Propagation of Sound Waves - Questions & Answers

(Choose the correct answer from the options given below).

1. The waves carry ........... energy of the vibrating particles with them so as to produce a sensation of hearing in our ears.
(a) light (b) chemical (c) mechanical (d) nuclear
Answer: (c) mechanical

2. Identify the correct statements :
(i) A material medium is necessary for the propagation of sound from one place to another.
(ii) The medium must have inertia.
(iii) The medium must be inelastic.
(iv) The medium must be frictionless.
(a) (i) and (ii) (b) (i), (ii) and (iii) (c) (i), (ii) and (iv) (d) (ii), (iii) and (iv)
Answer: (c) (i), (ii) and (iv)

3. The correct statement is :
(a) Sound and light both require medium for propagation
(b) Sound can travel in vacuum, but light cannot
(c) Sound needs medium, but light does not need medium for its propagation
(d) Sound and light both can travel in vacuum.
Answer: (c) Sound needs medium, but light does not need medium for its propagation

4. Sound in air propagates in form of :
(a) longitudinal wave (b) transverse wave (c) both longitudinal and transverse waves (d) neither longitudinal nor transverse wave.
Answer: (a) longitudinal wave

5. The S.I. unit of wavelength is :
(a) metre (b) hertz (c) second (d) m/s
Answer: (a) metre

6. The position of maximum upward displacement of medium particles in a transverse wave is known as :
(a) trough (b) compression (c) rarefaction (d) crest
Answer: (d) crest

7. The wave in which the particles of a medium vibrate about their mean position in a direction ............ to the direction of propogation of the wave is called transverse wave.
(a) parallel (b) similar (c) perpendicular (d) none of the above
Answer: (c) perpendicular

8. In a longitudinal wave, the distance between two consecutive ............ is equal to ............ wavelength(s).
(a) compressions, one (b) compressions, two (c) crests, one (d) crests, two
Answer: (a) compressions, one

9. Transverse waves can be produced inside :
(a) solids (b) liquids (c) gases (d) both (b) and (c)
Answer: (a) solids

10. Longitudinal waves can be produced in :
(a) solids (b) liquids (c) gases (d) All of the above
Answer: (d) All of the above

11. The maximum displacement of the medium particle on either side of its mean position is called ....... of the wave.
(a) frequency (b) time period (c) wavelength (d) amplitude
Answer: (d) amplitude

12. At compressions, the density and pressure of a medium is .........., while at rarefactions the density and pressure of a medium is ...........
(a) minimum, minimum (b) minimum, maximum (c) maximum, maximum (d) maximum, minimum
Answer: (d) maximum, minimum

13. Wave velocity is also defined as the ............. with which ............ is transferred from one place to another place by wave motion.
(a) speed, energy (b) speed, compression (c) speed, rarefaction (d) velocity, pressure
Answer: (a) speed, energy

14. For sound to travel through medium, it need not :
(a) be rigid (b) be elastic (c) possess inertia (d) be frictionless
Answer: (a) be rigid

15. The speed of sound in air is not affected by change in :
(a) temperature (b) moisture (c) pressure (d) density
Answer: (c) pressure

16. The speed of sound in a gas is given by :
(a) V = √(P / γρ) (b) V = √(γP / ρ) (c) V = √(γρ / P) (d) V = √(γpP)
Answer: (b) V = √(γP / ρ)

17. The speed of sound in air at 0°C is nearly :
(a) 1450 m s^-1 (b) 450 m s^-1 (c) 5100 m s^-1 (d) 330 m s^-1
Answer: (d) 330 m s^-1

18. The speed of sound in a gas is related to temperature (in Kelvin) as :
(a) directly proportional to temperature.
(b) directly proportional to the square root of temperature.
(c) inversely proportional to temperature.
(d) inversely proportional to the square root of temperature.
Answer: (b) directly proportional to the square root of temperature.

19. Factors affecting the speed of sound are :
(a) temperature (b) density (c) pressure (d) only (a) and (b)
Answer: (d) only (a) and (b)

1. What causes sound ?
Answer: Sound is caused due to vibrations of a body.

2. Complete the following sentence : Sound is produced by a ............. body.
Answer: Sound is produced by a vibrating body.

3. There is no atmosphere on moon. Can you hear other people on the moon's surface ?
Answer: No.

4. Choose the correct word/words to complete the following sentence :
When sound travels in a medium ...... (the particles of the medium, the source, the disturbance, the medium) travels in form of a wave.
Answer: When sound travels in a medium the disturbance travels in form of a wave.

5. Name the two kinds of waves in form of which sound travels in a medium.
Answer: Transverse wave and longitudinal wave.

6. What is a longitudinal wave ? In which medium: solid, liquid or gas, can it be produced ?
Answer: The wave in which the particles of a medium vibrate about their mean positions, in the direction of propagation of sound, is called a longitudinal wave. It can be produced in all types of media (solid, liquid and gas).

7. What is a transverse wave ? In which medium: solid, liquid or gas, can it be produced ?
Answer: The wave in which the particles of a medium vibrate about their mean positions, in a direction perpendicular to the direction of propagation of the wave, is called the transverse wave. It can be produced only in solids and at the surface of liquids.

8. Define the term amplitude of a wave. Write its S.I. unit.
Answer: The maximum displacement of the particle of the medium on either side of its mean position is called the amplitude of the wave. Its S.I. unit is metre (m).

9. How is the frequency of a wave related to its time period?
Answer: Frequency = 1 / time period (or f = 1 / T).

10. Arrange the speed of sound in gases V_g, solids V_s and liquids V_l in an ascending order.
Answer: V_g < V_l < V_s

11. State the speed of (i) light and (ii) sound in air ?
Answer: (i) Speed of light = 3 × 10⁸ m s^-1 (ii) Speed of sound = 330 m s^-1

12. Answer the following :
(a) Can sound travel in vacuum ?
(b) How does the speed of sound differ in different media ?
Answer: (a) No. (b) The speed of sound is maximum in solids, less in liquids and least in gases.

13. Flash of lightning reaches us earlier than the sound of thunder. Explain the reason.
Answer: Light travels much faster than sound. (Speed of light is 3 × 10⁸ m s^-1 whereas speed of sound in air is only about 330 m s^-1).

14. How does the speed of sound in air vary with temperature?
Answer: The speed of sound increases by 0.61 m s^-1 for each 1°C rise in temperature.

15. Complete the following sentences :
(a) Sound cannot travel through ...............; it requires a ...............
Answer: Sound cannot travel through vacuum; it requires a medium.

(b) When sound travels in a medium, the particles of medium ............... but the disturbance ...............
Answer: When sound travels in a medium, the particles of medium do not move ahead but the disturbance moves ahead.

(c) A longitudinal wave is composed of compression and ...............
Answer: A longitudinal wave is composed of compression and rarefaction.

(d) A transverse wave is composed of crest and ...............
Answer: A transverse wave is composed of crest and trough.

(e) Wave velocity = ...................... × wavelength.
Answer: Wave velocity = frequency × wavelength.

1. State three characteristics of the medium required for the propagation of sound ?
Answer: (i) The medium must be elastic. (ii) The medium must have inertia. (iii) The medium should be frictionless.

2. Explain with an example, the propagation of sound in a medium.
Answer: When a thin metal strip is fixed at its lower end and pushed to vibrate, its forward motion pushes and compresses the air layers in front of it (creating compression). As it returns and moves backwards, it creates a space of very low pressure (rarefaction). These compressions and rarefactions move forward in the air as a longitudinal wave, transferring the disturbance and energy through the medium.

3. Explain the meaning of terms compression and rarefaction in relation to a longitudinal wave.
Answer: In a longitudinal wave, compression is the region where the particles of the medium are closer together (density and pressure are maximum). Rarefaction is the region where the particles of the medium are spread apart (density and pressure are minimum).

4. What do you mean by the term frequency of a wave ? State its S.I. unit.
Answer: The number of vibrations made by a particle of a medium in one second is called the frequency of wave. Its S.I. unit is second^-1 (symbol s^-1) or hertz (symbol Hz).

5. Define the term wave velocity. Write its S.I. unit.
Answer: The distance travelled by a wave in one second is called its wave velocity or wave speed. Its S.I. unit is metre per second (m s^-1).

6. Draw displacement-time graph of a wave and show on it the amplitude and time period of wave.
Answer: [Drawing of a transverse sine wave on a graph with Time (t) on the X-axis and Displacement on the Y-axis. The maximum upward or downward peak from the axis is marked as amplitude 'a'. The total width of one complete wave cycle on the X-axis is marked as Time period 'T'.]

7. Draw a displacement-distance graph of a wave and mark on it, the amplitude of wave by the letter a and wavelength of wave by the letter λ.
Answer: [Drawing of a transverse sine wave on a graph with Distance (x) on the X-axis and Displacement on the Y-axis. The maximum upward or downward peak from the axis is marked as amplitude 'a'. The total width of one complete wave cycle on the X-axis is marked as wavelength 'λ'.]

8. State two properties of the medium on which the speed of sound in it depends.
Answer: The speed of sound in a medium depends on (i) the elasticity (E) of the medium, and (ii) the density (ρ) of the medium.

9. Compare approximately the speed of sound in air, water and steel.
Answer: 1 : 4 : 15 (Speed is roughly 330 m s^-1 in air, 1450 m s^-1 in water, and 5100 m s^-1 in steel).

10. If you place your ear close to an iron railing which is struck some distance away, you hear the sound twice. Explain why ?
Answer: Sound travels in iron much faster than in air. So first the sound travelling through the iron rail is heard and then the sound travelling through the air is heard.

11. The sound of an explosion on the surface of a lake is heard by a boat man 100 m away and by a diver 100 m below the point of explosion.
(a) Who would hear the sound first : boat man or diver?
(b) Give a reason for your answer in part (i).
(c) If sound takes time t to reach the boat man, how much time approximately does it take to reach the diver ?
Answer: (a) diver (b) sound travels faster in water than in air (c) 0.25 t.

12. How does the speed of sound change with change in (i) amplitude and (ii) wavelength, of sound wave ?
Answer: (i) No change (ii) No change.

13. In which medium the speed of sound is more : humid air or dry air ? Give a reason to your answer.
Answer: Humid air. Reason : Density of air decreases with the increase of moisture, and speed of sound is inversely proportional to the square root of density (V ∝ 1/√ρ).

1. (a) What is sound ? How is it produced ?
(b) Describe a simple experiment which demonstrates that the sound produced by a tuning fork is due to vibrations of its arms.
Answer: (a) Sound is a form of energy that produces the sensation of hearing in our ears. It is produced when a body vibrates.
(b) Take a tuning fork and strike its one arm on a rubber pad and bring it near a table tennis ball suspended by a thread. It is noticed that the arm of the vibrating tuning fork causes the ball to jump to and fro, and sound is heard. When its arm stops vibrating, the ball becomes stationary and no sound is heard.

2. Describe in brief, with the aid of a labelled diagram, an experiment to demonstrate that a material medium is necessary for the propagation of sound.
Answer: Bell jar experiment: An electric bell is suspended inside an air-tight glass bell jar connected to a vacuum pump. When the circuit is completed by pressing the key, the hammer strikes the gong and sound is heard. As air is gradually withdrawn using the vacuum pump, the loudness of sound decreases. Finally, no sound is heard when the entire air has been drawn out, although the hammer is still seen striking the gong. This demonstrates that a material medium is necessary for sound propagation.

3. Describe an experiment to show that in wave motion, only energy is transferred, but particles of medium do not leave their positions.
Answer: If we drop a piece of stone in the still water of a pond, a disturbance spreads outwards in the form of ripples. If we place a piece of cork on the water surface at some distance away, we notice that the cork does not move ahead, but it moves up and down at its place, while the wave moves ahead. The particles of water transfer their energy to neighbouring particles and return to their mean positions.

4. How do the following factors affect, if at all, the speed of sound in air :
(i) frequency of sound, (ii) temperature of air, (iii) pressure of air, (iv) moisture in air ?
Answer: (i) No effect (ii) Speed of sound increases with the increase in temperature (iii) No effect (iv) Speed of sound increases with the increase of moisture in air.

5. How are the wave velocity V, frequency f and wavelength λ of a wave related ? Derive the relationship.
Answer: V = f × λ.
Derivation: By the definition of wavelength, Wavelength λ = Distance travelled by the wave in one time period i.e., in T second = Wave velocity × Time period = V × T. Or V × T = λ. But T = 1 / f. Therefore, V × (1/f) = λ or V = fλ.

6. Describe a simple experiment to determine the speed of sound in air. What approximation is made in the method described by you ?
Answer: Choose two places A and B at high altitudes facing each other, at a known distance d apart (say, about 1 km). Observer at place A fires a gun. Observer at B starts a stop watch immediately on seeing the flash and stops it when he hears the sound, recording time t₁. Then observer at B fires the gun, and observer at A measures time t₂ similarly. The average time t = (t₁ + t₂)/2 is the time taken by sound to travel distance d. Speed of sound is V = d / t.
Approximation: We assume that light takes negligible time and sound takes the entire measured time to reach the observer, as the speed of light is about a million times faster than sound.

1. The heart of a man beats 75 times a minute. What is its (a) frequency and (b) time period ?
Answer: (a) 1.25 s^-1, (b) 0.8 s.

2. The time period of a simple pendulum is 2 s. Find its frequency.
Answer: 0.5 Hz.

3. The separation between two consecutive crests in a transverse wave is 100 m. If wave velocity is 20 m s^-1, find the frequency of wave.
Answer: 0.2 Hz.

4. A longitudinal wave travels at a speed of 0.3 m s^-1 and the frequency of wave is 20 Hz. Find the separation between the two consecutive compressions.
Answer: 1.5 × 10^-2 m (or 1.5 cm).

5. A source of wave produces 40 crests and 40 troughs in 0.4 s. What is the frequency of the wave ?
Answer: 100 Hz.

6. An observer A fires a gun and another observer B at a distance 1650 m away from A hears its sound. If the speed of sound is 330 m s^-1, find the time when B will hear the sound after firing by A.
Answer: 5 s.

7. The time interval between a lightning flash and the first sound of thunder is 5 s. If the speed of sound in air is 330 m s^-1, find the distance of flash from the observer.
Answer: 1650 m.

8. A boy fires a gun and another boy at a distance hears the sound of fire 2.5 s after seeing the flash. If speed of sound in air is 340 m s^-1, find the distance between the boys.
Answer: 850 m.

9. An observer sitting in line of two tanks, watches the flashes of two tanks firing at each other at the same time, but he hears the sounds of two shots 2 s and 3.5 s after seeing the flashes. If distance between the two tanks is 510 m, find the speed of sound.
Answer: 340 m s^-1.

10. How long will sound take to travel in (a) an iron rail and (b) air, both 3.3 km in length ? Take speed of sound in air to be 330 m s^-1 and in iron to be 5280 m s^-1.
Answer: (a) 0.625 s, (b) 10 s.

11. Assuming the speed of sound in air equal to 340 m s^-1 and in water equal to 1360 m s^-1, find the time taken to travel a distance 1700 m by sound in (i) air and (ii) water.
Answer: (i) 5 s (ii) 1.25 s.

(Choose the correct answer from the options given below).

1. A man can hear a sound of frequency :
(a) 1 Hz (b) 1000 Hz (c) 200 kHz (d) 5 MHz
Answer: (b) 1000 Hz

2. The speed of ultrasound in air is :
(a) the same as audible sound (b) less than audible sound (c) more than audible sound (d) equal to the speed of light
Answer: (a) the same as audible sound

3. Echo-cardiography is used to obtain the image of :
(a) lungs (b) liver (c) heart (d) kidney
Answer: (c) heart

4. Elephants produce ......... sound :
(a) ultrasonic (b) infrasonic (c) supersonic (d) none of the above
Answer: (b) infrasonic

5. ......... can hear the highest frequency range of sound.
(a) Whales (b) Dogs (c) Cats (d) Bats
Answer: (d) Bats

6. Choose the correct statement :
(a) Ultrasonic waves travel faster than supersonic waves.
(b) Sonic waves travel faster than supersonic waves.
(c) Sonic waves travel faster than ultrasonic waves.
(d) Sonic and ultrasonic waves travel with the same speed.
Answer: (d) Sonic and ultrasonic waves travel with the same speed.

7. The properties of ultrasound that make it useful are :
(a) high power and high speed (b) high power and good directivity (c) high frequency and high speed (d) high frequency and bending around the objects.
Answer: (b) high power and good directivity

8. Sonar makes use of :
(a) infrasonic sound (b) ultrasound (c) supersonic sound (d) light.
Answer: (b) ultrasound

1. What do you mean by the audible range of frequency?
Answer: The range of frequency within which the sound can be heard by a human being is called the audible range of frequency.

2. What is the audible range of frequency for humans ?
Answer: 20 Hz to 20 kHz.

3. For which range of frequencies are the human ears most sensitive ?
Answer: 2000 Hz to 3000 Hz.

4. Which has a higher frequency - ultrasonic sound or infrasonic sound ?
Answer: Ultrasonic sound.

5. Complete the following sentences :
(a) An average person can hear sounds of frequencies in the range ............ to ............
Answer: An average person can hear sounds of frequencies in the range 20 Hz to 20 kHz.

(b) Ultrasound is of frequency ............
Answer: Ultrasound is of frequency above 20 kHz.

(c) Infrasonic sound is of frequency ............
Answer: Infrasonic sound is of frequency below 20 Hz.

(d) Bats can produce and hear ............ sound.
Answer: Bats can produce and hear ultrasonic sound.

(e) Elephants produce ............ sound.
Answer: Elephants produce infrasonic sound.

6. Name the sounds of the frequencies given below:
(a) 10 Hz (b) 100 Hz (c) 1000 Hz (d) 40 kHz
Answer: (a) infrasonic (b) audible (c) audible (d) ultrasonic.

7. What is ultrasound ?
Answer: Sound of frequency above 20 kHz.

8. State the approximate speed of ultrasound in air.
Answer: 330 m s^-1.

1. Can you hear the sound produced due to vibrations of a seconds' pendulum ? Give reason.
Answer: No. Reason : The frequency of sound produced due to vibrations of seconds' pendulum is 0.5 Hz which is an infrasonic sound.

2. State two properties of ultrasound that make it useful to us.
Answer: (i) High energy content, (ii) high directivity.

3. State two applications of ultrasound.
Answer: (1) Bats use it to avoid obstacles in their path and locate prey. (2) It is used for cleaning minute objects such as the parts of watches and electronic components in a cleaning solution.

1. Differentiate between infrasonic, sonic, ultrasonic and supersonic sounds.
Answer: Infrasonic sounds have a frequency below 20 Hz. Sonic (audible) sounds are in the frequency range of 20 Hz to 20,000 Hz. Ultrasonic sounds have a frequency above 20,000 Hz. Supersonic refers not to a frequency, but to the speed of an object moving faster than the speed of sound in air (e.g., Concorde jet planes).

2. Explain how do bats locate the obstacles and prey in their way.
Answer: Bats produce and hear ultrasound which returns after striking an obstacle or prey in their path. By hearing the reflected sound, they judge the direction of the obstacle, and from the time interval between when they produce the ultrasound and receive it back, they judge the distance of the obstacle.

(Choose the answer based on the codes given below.)
(a) Both A and R are true and R is the correct explanation of A
(b) Both A and R are true and R is not the correct explanation of A
(c) Assertion is false but reason is true
(d) Assertion is true but reason is false

(i) Assertion (A) : Sound waves cannot travel through vacuum.
Reason (R) : Sound travels in the form of longitudinal waves.
Answer: (b) Both A and R are true and R is not the correct explanation of A.

(ii) Assertion (A) : The speed of sound is maximum in gases when compared with speed in solids.
Reason (R) : The speed of sound in a medium depends on the density of medium.
Answer: (c) Assertion is false but reason is true.

(iii) Assertion (A) : Sound travels faster in humid air than in dry air.
Reason (R) : The increase of moisture in air tends to decrease the density of air.
Answer: (a) Both A and R are true and R is the correct explanation of A.

(iv) Assertion (A) : Two persons on the surface of moon can not talk to each other.
Reason (R) : The speed of sound decreases with the decrease in temperature.
Answer: (b) Both A and R are true and R is not the correct explanation of A.

(v) Assertion (A) : The speed of ultrasound waves is higher than supersonic waves.
Reason (R) : Supersonic is used for objects travelling with speed greater than speed of sound in air.
Answer: (c) Assertion is false but reason is true.

1. Three observers hear the sound of an explosion from the same distance but through different media.
Observer A hears the sound through air, B hears it through water, and C hears it through steel.

(a) Arrange the observers in the order in which they would hear the sound first.
Answer: C → B → A.

(b) In which medium will the sound travel the fastest and why?
Answer: Steel. Sound travels fastest in solids because they are much more elastic than liquids and gases.

(c) On a cold morning, sound from a distant train is heard more clearly than during noon. Give reason.
Answer: On a cold morning, the air near the ground is colder and denser, allowing the sound waves to travel more effectively near the surface without scattering upwards as heavily as it does in warmer, less dense air during noon.

(d) "Solids are heavier, therefore sound should travel slower in them." Analyse this statement.
Answer: This statement is incorrect. While density does inversely affect sound speed in gases, in solids, the dominant factor is elasticity. Solids are much more elastic than liquids and gases, which makes sound travel fastest in solids despite their higher density.

(e) If sound could travel in vacuum, what property of sound would have to change?
Answer: Sound is a mechanical wave that requires a material medium to propagate via particle vibration. To travel in a vacuum, its fundamental nature would have to change to that of an electromagnetic wave, which does not require a medium.