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Chapter 13 SoundDate: _______________

Section A: Multiple Choice Questions

1 When a sound wave propagates through a reflective object of size comparable to the wavelength of

the sound,

A. most of the wave is diffracted. B. most of the wave is refracted.

C. most of the wave is reflected. D. none of the wave is diffracted.

2 A violin string is made to vibrate by a bow to produce a note. How to lower the pitch of the note?

A. Decrease the amplitude of vibration of the string.

B. Increase the amplitude of vibration of the string.

C. Decrease the frequency of vibration of the string.

D. Increase the frequency of vibration of the string.

3 The loudness of a sound wave is determined by

A. its amplitude. B. its frequency.

C. its wavelength. D. the medium of the propagation of the sound.

4 In a mini-concert of a rock band, a 120 dB sound intensity level is detected in the position of the front

row. In a room, if a speaker produces a sound of 100 dB intensity level, how many speakers would

be needed to produce the same intensity level as that in the front row of the concert?

A. 10 B. 20

C. 100 D. 200

5 A train whistle has an acoustic power output of 100 W. If the sound energy spreads out spherically,

what is the intensity level in dB at a distance of 100 m from the train?

(Given that I0 = 1012 W m

2.)

A. 55 dB B. 89 dB

C. 95 dB D. 120 dB

6 The frequency of a sound wave is determined by

A. the speed of the wave. B. the source of the wave.

C. the medium of the wave. D. the loudness of the wave.

7 Bees which are not carrying honey fly with a flapping frequency of 440 Hz while those which are

carrying honey fly with a flapping frequency of 300 Hz. The sound made by bees which are not

carrying honey has

A. a lower pitch. B. a higher pitch.

C. a smaller loudness. D. a greater loudness.

8 Which of the following statements about sound waves is correct?

A. The travel of sound waves is not affected by the medium through which it travels.

B. Sound waves travel faster in air than in liquid.

C. Sound waves travel faster in solid than in air.

D. Sound waves cannot travel through a solid.

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9 Which of the following is not a mechanical wave?

A. Sound wave B. Radio wave

C. Wave in spring D. Water wave in the deep ocean

10 What is the approximate range of audible frequencies for a human ear?

A. 20 Hz 20 kHz B. 20 kHz 200 kHz

C. 200 kHz 2 000 kHz D. 2000 kHz 20 000 kHz

11 The regions of compressions and rarefactions of a sound wave are established because

A. the sound wave undergoes diffraction behind obstacles.

B. the reflected sound wave at the fixed end interferes with the incident wave.

C. the speed of the sound wave changes as it travels through a medium.

D. the longitudinal movement of air molecules produces pressure fluctuations.

12 The frequency of a sound in a 8 m long metal rod is 250 Hz. There are two completed sound waves

in the rod. The speed of sound in this metal is

A. 250 m s1. B. 500 m s

1.

C. 750 m s1. D. 1 000 m s

1.

Section B: Questions1 In a science fiction film, people in a spaceship in the outer space could hear the sound of

another spaceship as it zooms past at high speed. The spaceship then travels through the

earths atmosphere and the frequency of the sound wave generated by it is 170 Hz.

However, its enemy cannot notice its movement on the earth through the radar system. It is

said to be invisible although it can be seen by the enemy when arriving their port and theyfound that the spaceship has a very flat surface.

(a) In the above science fiction film, what is wrong from the view of physics? Explain

briefly.

Ans:

(b) What is the wavelength of the sound wave produced by the spaceship? Given that

the speed of sound in air is 340 m s1.

Ans:

(c) Explain why the spaceship cannot be detected by the radar system.

Ans:

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2 (a) The frequencies of a G note and a C note are 384 Hz and 256 Hz respectively. If the

speed of sound in air is 340 m s1, calculate the wavelengths of the notes.

Ans:

(b) Which note has a higher pitch? Explain briefly.

Ans:

(c) If the ratio of frequencies of two notes is simple as follows, then the sound is said to

be pleasant. Some simple ratios are Octave, 2 : 1; Major Fifth, 3 : 2; Major Fourth, 4 :

3. If the G note and the C note in (a) is played together, do you think the resulting

sound is pleasant?

Ans:

3 A sonic device is used to measure the length of a room. It is placed in the middle of a room

and measures a time interval of 0.08 s between transmitting a sound pulse and receiving the

echo that is reflected from one of the walls. The speed of sound in air is 340 m s1.

(a) Calculate the length of the room.

Ans:

(b) State the difference between audible sound and ultrasound.

Ans:

(c) State two differences between light and ultrasound.Ans:

(d) State two other applications of ultrasound.

Ans:

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4 The figure shows a spring.

(a) (i) Describe how to demonstrate the motion of the air molecules in a sound wave

by the spring.

Ans:

(ii) If a sound wave in air is made for every 0.5 s, calculate the wavelength of the

sound. (Given that the speed of sound in air is 340 m s1.)

Ans:

(b) State the main difference between transverse wave and longitudinal wave. And give

two examples of each kind of waves.

Ans:

(c) A student talks to his friend and he says that in order for his friend to hear him, air

molecules must move from the lip of him to the ears of his friend. Do you agree with

him? Explain briefly.Ans:

End

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