky, buzzing of a mosquito, a gun shot etc. These sounds stimulate the auditory nerve in the human ear and the brain interprets the sound. Now let us define sound.
Sound is a form of energy which produces the sensation of hearing in our ears.
Perform the following activities to produce sound.
1. Take a plastic scale or ruler from your geometry box. Hold it flat on your desk or table with about half its length protruding
(stick out from the surface) over the edge. Now bend it down and release it. It will move up and down rapidly (i.e. it will vibrate)
and produce the sound at the same time. The sound will last as long as the vibration (i.e. rapid up and down motion) of the scale continues.
2. Take a tuning fork. Hold it from its stem and strike it with a rubber pad or hammer. You will observe that the prongs of the tuning fork
vibrate and at the same time sound is produced (Figure).
3. Place your finger lightly on your throat near the vocal cords as shown in figure. Now say "Ah" for few seconds. You will feel the
vibration in your finger as long as you say "Ah".
4. Tie a thin metallic string rigidly at the two ends of a table as shown in figure. Now, pluck the string from the middle and release it.
The string begins to vibrate up and down and at the same time, sound is heard. 
Conclusion: From these activities, we come to the conclusion that the sound is produced by the vibrating objects or bodies.
When a drum is beaten, then the skin of drum vibrates and sound is produced. When the strings of a guitar are plucked and released, they vibrate and produce sound. When air is blown into the flute, pipe, clarinet, saxophone etc., it vibrates in the tube of the instrument and hence sound is produced. Sound is also produced when the birds flap their wings during the flight.
The movement of the disturbance through a medium due to the repeated periodic motion of the particles of the medium about their mean positions is known as a wave.
² MECHANICAL WAVE
A mechanical wave is a periodic disturbance which requires material medium (i.e. solid, liquid or gas) for its propagation.
In other words, waves that are characterised by the motion of particles of a medium are called mechanical waves.
Examples of mechanical waves
(i) Sound waves in air (ii) Water waves
(iii) Waves produced due to the earthquake (known as seismic waves)
(iv) Waves produced by supersonic jet planes (known as shock waves)
(v) Waves produced in a stretched string. (vi) Waves produced in a slinky or long spring.
Types of waves
Waves are of two types : (i) Transverse Wave, (ii) Longitudinal Wave
1. TRANSVERSE WAVE
If the particles of a medium vibrate or oscillate about their mean positions at right angles to the direction of propagation of the disturbance then the wave is called transverse wave.
Examples : Movement of string of a sitar or violin, membrane of a tabla or dholak, movement of a kink on a rope.
Activity
Describe an activity to show the formation of a transverse wave.
Fix one end of a thin rope and give up and down jerk to the free end of the rope.
The rope oscillates or vibrates up and down as shown in figure. The disturbance travels from the free end to the fixed end but the rope vibrates up and down. This wave is known as transverse wave.
A transverse wave travelling on the surface of water is shown in figure.
When transverse wave travels through the medium, the shape of the medium changes. At some positions, the particles of the medium rise (or elevate) above their mean positions and at some positions, the particles of the medium go down (or depressed) below their mean positions.
The point on the elevation of the medium whose distance from the mean position is maximum is known as crest (C). On the other hand, the point on the depression of the medium whose distance from the mean position is maximum is known as trough (T). Thus, crests and troughs are formed when a transverse wave travels through a medium (Figure).
WAVELENGTH (OR LENGTH OF A WAVE)
The distance between two successive crests or between two successive troughs is known as the wavelength of a transverse wave.
OR
The distance between two successive particles of the medium which are in phase is called wavelength of the wave. It is denoted by l (lambda)
2. LONGITUDINAL WAVE
If the particles, of a medium vibrate or oscillate to and fro about their mean positions along the direction of propagation of the disturbance then the wave is called longitudinal wave.
Examples :- Sound wave, Organ pipes, Vibration on resonance apparatous
ACTVITY
Describe an activity to show the formation of longitudinal wave.
Take a slinky or a long spring which can be easily compressed and extended as shown in figure (a). Fix one end of the slinky with a rigid
support. push the free end of the slinky in the downward direction and release it. It is observed that the slinky begins to move up and
down (i.e. "to and fro") as shown in figure (b). The disturbance travels from the free end to the fixed end and the parts of the slinky vibrate
along the direction of the propagation the disturbance. This wave is known as longitudinal wave When a longitudinal wave passes through a medium,
the medium is divided into the regions of compressions (C) and rarefactions (R) as shown in figure (b).
COMPRESSION
The part or region of a medium, where the density of the medium is maximum or where the particles of the medium are very close to each other is known as compression. It is denoted by C.
RAREFACTION
The part or region of a medium, where the density of the medium is minimum or where the particles of the medium are far apart from each other is known as rarefaction. It is denoted by R.
PROPAGATION OF SOUND
A vibrating body produces sound. Now we shall study, how the sound travels from one place to another place.
When a body vibrates, then the particles of the medium (say air) around the vibrating body are set into vibrations. The particles of the medium which are very close to the vibrating body are pushed away from the body. These particles of the medium strike against the neighbouring particles. Hence the number of particles of the medium in the region where the displaced particles strike against the neighbouring particles is large. This region is known as compression (C). Since pressure is directly proportional to the number of particles, so the compression is a region of high pressure or high density. When the vibrating body moves backward, a region of emptiness known as rarefaction (R) or a region of low pressure or Low density is created. The displaced particles of the medium rebound into the region of low pressure or rarefaction. At the same time, compression is followed outwards. Therefore, when a body vibrates to produce sound, compressions and rarefactions follow one another as the sound waves travel through the' medium away from the vibrating body. When a sound wave travels through a medium, the particles of the medium simply vibrate about their rest positions and they do not move from one place to another place in the medium.
Figure represents the regions of compressions (or high pressures) and o rarefactions (or low pressures) as the sound propagates in the medium.
SOUND NEEDS A MEDIUM TO TRAVEL
We have learnt that sound travels from one place to another place when the energy is transferred from one particle to another particle of a medium like air or gas, liquid, solid etc. It means, sound needs a material medium for its propagation. In other words, sound cannot travel through vacuum.
DEMONSTRATION TO SHOW THAT SOUND WAVES CANNOT TRAVEL THROUGH VACCUM
Put an electric bell inside a closed Bell jar connected with a vacuum pump. Initially, air from the jar is not taken out. Connect the electric bell with a battery (Figure). It rings and the sound produced is heard by us.
Now start evacuating the air from a Bell jar using a vacuum pump, we will hear less and less sound. i.e. the loudness of the sound decreases. When there is no air in the Bell jar, we do not hear sound. This activity demonstrates that sound waves require material medium (in this case air) for its propagation.
SOUND WAVES ARE LONGITUDENAL WAVES
When a sound wave travels through the material medium, then compressions and rarefactions follow one another. The particles of the medium through which a sound wave travels vibrate to and fro about their mean positions parallel to the direction of propagation of the sound wave. Since the wave is known as longitudinal wave, if the particles of the medium vibrate to and fro about their mean positions parallel to the direction of propagation of the wave, therefore, the sound waves are longitudinal waves.
CHARACTERISTICS OF A SOUND WAVE
When a sound wave travels through a material medium, then the density or pressure of the medium changes continuously from maximum value to minimum value and vice-versa. Thus, the sound wave propagating in a medium can be represented as shown in figure.
Now, we shall discuss the characteristics or quantities to describe a sound wave.
(i) Amplitude : The maximum displacement of a vibrating body or particle from its rest position (i.e. mean position) is called amplitude.
(ii) Wavelength (or length of a wave) : The distance between two successive 1 regions of high pressure or high density (or compressions) or the distance between two successive regions of low pressure or low density (or rarefactions) is known as wavelength of a sound wave. It is denoted by l (read as lambda). In S.I., unit of wavelength is metre (m).
(iii) Frequency : The number of oscillations or vibrations made by a vibrating body or particles of a medium in one second is known as the frequency of a wave. It is denoted by u (read as Neu). In S.I., unit of frequency is hertz (Hz).
1 hertz = one oscillation completed by a vibrating body or a vibrating particle in one second.
(iv) Time period : Time taken by a vibrating particle or a body to complete one vibration or oscillation is known as time period. It is denoted by T.
In S.I., unit of time period is second(s).
Relation between Frequency and time period Let T = time period of a vibrating body.
Then number of oscillations completed in T second = 1
number of oscillations completed in 1 second = 1/T
But number of oscillations completed in 1 second = frequency (v)
(f) v = 1/T, frequency = 1/Time period
(iv) Pitch or Shrillness : Pitch is the characteristic (i.e., typical feature) of a sound that depends on the frequency received by a human ear.
A sound wave of high frequency has high pitch and a sound wave of low frequency has a low pitch.
You must have noticed that the voice of a woman has higher pitch than the voice of a man. Thus, the frequency of woman's voice is higher than the frequency of man's voice.
A sound wave of low pitch (i.e. low frequency) is represented by figure (a) and a sound wave of high pitch (i.e. high frequency) is represented by figure (b)
(v) Loudness : Loudness of a sound depends on the amplitude of the vibrating body producing the sound.
A sound produced by a body vibrating with large amplitude is a loud sound. On the other hand, a sound produced by a body vibrating with small amplitude is a feeble or soft sound. Loud sound and soft or feeble sound are represented as shown in Figure (a) and (b) respectively.
Loudness is a subjective quantity : It depends on the sensitivity or the response of our ears. A loud sound to a person may be a feeble sound for another person who is hard of hearing.
(vi) Timbre or quality : Quality or timbre is a characteristic (i.e., a typical feature) of a sound which enables us to distinguish between the sounds of same loudness and pitch.
This characteristic of sound helps us to recognise our friend from his voice without seeing him.
The quality of two sounds of same loudness and pitch produced by two different sources are distinguishable because of different waveforms produced by them. The waveforms produced by a vibrating tuning fork, violin and flute (Bansuri) are shown in figure.
(vii) Intensity : Intensity of a sound is defined as the sound energy transferred per unit time through a unit area placed perpendicular to the direction of the propagation of sound.
That is, intensity of sound = Soundenergy/Time * Area
Intensity of a sound is an objective physical quantity. It does not depend on the response of our ears.
In S.I., unit of intensity of sound is joule s–1 m–2 or watt m–2 ( 1Js–1 = 1W)
RELATIONSHIP BETWEEN WAVE VELOCITY, FREQUENCY AND WAVELENGTH FOR A PERIODIC WAVE
What is the relationship between wave velocity, frequency and wavelength
SPEED OF SOUND IN DIFFERENT MEDIA
We have seen above that sound can travel through solids, liquids and gases. The question which comes to mind is how fast does sound travel? Sound travels at different speeds in different media.
The speed of sound depends on the following factors :
As per definition,
Speed of sound = 
The speed of light in the air (or more correctly in vacuum) is 3 × 108 m/ s, (3lakh kilometre per second).
CONCLUSION
Speed of sound in solids is greater than the speed of sound in liquids and the speed of sound in liquids is greater than the speed of sound in gases.
SPEED OF SOUND IN VARIOUS MEDIA
REFLECTION OF SOUND
When a sound wave travelling in a medium bounces back to the same medium after striking the second medium, reflection of sound wave is said to take place. The reflection of sound wave is similar to the bouncing back of a rubber ball after striking a wall or the surface of a floor.
Just like light, sound is reflected by the solid and liquid surfaces. The reflection of sound obeys the laws of reflection.
The laws of reflection of sound are as follows :
(i) Incident angle = Reflected angle and (ii), The incident direction of sound, reflected direction of sound and the normal to the point of incidence all lie in the same plane.
ECHO
If we clap our hands while standing at some distance from a high and huge wall or a hill, we hear the clapping of our hands again after some short interval of time. The sound of clap heard by us is known as echo. Echo is produced due to the reflection of sound.
Thus, echo is a repetition of sound due to the reflection of original sound by a large and hard obstacle.
CONDITIONS FOR THE PRODUCTION OF AN ECHO
1. Time gap between the original sound and the reflected sound
We can hear the two sounds separately if the time gap between these two sounds is more than 1/10 s or 0·1 s. The time interval equal to 0·1 s is known as persistance of hearing. This means, the impression of any sound heard by us remains for 0·1 s in our brain. If any other sound enters our ears before 0·1 s, then the second sound will not be heard by us. Thus, the echo will be heard if the original sound reflected by an obstacle reaches our ears after 0·1 s.
2. Distance between the source of sound and obstacle
Minimum distance between the observer and the obstacle for echo to be heard
Let
Distance between the observer and the obstacle = d
Speed of sound (in the medium) = v
Time after which echo is heard = t
Then, t = 
or d = 
We know
Thus, the minimum distance (in air at 25°C) between the observer and the obstacle for the echo to be heard clearly should be 17.2 m.
The speed of sound increases with a rise in temperature. Therefore, the minimum distance in air between the observer and the obstacle for an echo to be heard clearly at temperatures higher than 25°C is more than 17.2m. In rooms having walls less than 17.2 m away from each other, no echo can be heard.
3. Nature of the obstacle : For the formation of an echo, the reflecting surface or the obstacle .must be rigid such as a building, hill or a cliff.
4. Size of the obstacle : Echoes can be produced if the size of the obstacle reflecting the sound is quite large.
REVERBERATION
The repeated reflection that results in the persistence of sound in a large hall is called reverberation.
Excessive reverberation in any auditorium/hall is not desirable because the sound becomes blurred and distorted. The reverberation can be minimised/reduced by covering the ceiling and walls with sound absorbing materials such as, fiber-board, rough plaster, draperies, perforated cardboard sheets etc.
When a sound is produced in a big hall, its waves reflects from the walls and travel back and forth. Due to this the sound does not vanish at once but it fades away gradually, that is the sound persists even after its production has been stopped. A small amount of reverberation is desirable in large halls or cinemas as it makes the sound pleasant and more effective. How ever too much reverberation is undesirable as it makes the sound confusing. To reduce reverberation the roof and walls of the hall are covered with sound absorbing materials like rough plaster and thick curtains. One may define reverberation as the persistance of sound due to repeated reflection and its gradual fadding away.
USES OF MULTIPLE REFLECTION OF SOUND
1. Megaphone : Megaphone is a device used to address public meetings. It is a horn-shaped. When we speak through megaphone, sound waves are reflected by the megaphone. These reflected sound waves are directed towards the
people (or audience) without much spreading.
2. Hearing Aid : Hearing aid is used by a person who is hard of hearing. The sound waves falling on hearing aid are concentrated into a narrow beam of sound waves by reflection. This narrow beam of sound waves is made to fall on the diaphragm of the ear. Thus, diaphragm of the ear vibrates with large amplitude. Hence, the hearing power of the person is improved.
3. Sound boards : Sound boards are curved surfaces (concave) which are used in a big hall to direct the sound waves towards the people sitting in a hall. The speaker is (i.e. source of sound) placed at the focus of the sound board as shown in figure.
Sound waves from the speaker are reflected by die sound board and these reflected waves are directed towards the people (or audience).
4. Stethoscope : Stethoscope is a device used by doctors to listen the sound produced by heart and lungs. The sound produced by heart beat and lungs of a patient reaches the ears of a doctor due to multiple reflection of sound.
5. Ceilings of concert halls are curved : The ceilings of concert halls and auditoriums are made curved. This is done so that the sound reaches all the parts of the hall after reflecting from the ceiling as shown in figure. Moreover, these ceilings are made up of sound absorbing materials to reduce the reverberation.
RANGE OF HEARING (AUDIBLE RANGE)
All vibrating bodies produce waves. Each wave has its own frequency. The frequency of a wave is equal to the frequency of the vibrating body producing sound. When a woman speaks, the waves produced by the vocal cords in her throat have different frequency than the frequency of the waves produced by the vocal cords of a man. Can human ears hear all the frequencies produced by the vibrating bodies ? The answer is No. In fact, normal human ears can hear only those waves whose frequency lies between 20 Hz and 20,000 Hz. The waves having frequency between 20 Hz and 20,000 Hz are known as sound waves. Thus, the audible range of frequency is 20 Hz to 20,000 Hz.
The waves having frequency less than 20 Hz and greater than 20,000 Hz cannot be heard by human ear.
INFRASONICS or INFRASOUND
The waves of frequency less than 20 Hz are known as infrasonic waves.
The infrasonic waves are produced by large vibrating bodies.
For example, infrasonic waves are produced by the vibration of the earth's surface during the earthquake. Some animals like elephants, rhinoseroses and whales etc. also produce infrasonic waves. These waves are not audible to a human ear.
It has been observed that animals behaviour becomes unusual just before the tremor is felt. This is because the animals has the ability to detect infrasonic waves produced at the time of tremor.
ULTRASONICS or ULTRASOUND
The waves of frequency greater than 20,000 Hz are known as ultrasonic waves or ultrasound. These waves are not audible to a human ear but they can be heard by animals and birds.
Bats can produce ultrasonic waves by flapping their wings. They can also detect these waves. The ultrasonic waves produced by the bats after reflection from the obstacles like buildings guide them to remain away from the obstacles during their flights. Hence, they can fly during night without hitting the obstacles. Bats also catch their prey during night with the help of ultrasonic waves. The ultrasonic waves produced by a bat spread out. These waves after reflecting from a prey sayan insect reach the bat. Hence, the bat can easily locate its prey.
Dolphins also produce ultrasonic waves. They can also detect the ultrasonic waves. They catch their prey like a fish due to their ability to detect the ultrasonic waves reaching them after reflecting from a fish.
APPLICATIONS OF ULTRASOUND (ULTRASONIC WAVES)
Ultrasonic waves have number of uses :
1. Ultrasonic vibrations are used for homogenising milk, i.e., the milk is agitated with ultrasonic vibrators. These vibrations break down the larger particles of the fat present in milk to smaller particles.
2. Ultrasonic vibrations are used in dish washing machines. In such machines, water and detergent are vibrated with ultrasonic vibrators. The vibrating detergent particles rub against the dirty utensils and thus clean them.
3. Ultrasonic vibrations produce a sort of depression in rats and cockroaches. Ultrasonic vibrators are used to drive rats and cockroaches from godowns.
4. Ultrasonic vibrations are used for imaging internal organs of human body. In fact they are even used to study the growth of foetus in mother's womb.
5. Ultrasonic vibrations are used in relieving pain in joints and muscles.
6. Ultrasonic vibrations are used in detecting flaws in articles made from metals. They are also used in finding the thickness of various parts of a metallic component.
Sonar
SONAR stands for Sound Navigation and Ranging.
It is a device which is used in the ships to locate rocks, icebergs, submarines, old ships sank in sea ete. It is also used to measure the depth of a sea.
PRINCIPLE: It is based on the principle of the reflection of sound wave (i.e. echo).
Determination of the Depth of a Sea using Sonar
A beam of ultrasonic waves from the transmitter of a SONAR fitted on the ship is sent towards the bottom of the sea. This beam is reflected back from the bottom of the sea and is received by the receiver of the SONAR on the ship.
The time taken by the ultrasonic waves to go from the ship to the bottom of the sea and then back to the ship is noted. Let it be 't' seconds. Therefore, the time taken by the ultrasonic waves to go from the ship to the bottom of the sea is (t/2) seconds.
Using the following formula S = v(t/2) we can find the depth of the sea.
Here, u = speed of ultrasonic wave in water.
S = depth of the sea
THE HUMAN EAR
In this article we will learn about the accoustics of hearing. We will see how a human ear converts sound energy into mechanical energy and then to a nerve impulse which is transmitted to the brain.
The human ear consists of (a) the outer ear (pinna), (b) the middle ear, (c) the inner ear. Each part has a specific task to perform. The outer ear, collects the sound and guides it to the middle ear. In the middle ear sound energy is converted into mechanical energy in the form of internal vibrations of the bone structure. These vibrations are then transferred into the inner ear which converts the vibrations into nerve impulses.
The outer ear has an approximately 2 cm tong ear canal. Here the sound is collected and amplified. It is in the form of pressure waves with alternate high pressure and low pressure regions.
The middle ear consists of eardrum (tympanic membrane) three tiny inter connected bones-the hammer (mallens), anvil (incus) and stirrup (stapes). The eardrum is a tightly stretched membrane. As the incoming pressure wave from the outer ear strikes, the ear drum starts to vibrate. A compression forces the eardrum inwards whereas a rarefaction forces the eardrum outwards. This means that the eardrum vibrates at the same frequency as that of the sound wave. The eardrum is connected to hammer which in turn is connected to anvil and stirrup. The motion of eardrum will set the hammer, anvil and stirrup into motion at the same frequency as that of eardrum. The three-bone system amplifies the sound further.
The stirrup is connected to the inner ear which consists of cochlea, semi circular canals and the auditory nerve. The vibrations are turned into electrical signals in inner ear which are sent to the brain via the auditory nerve. The brain interprets the sound by the electrical impulses it receives.
Some suggestions to keeps the ears healthy are given below:
Never insert any pointed object into the ear. It can damage the eardrum and make a person deaf.
Never shout loudly or produce a loud sound into someone's ear.
Never hit anyone hard on his/her ear.
SOLVED EXAMPLES
Ex.1 A source of wave produces 40 crests and 40 troughs in 0.4 second. Find the frequency of the wave.
Sol. Number of crests and troughs produced by the wave = 40
Number of waves formed = 40
Time taken = 0.4 s
Frequency = ?
Number of waves produced in one second =40/0.4s = 100 s–1
Frequency of the wave = 100 Hz
Ex.2 A person has a hearing range from 20 Hz to 20 kHz. What are the typical wavelengths of sound waves in air corresponding to these two frequencies? Take the speed of sound in air as 344 m s–1.
Sol. Hearing range = 20 Hz to 20 kHz (= 20000 Hz)
Speed of sound in the air = 344 m s–1
For a wave,
Ex.3 Calculate the wavelength of a sound wave whose frequency is 220 Hz and speed is 440 m/ s in a given medium.
Sol. Frequency, v = 220 Hz
Speed of sound, v = 440 m/ s
The wavelength can be described by the relationship,
Wave velocity = Wavelength of the wave x Frequency of the wave
Therefore, wavelength of the sound wave is 2 m.
Ex.4 A person is listening to sound of 50 Hz sitting at a distance of 450 m from the source of sound. What is the time interval between successive compressions from the source reaches him?
Sol. Frequency of the sound = 50 Hz
Distance from the source = 450 m
Time between the successive compressions is equal to time taken by the sound to travel a distance equal to its wavelength. Thus, we have to find out the time period we know,
The successive compressions will reach the person after every 0.02 s.
Ex.5 A human heart, on an average, is found to beat 75 times a minute. Calculate its frequency.
Ex.6 A boat at anchor is rocked by waves whose consecutive crests are 100 m apart. The wave velocity of the moving crests is 20 m/ s. What is the frequency of rocking of the boat?
Sol. Distance between two consecutive crests = 100 m
Wave velocity v = 20 m/ s
The distance between two consecutive crests is equal to the wavelength of the
wave. So,
So, the frequency of rocking of the boat is 0.2 s–1.
Ex.7 A longitudinal wave is produced on a toy slinky. The wave travels at a speed of 30 cm/ s and the frequency of the wave is 20 Hz. What is the minimum separation between the consecutive compressions of the slinky?
Sol. Wave speed, v = 30 cm/ s
Frequency of the wave, v = 20 Hz = 20 s–1
The minimum separation between the consecutive compressions is equal to the wavelength. Therefore,
Thus, the minimum separation between the consecutive compression of the slinky is 1.5 cm.
Ex.8 A bat can hear sound at frequencies up to 120 kHz. Determine the wavelength of sound in the air at this frequency. Take the speed of sound in the air as 344 m/s.
Ex.9 A gun is fired in the air at a distance of 660 m, from a person. He hears the sound of the gun after 2 s. What is the speed of sound?
Sol. Distance travelled by sound = 660 m, Time taken by the sound = 2 s, Speed of sound in air = ?
Ex.10 A child hears an echo from a cliff 4 seconds after the sound from a powerful cracker is produced. How far away is the cliff from the child? Velocity of sound in air at 20°C is 344 m/ s.
Sol. Let the distance between the child and the cliff be d. Then,
Total distance travelled by the sound = 2d
Total time taken by the sound = 4 s
Then,
Ex.11 A ship sends on a high frequency sound wave and receives an echo after 1 second. What is the depth of the sea? Speed of sound in water is 1500 m/ s.
Sol. Let,
Depth of the sea = d
So, Total distance travelled by the sound wave = 2d
Time taken by sound to travel both ways = 1 s
As per definition,
Ex.12 A sonar echo takes 2.2 s to return from a whale. How far away is the whale?
Sol. Total time taken by the signal = 2.2 s
Ex.13 Using the SONAR, sound pulses are emitted at the surface of water. These pulses after being reflected from the bottom are detected. If the time interval from the emission to the detection of the sound pulses is 2 seconds, find the depth of the water. Velocity of sound in water = 1498 m/s.
Sol. Let, depth of the water from the earth's surface be d. Then,
Total distance travelled by the pulse = 2d
Total time taken by the pulse = 2 s
As per definition,
Ex.14 A wave moves a distance of 8 m in 0.05 s.
(a) Find the velocity of the wave.
(b) What is the wavelength of the wave if its frequency is 200 Hz ?
Ex.15 Two children are at opposite ends of an iron pipe. One strikes his end of the iron pipe with a stone. Find the ratio of times taken by the sound waves in air and in iron to reach the other child. Given velocity of sound in air is 344 ms–1 and that in iron is 5130 ms–1.
Ex.16 A boat at anchor is rocked by waves whose consecutive crests are 100 m apart. The wave velocity of the moving crests is 20 m/s. What is the frequency of rocking of the boat?
Ex.17 A stone is dropped into a well 44.1 m deep. The splash is heard 3.13 seconds after the stone is dropped. Find the velocity of sound in air.
Ex.18 Using sonar, sound pulses are emitted at the surface of water. These pulses after being reflected from water bottom are detected. If the time interval from the emission to the detection of the sound pulses is 2 seconds, find the depth of the water. [speed of sound in water = 1531 m/s given].
Ex.19 A gun is fired at a distance. Why is the sound heard after the flash is seen?
Ans. The velocity of sound in air is 344 m/ s, whereas the velocity of light is 3 × 108 m/ s. So, light waves travel much faster than the sound waves. As a result, the sound due to gun fire is heard after the flash is seen.
It is due to this reason that during a thunder storm, one sees the lightning much before one hears the thunder (sound).
Ex.20 Which of the following is carried by the waves from one place to another?
(a) mass (b) velocity (c) wavelength (d) energy
Ans. The correct answer is (d).
Ex.21 At the surface of the moon, there is no atmosphere. Suppose you and your friend land on the moon. Would you and your friend be able to talk to each other? Why?
Ans. No. People cannot talk, on the moon. This is because there is no atmosphere (or medium) on the moon, and the sound needs a medium to travel.
Ex.22 Sound waves are
(a) longitudinal
(b) transverse
(c) partly longitudinal and partly transverse
(d) sometimes longitudinal, sometimes transverse
Ans. Sound waves are longitudinal waves. So, answer (a) is correct.
Ex.23 State two properties of the medium required for wave propagation.
Ans. Any medium required for wave propagation should have the following characteristics.
(i) It should be a material medium.
(ii) The medium should be elastic.
Ex.24 Why are sound waves longitudinal in nature?
Ans. The sound wave is longitudinal because it propagates in any material medium as a series of compressions and rarefactions.
During the propagation of a sound wave, the particles of the medium oscillate back and forth about their mean position in the direction of sound propagation.
Q.1 What is sound ?
Ans. Sound is a form of energy which produces the sensation of hearing in our ears.
Q.2 What does wave transfer ?
Ans. Wave transfers energy.
Q.3 What is a mechanical wave ?
Ans. A waves that are characterised by the motion of particles of a medium are called mechanical waves. These waves require material medium for their propagation.
Q.4 What is a traverse wave ?
Ans. If the particles of the medium vibrate about their mean positions at right angles to the direction of propagation of the disturbance, then the wave is called transverse wave..
Q.5 What is a longitudinal wave ?
Ans. If the particles of a medium vibrate or oscillate to and fro about their mean positions along the direction of propagation of the disturbance, then the, wave is called longitudinal wave.
Q.6 How a sound is produced ?
Ans. Vibrating bodies produce sound.
Q.7 What do you understand by the terms "compression" and "rarefaction" ?
Ans. A region of high pressure of a medium when wave travels through it is called compression. A region of low pressure of a medium when wave travels through it is called rarefaction.
Q.8 What happens to the medium through which sound travels ?
Ans. A medium is divided into the regions of high pressure or high density and regions of low pressure or low density called compressions and rarefactions respectively.
Q.9 What do you understand by the wavelength of a sound wave ?
Ans. Distance between two successive compressions or successive rarefactions is called wavelength of a sound wave.
Q.10 What do you understand by the frequency of a sound wave ?
Ans. The number of vibrations or oscillations made by a vibrating body in one second is called the frequency of a sound wave.
Q.11 Name the physical quantity which determines the pitch of a sound.
Ans. Frequency of a sound wave.
Q.12 Name the physical quantity which determines the loudness of a sound.
Ans. Amplitude of the vibrating body.
Q.13 What do you understand by the pitch of a sound ?
Ans. Pitch of a sound is the characteristic of sound that depends on the frequency received by a human ear.
Q.14 What do you understand by the loudness of a sound ?
Ans. Amplitude of the vibrating body determines the loudness of the sound. Large is the amplitude of vibration, large is the loudness of the sound produced.
Q.15 Define the characteristic "timbre" or "quality" of a sound.
Ans. Quality or timber is a characteristic of a sound which enables us to distinguish between two sounds of some loudness and pitch.
Q.16 Which characteristic of a sound helps you to identify your friend by his voice while sitting with others in a dark room ?
Ans. Timbre or quality of sound.
Q.17 What do you mean by the intensity of sound ?
Ans. Sound energy per unit time per unit area is known as the intensity of sound.
Q.18 Write the S.I. unit of intensity of sound.
Ans. Js–1 m–2 or Wm–2 (Watt m–2).
Q.19 Out of which of the following media, the speed of sound is maximum : solid, liquid, gas.
Ans. The speed of sound is maximum in solid medium.
Q.20 Sound travels faster as the temperature of the medium increases. Why ?
Ans. Speed of sound increases with increase in temperature.
Q.21 By what amount, the speed of sound in air increases with 1°C rise in temperature of the air?
Ans. 0.61 ms–1 or 61 cm s–1
Q.22 What do you understand ‘by supersonic speed
Ans. The speed of an object moving faster than the speed of sound is known as supersonic speed.
Q.23 What is a shock wave ?
Ans. The sound waves produced by an object moving with a speed greater than the speed of sound are known as shock waves.
Q.24 What do you understand by Sonic boom ?
Ans. A loud sound produced by shock waves is known as sonic boom.
Q.25 What do you understand by the reflection of sound ?
Ans. The bouncing back of a sound wave after striking a solid surface is called reflection of sound.
Q.26 What is an echo ?
Ans. Echo is a repetition of sound due to the reflection of original sound by a large and hard obstacle.
Q.27 What should be the minimum distance between the source of sound and the obstacle to hear an echo ?
Ans. 17 metres.
Q.28 What do you understand by the reverberation ?
Ans. The phenomenon of prolongation of original sound due to the multiple reflection of sound waves even after the source of sound stops producing sound is called reverberation.
Q.29 What is reverberation time
Ans. The time interval during which original sound appears to prolong.
Q.30 What is the audible range ?
Ans. Audible range is 20 Hz to 20,000 Hz.
Q.31 What are infrasonic waves ?
Ans. The waves of frequency less than 20 Hz are called infrasonic waves.
Q.32 What are ultrasonic waves ?
Ans. The waves of frequency greater than 20,000 Hz are called ultrasonic waves.
Q.33 What type of waves are produced by bats
Ans. Ultrasonic waves.
Q.34 What does SONAR stand for ?
Ans. SONAR stands for Sound Navigation And Ranging.
Q.35 What is the basic principle which SONAR works ?
Ans. SONAR works on the principle of reflection of waves (i.e. echo).
Q.1 Explain why echoes can’t be heard in a small room.
Ans. For hearing echo, there should be at least a distance of 17 m between the source of sound and the body from which sound is reflected. In small rooms this is not the case, hence echoes are not heard.
Q.2 Why can we hear echoes in long galleries and big halls?
Ans. For hearing echo, there should be at least a distance of 17 m between the source of sound and the body from which sound is reflected. In big rooms and galleries this is so, hence echoes are not heard.
Q.3 Two astronauts cannot hear each other on the moon. Why?
Ans. Material medium is necessary for the propagation of sound. On the moon there is vacuum i.e., no air, therefore, sound cannot propagate on the moon. Thus the astronauts cannot hear each other.
Q.4 Explain why there is usually a time delay between observing a flash and hearing a thunder.
Ans. This is because velocity of light is much greater than the velocity of sound.
Q.5 Bats have no eyes still they can ascertain distances, directions nature and size of the objects. Explain why.
Ans. Bats, have special types of wings. When they fly they produce ultrasonic waves. These waves are received by the ears of bat after they have been reflected by the object. The ears of the bat are so sensitive and trained that they not only get information of distance of the obstacle but also that of the nature of the reflecting surface.
Q.6 Sound is produced due to a vibratory motion, then why a vibrating pendulum does not produce sound?
Ans. The frequency of the vibrating pendulum does not lie within the audible range (20 Hz to 20,000 Hz) and hence it does not produce audible sound.
Q.7 A loud sound can be heard at a large distance but a feeble or soft sound cannot be heard at a large distance. Explain why.
Ans. Sound is a form of energy which is transferred from one place to another. Sound energy is directly proportional to the square of the amplitude of the vibrating body, louder the sound; larger is its energy. As the sound travels through a medium, sound with small energy is absorbed after travelling a small distance in the medium but sound with large energy will be absorbed after travelling a large distance in the medium. Therefore, loud sound can he heard at a large distance but feeble sound cannot be heard at a large distance.
Q.8 Two sound waves A and B are shown in the figure. Identify the sound wave-having
(i) high frequency (ii) low frequency.
Ans. (i) Wave B has high frequency as it repeats itself after smaller intervals of time.
(ii) Wave A has low frequency as it repeats itself after longer intervals of time.
Q.9 Two sound waves A and B are shown in figure. Identify the sound wave having (i) small amplitude (ii) large amplitude.
Ans. (i) Sound wave B has small amplitude. ( ii) Sound wave A has large amplitude.
Q.10 A sound wave travelling in a medium is represented as shown in figure.
(i) Which letter represents the amplitude of the sound wave?
(ii) Which letter represents the wavelength of the wave?
Ans. 
(i) Letter P represents the amplitude of the sound wave.
(ii) Letter Q represents the wavelength of the sound wave.
NCERT QUESTIONS WITH SOLUTIONS
Q.1 How does the sound produced by a vibrating object in a medium reach your ear ?
OR
Explain how sound is produced by your school bell.
Ans. Air is the commonest material through which sound propagates. When a vibrating object, like prongs of tuning fork move forward, they push the molecules of the air infront of them. This is turn compresses the air, thus creating a region of high pressure and high density called compression. This compression in the air travels forward. When the prongs of the tuning fork move backward, they create a region of low pressure in the air, commonly called rarefaction. This region has low pressure low density and more volume. As the tuning fork continues to vibrate, the regions of compression in the air alternate with the regions of rarefaction. These regions alternate at the same place. The energy of vibrating tuning fork travels outward. This energy which reaches the ears, makes the ear drums to vibrate and thus we hear sound.
Q.2 A sonar device on a submarine sends out a signal and receives an echo 5 s later. Calculate the speed of sound in water if the distance of the object from the submarine is 3625 m.
Ans. Speed of sound =
Q.3 Why are sound waves called mechanical waves ?
Ans. Some mechanical energy is required to make an object vibrate. Sound energy cannot be produced on its own. the mechanical energy of vibrating object travels through a medium and finally reaches the ear. Therefore, the sound waves are called mechanical waves.
Q.4 Suppose you and your friend are on the moon. Will you be able to hear any sound produced by your friend?
Ans. No, I will not be able to hear sound, because Moon has no atmosphere. Therefore, no sound waves can travel to your ears and therefore, no sound is heard.
Q.5 Which wave property determines
(a) loudnes (b) pitch ?
Ans. (a) The amplitude of the wave determines the loudness; more the amplitude of a wave, more is-the loudness produced.
(b) The pitch is determined by the frequency of the wave. Higher the frequency of a wave, more is its pitch and shriller is the sound.
Q.6 Guess which sound has a higher pitch; guitar or car horn ?
Ans. Car horn has a higher pitch than a guitar, because sound produced by the former is shriller than the later.
Q.7 What are wavelength, frequency, time period and amplitude of a sound wave ?
Ans. Wavelength : It is the linear distance between two consecutive compressions or two consecutive rarefactions.
Frequency : The number of compressions or rarefactions taken together passing through a point in-one second is called frequency.
Time Period : It is the time taken by two consecutive compressions or rarefactions to cross a point.
Amplitude : It is the magnitude of maximum displacement of a vibrating particle about its mean position.
Q.8 How are the wavelength and frequency of a sound wave related to its speed ?
Ans. Speed of sound = Frequency × Wavelength
Q.9 Calculate the wavelength of a sound wave whose frequency is 220 Hz and speed is 440 ms–1 in a given medium.
Ans. Frequency = 220 Hz
Speed of sound 440 ms–1
We know speed of sound = Frequency × wavelength
Þ Wavelength = = 2m
Q.10 A person is listening to tone of 500 Hz sitting at a distance of 450 m from the source of the sound. What is the time interval between successive compressions from the source ?
Ans. Time interval = 1/Frequency = 1/500 = 2*10-3
Q.11 Distinguish between loudness and intensity of sound.
Ans. The loudness depends on energy per unit area of the wave and on the response of the ear but intensity depends only on the energy per unit area of the wave and is independent of the response of the ear.
Q.12 In which of the three media, air, water or iron, does sound travel the fastest at a particular temperature?
Ans. Sound travels fastest in iron as compared to water and air.
Q.13 An echo is returned in 3s. What is the distance of the reflecting surface from the source, given that the speed of sound is 342 ms–1 ?
Ans. Distance of reflecting body from the source of sound = speed of sound*time/2=342*3/2=513
Q.14 Why are the ceiling of concert halls curved ?
Ans. The ceilings of concert halls are curved because sound after reflection from it reaches all the corners of the hall and is audible to each person in the hall.
Q.15 What is the audible range of the average human ear ?
Ans. An average human ear can hear sound waves between frequencies 20 Hz to 20,000 Hz.
Q.16 What is the range of frequencies associated with
(a) Infrasound ? (b) Ultrasound ?
Ans. (a) Infrasound : Sound waves between the frequencies 1 to 20-Hz-
(b) Ultrasound : Sound waves of the frequencies above 20,000 Hz.
Q.17 A submarine emits a sonar pulse, which returns from an underwater cliff in 1.02 s. If the speed of, sound in salt water is 1531 ms–1, how far away is the cliff ?
Ans. Distance of cliff =speed of sond*time/2=1531*1.02/2=780.81m
Q.18 What is sound and how is it produced ?
Ans. Sound is mechanical energy which produces sensation of hearing. When an object is set into vibrations, sound is produced.
Q.19 Give an experiment to show that sound needs a material medium for its Propagation.
Ans. Take an electric circuit which consists of a cell, a switch and an electric bell arranged inside a bell jar, which stands on the plat
form of an evacuating pump.
The switch of the bell is pressed to close the electric circuit. When there is air within the bell jar, sound is heard. Air is now pumped
out of the bell jar. When the air is completely removed from the bell jar, no sound is heard as it is obvious from fig. because the medium
of air which has to carry energy from the bell to the bell jar, is removed. It shows that sound needs material medium for its propagation.
Q.20 Why is sound wave called a longitudinal wave ?
Ans. Sound wave is called longitudinal wave because the particles of the medium vibrate in the direction of the propagation of wave.
Q.21 Which characteristic of the sound helps you to identify your friend by his voice while sitting with Miens in a dark room ?
Ans. The characteristic of sound is quality or timbre.
Q.22 Flash and thunder are produced simultaneously. But thunder is heard a few seconds after the flash is seen, why ?
Ans. The speed of light is 3 × 108 ms–1 and the speed of sound is 344 ms–1 in air. Thus, flash of lightning is seen at first, but sound takes few seconds to reach the ears.
Q.23 A person has a hearing range from 20 Hz to 20 kHz. what are the typical wavelengths of sound waves in air corresponding to these two frequencies ? Take the speed of sound in air as 344 ms–1.
Ans. Wavelength of sound of frequency 20 Hz
wavelength of sound of frequency 20,000 Hz
Q.24 Two children are at opposite ends of an aluminium rod. One strikes the end of the rod with a stone. Find the ratio of times taken by the sound wave in air and in aluminium to reach the second child. Given velocity of sound in air and aluminium are 346 ms–1 and 6420 ms–1 respectively.
Q.25 The frequency of a source of sound is 100 Hz. How many times does it vibrate in a minute ?
Ans. No. of vibrations produced in 1 s = 100 Þ No. of vibration produced in 60 (sec)
(1 min) = 100 × 60 = 6000
Q.26 Does sound follow the same laws of reflection as light does ? Explain.
Ans. Yes, sound and light follow the same laws of reflection given below :
(a) Angle of incidence at the point of incidence = Angle of reflection.
(b) At the point of incidence the incident sound wave, the normal and the reflected sound wave lie in the same plane.
Q.27 When a sound is reflected from a distant object, an echo is produced. Let the distance between the reflecting surface and the source of sound production remain the same. Do you hear echo sound on a hotter day ?
Ans. If the temperature rises the speed of sound will increase. This is turn will increase the minimum distance required for hearing an echo. No echo is heard because, the distance between the source of sound and reflecting body does not increase.
Q.28 Give two practical applications of reflection of sound waves.
Ans. (i) Megaphones are designed to send sound waves in particular direction are based on the reflection of sound.
(ii) In stethoscope the sound of patient’s heartbeat reaches the doctor’s ears by multiple reflection in the tubes.
Q.29 A stone is dropped from the top of a tower 500 m high into a pond of water at the base of the tower. When is the splash heard at the top ? Given, g = 10 ms–2 and speed of sound = 340 ms–1.
Q.30 A sound wave travels at a speed of 339 ms–1. If its wavelength is 1.5 cm, what is the frequency of the wave? Will it be audible ?
Ans. Speed of sound wave = 339 ms–1
Wavelength of sound wave = 1.5 cm = 0.015 m
\ Frequency of sound wave = speed of sound/wavelength = 339/0.015=22600 Hz
The sound will not be audible, because human beings can hear only upto 20,000 Hz.
Q.31 What is reverberation? How can it be reduced ?
Ans. Reverberation is the repeated multiple reflections of sound in any big enclosed space. It can be reduced by covering the ceiling and walls of the enclosed space with some absorbing materials like fibre board, loose woollens etc.
Q.32 What is loudness of sound ? What factors does it depend on ?
Ans. Loudness of sound is the effect produced in the brain by the sound of different frequencies. The loudness of the sound depends on the distance of the observer from the source of sound ; lesser the distance louder the sound. It increases with the increase in amplitude and the area of the vibrating body.
Q.33 Explain how bats use ultrasound to catch their prey.
Ans. The bats produce high pitched ultrasonic waves which are not heard by human beings. The ultrasonic waves on striking the insect send back an echo, which is heard by the bat. As the echo is heard by the bat it hoves on the insect and catches it.
Q.34 How is ultrasound used for cleaning ?
Ans. The object to be cleaned is put in a tank fitted with ultrasonic vibrator. The tank is filled with water containing detergent. As the ultrasonic vibrator is switched on the detergent rub against the object at a very high speed and hence clean it.
Q.35 Explain the working and application of a sonar.
Ans. SONAR is a device for determining water depth and locating underwater objects like reefs, submarines and schools of fish.
On striking the bottom of the ocean, the ultrasonic wave is reflected upward toward the ship. This wave is received by a suitable receiver. The time of travel from the source of sound to the receiver is noted. We can calculate the depth of ocean floor if the velocity of sound in water is known
Depth of ocean floor = 
For Example : If it takes 2.4 s to record echo by the sonar
Velocity of sound in sea water = 1450 ms–1
Depth of ocean floor = 
EXERCISE -I
Q.1 A human heart, on an average, is found to beat 75 times in a minute. Calculate its frequency.
Q.2 Calculate the wavelength of a sound wave whose frequency is 220 Hz and speed is 440 m/s in a given medium
Q.3 A person is listening to a tone of 500 Hz sitting at a distance of 450 m from the source of the sound. What is the time interval between successive compressions from the source?
Q.4 A stone is dropped from the top of a tower 500 m high into a pond of water at the base of the tower. When is the splash heard at the top? Given, g = 10 ms–2 and speed of sound = 340 ms–1.
Q.5 A sound wave travels at a speed of 339ms–1. If its wavelength is 1.5 cm, what is the frequency of the wave? Will it be audible?
Q.6 A sonar device on a submarine sends out a signal and receives an echo 5 s later. Calculate the speed of sound in water if the distance of the object from the submarine is 3625 m.
Q.7 What is the frequency of a wave with a time period of 0.05 s?
Q.8 A bat can hear sound of frequency 100 kHz. Find the wavelength of the sound wave in air corresponding to this frequency. Given, speed of sound in air = 344 ms–1.
Q.9 A boy heard a sound of frequency 100 Hz at a distance of 500 m from the source of sound. What is the time period of oscillating particles of the medium?
Q.10 The water waves are produced at a frequency of 40 Hz. If the wavelength of these waves is 2· 5 cm, calculate the speed of the waves.
Q.11 A radio station transmits waves of wavelength 200 m. If the speed of the waves is 3 × 108 m/s, find the frequency of the radio station.
Q.12 Calculate the time taken by a sound wave of frequency 1000 Hz and wavelength 50 cm to travel a distance of 500 m.
Q.13 Audible range of frequencies is 20 Hz to 20,000 Hz. Find the range of wavelengths corresponding to this frequency. Given, velocity of sound = 340 ms–1.
Q.14 A rock at the bank of a coast is struck by water waves. Find the frequency of the waves striking the rock, if the distance between two consecutive crests or troughs is 50 metre. Given, velocity of water wave = 50 ms–1.
Q.15 A long spring whose one end is rigidly fixed is stretched from the other end and then left. Longitudinal waves of frequency 10 Hz are produced. If the velocity of the wave is 25 ms–1, find the distance between two consecutive compressions in the spring.
Q.16 Find the velocity of the wave shown in the figure below.
Q.17 A man stands at a distance of 112 m from a vertical wall. On blowing a whistle, he hears the echo after 0.7 second. Calculate the speed of sound in air.
Q.18 A boy standing in front of a wall at 17 m produces 10 claps per second. He notice that the sound of his clapping coincides with the echo. Echo is heard only once when clapping is stopped. Calculate the speed of sound.
Q.19 A man stands at a distance of 25 m from a high wall. He hears the echo off the high wall, produced by a clap of his hands. If the velocity of sound is 330 ms–1, what would be the time interval between his original clap and hearing of the echo ?
Q.20 A man fired a gun standing between two parallel cliffs. He heard two successive echoes after 3 and 5 seconds respectively. What is the distance between the cliffs. (Velocity of sound = 330 ms–1)
Q.21 A man makes a short and loud sound in front of a hill and the echo is heart after 3 seconds. On moving closer to the hill by 165 m, the echo is heard after 2 seconds. Calculate the velocity of sound and the distance of hill from the first position.
Q.22 A hospital uses an ultrasonic scanner to locate tumours in a tissue. What is the wavelength of sound in a tissue in which the speed of sound is 1.7 km s–1 ? The operating frequency of the scanner is 4.2 MHz.
Q.23 In a ripple tank, 10 full ripples/s are produced. The distance between a trough and a crest is 15 cm. Calculate:
(i) the frequency, (ii) the wavelength and (iii) the velocity of the ripples
Q.24 Velocity of light in vacuum is 3 × 108 ms–1 and that in water 2.3 × 108 ms–1. If the wavelength of a wave in vacuum is 6 × 10–7 m, what will be its wavelength in water ? What is the frequency of the wave ?
Q.25 A source of sound produces waves of wavelength 0.80 m in air. The same source of sound produces waves of wavelength 4.0 m in water. If the velocity of sound in air = 332 ms–1, find the velocity of sound in water.
Q.26 The wave form represent's a transverse wave produced on a taut string AB which is vibrated at A.
(i) What is the amplitude of the waves ?
(ii) Mark the wavelength l on the diagram. State its value.
(iii) Calculate the time period of the vibration.
(iv) Calculate the frequency of the source.
(v) Calculate the velocity of the transverse wave.
Q.27 The following graph shows the displacement vs distance of a pulse on a rope at two different times. Find the speed of the pulse.
Q.28 Two sound wave in air have wavelength ratio 1 : 3. Find their frequency ratio.
Q.29 A source produces 15 crests and 15 through in 3 seconds. When the second crest is produced, the first is 2 cm away from the source. Calculate
(i) the wavelength (ii) the frequency, and (iii) the wave speed.
Q.30 Shown in figure a displacement-distance time graph for a wave. The wave velocity is
320 ms–1. Determine
(i) wavelength (ii) frequency, and (iii) amplitude
Q.1 Sound waves are transverse whereas light waves are longitudinal in nature.
Q.2 Distance between a crest and the next trough in a wave motion is l/4.
Q.3 The audible range of frequencies is 20Hz - 20 kHz.
Q.4 The frequency of a sounding body of time period 0.01 sec is 100 Hz.
Q.5 The minimum distance between a source and the reflector of sound should be 34 m.
Q.6 Sound waves travel faster in air than in water.
Q.7 Sound travels faster at a higher temperature than at a lower temperature.
Q.8 Sound and light travel in a medium in the form of crests and troughs and rarefactions and compressions respectively.
Q.9 Light waves require no medium for their propagation.
Q.10 Sound produced by a sounding body of frequency 300 Hz covers 34 m in the time the sounding body produces 30 vibrations.
Q.11 Velocity of sound in air is more for higher frequency than for lower frequency.
Q.12 The wavelength of 5 MHz ultrasound in a medium where the velocity is 1540 ms–1 is 0.3 mm.
Q.1 A wave is a ____________ in a medium.
Q.2 Waves transport ____________ and not ____________ from one region to the other.
Q.3 If the particles of the medium vibrate ____________ to the direction of propagation of the wave then the wave is called a transverse wave.
Q.4 Transverse waves can’t be transmitted inside ____________
Q.5 If the particles of the medium vibrate ____________ to the direction of the propagation of the wave then the wave is called longitudinal wave.
Q.6 The speed of sound in a medium depends upon ____________ of the medium and ____________ of the medium.
Q.7 Sound travels more than ____________ times faster in water and more than ____________ times faster in steel than it does in air.
Q.8 Ultrasonic are the frequencies which are ____________ than 20,000 Hz.
Q.9 Infrasonic are the frequencies which are lesser than ____________.
Q.10 The audible range for a normal human is ____________ to 20,000 Hz
Q.11 An echo is the phenomenon of ____________ of sound of a source by reflection from an obstacle.
Q.12 The sensation of sound lasts on our ears for ____________ of a second. This is called persistence of hearing.
Q.13 The minimum distance between a source and the reflector of sound to hear an echo is ____________.
Q.14 SONAR is a technique of determining locating objects ____________.
Q.1 COLUMN–I COLUMN–II
(A) Sound waves. (p) electromagnetic wave.
(B) Waves in strings. (q) longitudinal wave.
(C) Waves in a slinky. (r) transverse wave.
(D) Light waves. (s) both transverse and longitudinal.
Q.2 COLUMN–I COLUMN–II
(A) Crest. (p) longitudinal wave.
(B) Compression. (q) v/l
(C) Frequency. (r) transverse wave.
(D) Wavelength. (s) distance between two consecutive rarefactions.
Each question, in this section, contains statements given in two columns which have to be matched. The statements in Column I are labelled A, B, C and D, while the statements in Column II are labelled p, q, r, s and t. Any given statement in Column I can have correct matching with ONE OR MORE statement(s) in Column II.
Q.3 COLUMN–I COLUMN–II
(A) Degree of sensation (p) loudness.
of sound.
(B) Amplitude. (q) quality.
(C) Frequency. (r) intensity.
(D) Timbre. (s) shrillness.
(t) pure note.
Q.4 COLUMN–I COLUMN–II
(A) Multiple reflection (p) galton whistle.
of sound.
(B) Infrasonics. (q) ear.
(C) Pinna. (r) stethoscope.
(D) Anvil. (s) sound boards.
(t) lesser than 20 Hz.
ANSWER KEY
1. 1.25 s–1 4. 11.47 sec
5. 22600 Hz, not be audible
6. 1450 m/s 7. 20 Hz
8. 3.44 mm 9. time period = 0.01 sec
10. 1 m/s 11. 1.5 × 106 Hz
12. t = 1 sec, v = 500 m/s
13. l = 17m, 0.017 m
14. 1 Hz 15. 2.5 m
16. 0.075 ms–1 17. 320 ms–1
18. 340 ms–1 19. 0.15 s
20. 1320 m
21. 330 ms–1, 450 m 22. 4.0× 10–4 m
23. (i) 10 Hz, (ii) 30 cm, (iii) 3 ms–1
24. 4.6 × 10–7 m 25. 1660 ms–1
26. (i) 0.2 m, (ii) 1 m, (iii) 0.5 s, (iv) 2 Hz, (v) 2 ms–1
27. 4.5 cm s–1 28. 3 : 1
29. (i) 2 cm ; (ii) 5 Hz ; (iii) 10 cm s–1
30. (i) 0.4 m ; (ii) 800 Hz ; (iii) 2 cm
1. False 2. False 3. True
4. True 5. False 6. False
7. True 8. False 9. True
10. True 11. False 12. True
1. disturbance 2. energy, matter
3. perpendicular 4. liquids and gases
5. parallel 6. nature, density
7. four, seven 8. greater
9. 20 Hz 10. 20 Hz
11. repetition 12. one-tenth
13. 17 metre 14. under water
1. (A ® q), (B ® r), (C ® s), (D ® p)
2. (A ® r), (B ® p), (C ® q), (D ® s)
3. (A ® p), (B ® p, r), (C ® s, t), (D ® q)
4. (A ® r, s), (B ® t), (C ® q), (D ® q)
Q.1 A part of longitudinal wave in which particles of medium are farther away than the normal particles is called:
(A) rarefaction (B) trough (C) compression (D) crest
Q.2 In case of a longitudinal wave, in the region of rarefaction :
(A) the volume of momentarily increases (B) the density momentarily decreases
(C) the pressure momentarily decreases (D) all the above
Q.3 In the region of compression or rarefaction, in a longitudinal wave the physical quantity which does not change is:
(A) pressure (B) mass (C) density (D) volume
Q.4 A slinky can produce in laboratory :
(A) transverse waves only
(B) longitudinal waves only
(C) both (A) and (B)
(D) none of the above
Q.5 In case of transverse wave :
(A) the hump on the + y axis is called crest
(B) the hump on the – y axis is called crest
(C) the highest point on the hump on + y axis is called crest
(D) the highest point on the hump on the – y axis is called crest
Q.6 In case of transverse wave:
(A) the hump on the – y axis is called trough
(B) the lowest point the hump on the – y axis is called trough
(C) the hump on + y axis is called trough
(D) the highest point on the hump on the + y axis is called trough
Q.7 The wavelength is the linear distance between the:
(A) two consecutive compressions
(B) two consecutive rarefactions
(C) one compression and one rarefaction
(D) both (A) and (B)
Q.8 In case of transverse wave the wavelength is the linear distance between :
(A) two consecutive troughs
(B) two consecutive crests
(C) one crest and one trough
(D) both (A) and (B)
Q.9 The change in density/pressure of a medium from maximum value to minimum value and again to maximum value, due to the propagation of a longitudinal wave is called complete:
(A) oscillation (B) frequency (C) amplitude (D) none of the above
Q.10 The number of oscillations passing through a point in unit time is called:
(A) vibration (B) frequency (C) wavelength (D) none of the above
Q.11 The SI unit of frequency is :
(A) hertz (B) gauss (C) decibel (D) none of the above
Q.12 If the frequency of a wave is 25 Hz, the total number of compressions and rarefactions passing through a point in 1 second is
(A) 25 e period of a wave in a medium is the time taken by:
(A) a com pression to pass through a point (B) a rarefaction to pass through a point
(C) an oscillation to pass through a point (D) none of the above
Q.14 Amplitude of a longitudinal wave in a medium:
(A) is the extent to which a medium gets compressed
(B) is the extent to which a medium gets rarefied
(C) either (A) or (B)
(D) none of the above
Q.15 Non-mechanical (electromagnetic) wave can propagate in :
(A) material medium as well as vacuum (B) in vacuum, but not in material medium
(C) in material medium but not in vacuum (D) neither in material medium nor in vacuum
Q.17 A longitudinal wave travels in water from west to east. The direction which the particles of medium move:
(A) east to west (B) west to east (C) north to south (D) south to north
Q.18 A stretched string is plucked gently to produce a note. The string is producing:
(A) longitudinal waves
(B) stationary waves
(C) transverse waves
(D) both (A) and (C)
Q.19 A stretched slinky is given a sharp push along its length. A wave travels from one end to another. The wave so produced is :
(A) transverse wave (B) longitudinal wave (C) stationary wave (D) none of the above
Q.20 A longitudinal sound wave in air consists:
(A) a number of rarefaction pulses one after the other
(B) a number of compression pulses one after the other
(C) compression and rarefaction pulses alternating with each other
(D) a rarefaction pulse followed by compression pulse, separated by some distance.
Q.21 The density of air at some point in a longitudinal sound wave is minimum at an instant. The pressure of air at that point is :
(A) minimum
(B) maximum
(C) equal to atmospheric pressure
(D) none of the above
Q.22 Which of the following is an elastic wave?
(A) light wave (B) radio waves (C) sound wave (D) microwaves
Q.23 Infrasonic vibrations have frequency:
(A) less than 10Hz
(B) less than 20 Hz
(C) between 20 and 20,000 Hz
(D) more than 20,000 Hz.
Q.24 The range of sonic waves is between:
(A) 20 Hz to 2000 Hz
(B) 20 Hz to 10,000 Hz
(C) 20 Hz to 15,000 Hz
(D) 20 Hz to 20,000 Hz
Q.25 The sound waves having a frequency more than 20,000 Hz are called:
(A) infrasonic waves (B) supersonic waves
(C) ultrasonic waves (D) hypersonic waves
Q.26 The animal which cannot hear ultrasonic waves is :
(A) bat (B) cow (C) dog (D) dolphin
Q.27 The bat hunts it prey by emitting and receiving reflected:
(A) super sonic waves
(B) ultrasonic waves
(C) sonic waves
(D) infrasonic waves
Q.28 A sonic boom is produced in the air when an aircraft flies at a speed:
(A) equal to the speed of sound
(B) more than the speed of sound
(C) less than the speed of sound
(D) climbs vertically
Q.29 Sound travels fastest in :
(A) vacuum (B) gases (C) solids (D) liquids
Q.30 A boy sitting in a boat fires a gun. An observer P is at a distance of 50 m from the boat. Another observer Q is a diver, who is 50 m under water. Both hear the sound of gun:
(A) P hears the sound first
(B) Q hears the sound first
(C) Both P and Q hear the sound at the same time
(D) none of the above.
Q.31 When the lightning strikes, we hear multiple of cracks of thunder. These multiple reflections of sound are called:
(A) echoes (B) reverberations (C) resonance (D) none of the above.
Q.32 Hearing of repetition of sound after reflection from a distant object is called:
(A) reverberation (B) resonance (C) echo (D) none of the above.
Q.33 For hearing an echo the minimum distance should be :
(A) less than 10m
(B) between 10m and 15 m
(C) 17 m or more
(D) none of the above.
Q.34 An echo is heard only, if the original sound after reflection should reach the ear in :
(A) less than s (B) less than s (C) more than s (D) none of the above.
Q.35 A bullet is moving at a speed, more than the speed of sound. It is said to be moving at :
(A) supersonic speed
(B) ultrasonic speed
(C) infrasonic speed
(D) sonic speed
Q.36 Naval ships called "destroyers" can detect submarines in the sea. The device used by these ships is called:
(A) ultra sonometer (B) sonar (C) ultrasonograph (D) none of the above .
Q.37 Which of the following properties of a sound wave are affected by the change in temperature of air:
(A) frequency (B) amplitude (C) wavelength (D) intensity
Q.38 Which of the following gases the sound travels fastest:
(A) hydrogen (B) helium (C) nitrogen (D) oxygen
Q.39 The waves used in sonography as :
(A) microwaves
(B) ultra-violet waves
(C) ultrasonic waves
(D) sound waves
Q.40 The crack of thunder is heard after few seconds the lightning flash, because:
(A) crack of thunder and lightning are not produced at same time
(B) light travels extremely fast as compared to sound
(C) sound waves slow down on passing through air
(D) none of the above
Q.41 Sound energy is basically:
(A) mechanical energy
(B) electromagnetic energy
(C) potential energy
(D) electrical energy
Q.42 The transfer of energy in a material medium due to the periodic motion of its particles is called:
(A) wave front (B) wave motion (C) pulse (D) none of the above
Q.43 Which is not the condition for hearing sound?
(A) There must be a vibrating body capable of transferring energy.
(B) There must be a material medium to pick up and propagate energy
(C) The medium must have a large density.
(D) There must be receiver to receive the energy and interpret it.
Q.44 An instrument commonly used in laboratory to produce a sound of some particular frequency is:
(A) sonar (B) electric bell (C) tuning fork (D) a stretched wire
Q.45 The sound waves in a medium are characterised by the:
(A) linear motion of particles in the medium (B) rotatory motion of particles in the medium
(C) oscillatory motion of particles in the medium (D) none of the above
Q.46 The sound waves which travel in the air are called:
(A) transverse waves
(B) longitudinal waves
(C) polarised waves
(D) none of the above
Q.47 When a sound wave travels in the air, the physical quantity which is transferred from one place to the other is:
(A) mass (B) force (C) momentum (D) energy
Q.48 In case of longitudinal waves, the particles of medium vibrate:
(A) in the direction of wave propagation (B) opposite to the direction of wave propagation
(C) at right angles to the direction of wave propagation
(D) none of the above
Q.49 In case of transverse waves the particles of a medium vibrate:
(A) in the direction of wave propagation
(B) opposite to the direction of wave propagation
(C) at the right angles to the direction of wave propagation
(D) none of the above
Q.50 A longitudinal waves consists of:
(A) crest and troughs in the medium (B) compressions and rarefactions in the medium
(C) both (A) and (B) (D) neither (A), nor (B)
Q.51 A transverse wave consists of:
(A) crests and troughs in the medium (B) compressions and rarefactions in the medium
(C) both (A) and (B) (D) neither (A), nor (B)
Q.52 The longitudinal waves can propagate only in :
(A) solids (B) liquids (C) gases (D) all the above
Q.53 The transverse waves can propagate only in :
(A) liquids (B) gases (C) solids (D) vacuum
Q.54 A part of the longitudinal wave in which particles of medium are closer than the normal particles is called:
(A) rarefaction (B) crest (C) trough (D) compression
Q.55 In the compression region of the medium in case of longitudinal wave:
(A) the volume monetarily decreases (B) the density momentarily increases
(C) the pressure monetarily increases (D) all the above
Q.56 In which of the following media will sound travel the fastest? [NTSE]
(A) solid (B) both solid and liquid (C) liquid (D) gas
Q.57 Sound waves in air are ______ waves.[NTSE]
(A) longitudinal (B) radio
(C) transverse (D) electromagnetic
Q.58 Sound waves cannot pass through: [NTSE]
(A) a solid-liquid mixture
(B) an ideal gas
(C) a liquid-gas mixture
(D) a perfect vacuum
Q.59 Out of the following, which frequency is not clearly audible to the human ear? [NTSE]
(A) 30 Hz (B) 30,000 Hz (C) 300 Hz (D) 3000 Hz
Q.60 The frequency of sound waves can be expressed in: [NTSE]
(A) Hz only (B) cycles/second only
(C) s–1 only (D) all the above
Q.61 Sound waves are: [NTSE]
(A) transverse mechanical waves
(B) longitudinal mechanical waves
(C) neither (A) nor (B)
(D) none of these
Q.62 The speed of sound wave in a given medium is: [NTSE]
(A) directly proportional to its frequency (B) inversely proportional to its frequency
(C) directly proportional to the square of its frequency
(D) independent of its frequency
Q.63 Supersonic means: [NTSE]
(A) frequencies less than 20 Hz
(B) same as ultrasonic
(C) frequencies much more than that of ultrasonics
(D) same as infrasonics
Q.64 The frequency of a wave is 5 Hz. It refers to (types of wave) [NTSE]
(A) ultrasonics (B) microwaves
(C) infrasonics (D) radio waves
Q.65 When a vibrating tuning fork is placed on a table, a large sound is heard. This is due to:
[NTSE]
(A) forced vibrations (B) resonance (C) beats (D) reflection
Q.66 A body produces sound only if it is: [NTSE]
(A) made of steel (B) made of glass (C) plucked (D) vibrating
Q.67 Velocity of sound is minimum in: [NTSE]
(A) nitrogen (B) hydrogen (C) air (D) carbon dioxide
Q.68 Sound takes some time to travel from one place to another. It will be maximum: [NTSE]
(A) at night (B) during summer (C) during winter (D) all the time same
Q.69 The sound propagates in a gaseous medium by: [NTSE]
(A) transverse waves
(B) longitudinal waves
(C) both (A) and (B)
(D) neither (A) and (B)
Q.70 Echo is produced due to: [NTSE]
(A) reflection of sound
(B) refraction of sound
(C) resonance
(D) none of these
Q.71 SONAR is based on the principle of: [NTSE]
(A) echo (B) resonance (C) reverberation (D) any one of the above
Q.72 The audible range of frequency is: [NTSE]
(A) 20 Hz to 20,000 Hz
(B) 40 Hz to 40,000 Hz
(C) 60 Hz to 60,000 Hz
(D) 10 Hz to 20,000 Hz
Q.73 Which of the following frequencies of sound cannot be heard by human beings? [NTSE]
(A) 5Hz (B) 20 Hz (C) 400 Hz (D) 1000 Hz
Q.74 Velocity of sound in air is about: [NTSE]
(A) 330 m/s (B) 360 m/s (C) 380 m/s (D) 400 m/s
Q.75 A echo will be heard if the minimum distance between the source of sound and the obstacle is: [NTSE]
(A) 1 m (B) 10 m (C) 15 m (D) 17 m
1. A 2. D 3. B 4. C
5. C 6. B 7. D 8. D
9. A 10. B 11. A 12. B
13. C 14. C 15. A 16. A
17. B 18. C 19. B 20. C
21. A 22. C 23. B 24. D
25. C 26. B 27. B 28. B
29. C 30. B 31. B 32. C
33. C 34. C 35. A 36. B
37. C 38. A 39. C 40. B
41. A 42. B 43. C 44. C
45. C 46. B 47. D 48. A
49. C 50. B 51. A 52. D
53. C 54. D 55. D 56. B
57. B 58. C 59. D 60. A
61. B 62. A 63. B 64. C
65. A 66. D 67. D 68. C
69. B 70. A 71. A 72. A
73. A 74. A 75. D
1. A part of longitudinal wave in which particles of medium are farther away than the normal particles is called:
(A) rarefaction (B) trough (C) compresion (D) crest
2. In case of a longitudinal wave, in the region of rarefaction :
(A) the volume of momentarily increases (B) the density momentarily decreases
(C) the pressure momentarily decreases (D) all the above
3. In the region of compression or rarefaction, in a longitudinal wave the physical quantity which does not change is:
(A) pressure (B) mass (C) density (D) volume
4. A slinky can produce in laboratory :
(A) transverse waves only (B) longitudinal waves only
(C) both (A) and (B) (D) none of the above
5. In case of transverse wave :
(A) the hump on the + y axis is called crest
(B) the hump on the – y axis is called crest
(C) the highest point on the hump on + y axis is called crest
(D) the highest point on the hump on the – y axis is called crest
6. In case of transverse wave:
(A) the hump on the – y axis is called trough
(B) the lowest point the hump on the – y axis is called trough
(C) the hump on + y axis is called trough
(D) the highest point on the hump on the + y axis is called trough
7. The wavelength is the linear distance between the:
(A) two consecutive compressions
(B) two consecutive rarefactions
(C) one compression and one rarefaction
(D) both (A) and (B)
8. In case of transverse wave the wavelength is the linear distance between :
(A) two consecutive troughs (B) two consecutive crests
(C) one crest and one trough (D) both (A) and (B)
9. The change in density/pressure of a medium from maximum value to minimum value and again to maximum value, due to the propagation of a longitudinal wave is called complete:
(A) oscillation (B) frequency (C) amplitude (D) none of the above
10. The number of oscillations passing through a point in unit time is called:
(A) vibration (B) frequency (C) wavelength (D) none of the above
11. The SI unit of frequency is :
(A) hertz (B) gauss (C) decibel (D) none of the above
12. If the frequency of a wave is 25 Hz, the total number of compressions and rarefactions passing through a point in 1 second is
(A) 25 (B) 50 (C) 100 (D) none of the above
13. Time period of a wave in a medium is the time taken by:
(A) a compression to pass through a point (B) a rarefaction to pass through a point
(C) an oscillation to pass through a point (D) none of the above
14. Amplitude of a longitudinal wave in a medium:
(A) is the extent to which a medium gets compressed
(B) is the extent to which a medium gets rarefied
(C) either (A) or (B) (D) none of the above
15. Non-mechanical (electromagnetic) wave can propagate in :
(A) material medium as well as vacuum (B) in vacuum, but not in material medium
(C) in material medium but not in vacuum (D) neither in material medium nor in vacuum
17. A longitudinal wave travels in water from west to east. The direction which the particles of medium move:
(A) east to west (B) west to east (C) north to south (D) south to north
18. A stretched string is plucked gently to produce a note. The string is producing:
(A) longitudinal waves (B) stationary waves (C) transverse waves (D) both (A) and (C)
19. A stretched slinky is given a sharp push along its length. A wave travels from one end to another. The wave so produced is :
(A) transverse wave (B) longitudinal wave (C) stationary wave (D) none of the above
20. A longitudinal sound wave in air consists:
(A) a number of rarefaction pulses one after the other
(B) a number of compression pulses one after the other
(C) compression and rarefaction pulses alternating with each other
(D) a rarefaction pulse followed by compression pulse, separated by some distance.
21. The density of air at some point in a longitudinal sound wave is minimum at an instant. The pressure of air at that point is :
(A) minimum (B) maximum (C) equal to atmospheric pressure (D) none of the above
22. Which of the following is an elastic wave?
(A) light wave (B) radio waves (C) sound wave (D) microwaves
23. Infrasonic vibrations have frequency:
(A) less than 10Hz (B) less than 20 Hz (C) between 20 and 20,000 Hz (D) more than 20,000 Hz.
24. The range of sonic waves is between:
(A) 20 Hz to 2000 Hz (B) 20 Hz to 10,000 Hz
(C) 20 Hz to 15,000 Hz (D) 20 Hz to 20,000 Hz
25. The sound waves having a frequency more than 20,000 Hz are called:
(A) infrasonic waves (B) supersonic waves
(C) ultrasonic waves (D) hypersonic waves
26. The animal which cannot hear ultrasonic waves is :
(A) bat (B) cow (C) dog (D) dolphin
27. The bat hunts it prey by emitting and receiving reflected:
(A) super sonic waves (B) ultrasonic waves (C) sonic waves (D) infrasonic waves
28. A sonic boom is produced in the air when an aircraft flies at a speed:
(A) equal to the speed of sound (B) more than the speed of sound
(C) less than the speed of sound (D) climbs vertically
29. Sound travels fastest in :
(A) vacuum (B) gases (C) solids (D) liquids
30. A boy sitting in a boat fires a gun. An observer P is at a distance of 50 m from the boat. Another observer Q is a diver, who is 50 m under water. Both hear the sound of gun:
(A) P hears the sound first (B) Q hears the sound first
(C) Both P and Q hear the sound at the same time (D) none of the above.
31. When the lightning strikes, we hear multiple of cracks of thunder. These multiple reflections of sound are called:
(A) echoes (B) reverberations (C) resonance (D) none of the above.
32. Hearing of repetition of sound after reflection from a distant object is called:
(A) reverberation (B) resonance (C) echo (D) none of the above.
33. For hearing an echo the minimum distance should be :
(A) less than 10m (B) between 10m and 15 m
(C) 17 m or more (D) none of the above.
34. An echo is heard only, if the original sound after reflection should reach the ear in :
(A) less than s (B) less than s (C) more than s (D) none of the above.
35. A bullet is moving at a speed, more than the speed of sound. It is said to be moving at :
(A) supersonic speed (B) ultrasonic speed (C) infrasonic speed (D) sonic speed
36. Naval ships called "destroyers" can detect submarines in the sea. The device used by these ships is called:
(A) ultra sonometer (B) sonar (C) ultrasonograph (D) none of the above .
37. Which of the following properties of a sound wave are affected b the change in temperature of air:
(A) frequency (B) amplitude (C) wavelength (D) intensity
38. Which of the following cases the sound travels fastest:
(A) hydrogen (B) helium (C) nitrogen (D) oxygen
39. The waves used in sonography as :
(A) microwaves (B) ultra-violet waves (C) ultrasonic waves (D) sound waves
40. The crack of thunder is heard after few seconds the lightning flash, because:
(A) crack of thunder and lightning are not produced at same time
(B) light travels extremely fast as compared to sound
(C) sound waves slow down on passing through air
(D) none of the above
