Chapter 13: Sound

MULTIPLE CHOICE QUESTIONS

1.A list of mediums is given below.
(i) wood  (ii) water  (iii) air  (iv) vacuum
In which of these mediums can sound travel?
(a) i & ii only   (b) i, ii & iii only   (c) iii & iv only    (d) ii, iii & iv only
Answer: (b) i, ii & iii only

[ Sound needs a material medium to travel. It can move through solids (wood), liquids (water), and gases (air), but not through vacuum.]

2.The loudness of sound depends on:
(a) its amplitude.   (b) its frequency.  (c) its time period.  (d) its speed.

Answer: (a) its amplitude

[ Loudness depends on amplitude. Greater amplitude means louder sound, while smaller amplitude means softer sound.]
3. Which of the following statements are correct?
(i) Sound is produced by vibrations.
(ii) Sound requires a medium for propagation.
(iii) Light and sound both require a medium for propagation.
(iv) Sound travels slower than light.
(a) i & ii only              (b) i, ii & iii only
(c) ii, iii & iv only        (d) i, ii & iv only

Answer: (d) i, ii & iv only

[ Sound is produced by vibrations, needs a medium to travel, and travels slower than light. Light does not require a medium.]

4. An object is vibrating at 50 hertz. What is its time period?
(a) 0.02 s       (b)  2 s   (c) 0.2 s   (d) 20.0 s

Answer: (a) 0.02 s

[ Here,  

We have, Time period ]
5. In order to reduce the loudness of a sound we have to
(a) decrease its frequency of vibration of the sound.
(b) increase its frequency of vibration of the sound.
(c) decrease its amplitude of vibration of the sound.
(d) increase its amplitude of vibration of the sound.
Answer: (c) decrease its amplitude of vibration of the sound

[ Loudness depends on amplitude. Reducing amplitude makes the sound softer (less loud).]

6.Loudness of sound is measured in units of
(a) decibel (dB)    (b) hertz (Hz)  (c) metre (m)  (d) metre/second (m/s)

Answer: (a) decibel (dB)

[ Loudness is measured in decibels (dB). Hertz (Hz) measures frequency.]

7.The loudness of sound is determined by the
(a) amplitude of vibration
(b) ratio of amplitude and frequency of vibration
(c) frequency of vibration
(d) product of amplitude and frequency of vibration
Answer: (a) amplitude of vibration

[ Loudness depends only on amplitude. Greater amplitude gives louder sound; frequency affects pitch, not loudness.]

8. 1 hertz is equal to
(a) 1 vibration per minute
(b) 10 vibrations per minute
(c) 60 vibrations per minute
(d) 600 vibrations per minute
Answer: (c) 60 vibrations per minute

[ 1 hertz means 1 vibration per second. So in 1 minute (60 seconds), it becomes 60 vibrations per minute.]

9.Pitch of sound is determined by its
(a) frequency     (b) amplitude  (c) speed     (d) loudness
Answer: (a) frequency

[ Pitch depends on frequency. Higher frequency means higher pitch, and lower frequency means lower pitch.]

10.Ultrasound has frequency of vibration
(a) between 20 and 20,000 Hz         (b) below 20 Hz
(c) above 20,000 Hz                        (d) between 500 and 10,000 Hz

Answer: (c) above 20,000 Hz

[ Ultrasound refers to sound waves having frequency higher than 20,000 Hz, which is beyond the human hearing range.]

VERY SHORT ANSWER QUESTIONS

11. Lightning can be seen the moment it occurs. Paheli observes lightning in her area. She hears the sound 5 s after she observed lightning. How far is she from the place where lightning occurs? (speed of sound = 330 m/s).
Answer:  Given, Speed of sound = 330 m/s and Time = 5 s

We have, Distance = Speed × Time

Distance = 330 m/s × 5 s = 1650 metres

She is 1650 metres (or 1.65 km) away from the lightning.

12. Does any part of our body vibrate when we speak? Name the part.
Answer: Yes, the vocal cords (present in the voice box or larynx) vibrate when we speak.

13. Boojho saw a cracker burst at night at a distance from his house. He heard the sound of the cracker a little later after seeing the cracker burst. Give reason for the delay in hearing the sound.
Answer: Light travels much faster than sound. So Boojho sees the cracker burst first, but sound takes more time to reach his ears through air, causing delay.

14. When we hear a sound, does any part of our body vibrate? Name the part.
Answer: Yes. When we hear sound, our eardrum (tympanic membrane) vibrates. These vibrations are then passed to the inner ear and help us hear the sound.

15. Name two musical instruments which produce sound by vibrating strings?

Answer: Two musical instruments are guitar and violin.

SHORT ANSWER QUESTIONS

16. A simple pendulum makes 10 oscillations in 20 seconds. What is the time period and frequency of its oscillation?
Answer: Given, number of oscillations = 10  and time = 20 s

and 

17. We have learnt that vibration is necessary for producing sound. Explain why the sound produced by every vibrating body cannot be heard by us?
Answer:  The sound produced by a vibrating body cannot be heard by us if its frequency is less than 20 Hz or more than 20,000 Hz. These sounds are called infrasonic and ultrasonic sounds respectively, which are inaudible to the human ear.

18. Suppose a stick is struck against a frying pan in vacuum. Will the frying pan vibrate? Will we be able to hear the sound? Explain.
Answer: Yes, the frying pan will vibrate because striking gives it energy. But we will not hear any sound in vacuum because there is no air or medium to carry sound waves to our ears.

19. Two astronauts are floating close to each other in space. Can they talk to each other without using any special device? Give reasons.
Answer: No, they cannot talk without a special device. Sound needs a medium (like air) to travel, but space is a vacuum. Therefore, sound waves cannot travel from one astronaut to the other.

20. List three sources of noise pollution in your locality.
Answer: Three common sources of noise pollution in a locality are:

(i) Traffic like cars, bikes and horns

(ii) Loudspeakers used in functions or public events

(iii) Construction work such as drilling and hammering

LONG ANSWER QUESTIONS

21. We have a stringed musical instrument. The string is plucked in the middle first with a force of greater magnitude and then with a force of smaller magnitude. In which case would the instrument produce a louder sound?

Answer: The instrument would produce a louder sound when plucked with a force of greater magnitude, because larger force produces larger amplitude of vibration, resulting in a louder sound.

22. How is sound produced and how is it transmitted and heard by us?

Answer: Sound is produced by the vibration of an object. These vibrations create sound waves that travel through a medium (like air) in the form of compressions and rarefactions. When these waves reach our ear, they make the eardrum vibrate, which is heard as sound by us.

23. An alarm bell is kept inside a vessel as shown in Fig. 13.1. A person standing close to it can distinctly hear the sound of alarm. Now if the air inside the vessel is removed completely how will the loudness of alarm get affected for the same person?

 

            Fig. 13.1

Answer: The person will not hear any sound because sound requires a medium (like air) to travel. When air is removed, a vacuum is created . Since sound cannot travel through a vacuum, it will not reach the person’s ears and the loudness will become zero .

24. The townhall building is situated close to Boojho’s house. There is a clock on the top of the townhall building which rings the bell every hour. Boojho has noticed that the sound of the clock appears to be much clearer at night. Explain.

Answer: At night, the air is cooler and denser near the ground. Sound travels faster and with less loss in denser, cooler air. There is also less noise and disturbance during night. Hence, the sound of the clock appears much clearer at night.

25. Suggest three measures to limit noise pollution in your locality.

Answer: Three measures to reduce noise pollution are:

(i) Avoid unnecessary use of loud horns and keep vehicle sound low

(ii) Control loudspeaker volume, especially during late hours

(iii) Plant more trees, as they help absorb sound and reduce noise level.