KFUPM-Physics102-032

Dr.Aljalal

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Recitation ch 18

Answers at the end

Question 1

A police car moves at a speed of 50.00 m/s behind

a truck that has a speed of 25.00 m/s as in figure 1.

The police siren has a frequency of 1200 Hz.

What is the frequency as heard by the truck driver ?

a 975.0 Hz

b 1125 Hz

c 1408 Hz

d 1302 Hz

e 1200 Hz

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Question 2

A point source of a sound wave has a power of 0.50 W.

At what distance from the source will the sound

level be 90 dB ?

a 40 m

b 80 m

c 34 m

d 6.3 m

e 8.9 m

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Question 3

Two speakers A and B are driven by a common oscillator at

256 Hz and face each other at a distance of 10.0 m (see

figure 2). A small detector is located midway between the

two speakers (at point O). Find the distance that the

detector has to move towards A along the line joining A

and B to detect the first minimum in the sound intensity.

[speed of sound in air = 343 m/s].

a 1.00 m

b 0.195 m

c 0.670 m

d 0.335 m

e 0.172 m

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Question 4

Two waves are given by the equations:

y1(x,t) = 5.0 sin( 0.25 x + 75 t)

y2(x,t) = 10.0 sin( 0.50 x + 150 t)

where x and y are in meters and t is in seconds. The

intensity ratio of I1/I2 of the two waves is:

a 4.

b 1/16.

c 1/3.

d 1/2.

e 1/4.

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Question 5

A pipe is closed at one end and open at the other end.

The length of the pipe is 1.00 m. The air column in the

pipe can resonate with a sound source of frequency:

[speed of sound in air = 343 m/s]

a 450 Hz

b 160 Hz

c 320 Hz

d 257 Hz

e 200 Hz

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Question 6

The average power of a sound source is 0.800 W.

Assume that the source is a point source which emits

sound equally in all directions.

At what distance will the sound level be 85.0 dB ?

a 57.0 m

b 16.7 m

c 14.0 m

d 24.3 m

e 8.50 m

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Question 7

Two trucks are moving toward each other. Truck A moves

at a speed of 13.8 m/s and truck B moves at a speed of

22.2 m/s. Truck A sounds its horn with a frequency of

500 Hz. What will be the frequency heard by truck B ?

[speed of sound in air = 343 m/s].

a 512 Hz

b 451 Hz

c 521 Hz

d 532 Hz

e 555 Hz

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Question 8

Which of the following statements is CORRECT ?

a Sound waves can travel in vacuum.

b The power intercepted by a sound detector does not

depend on the area of the detector.

c Electromagnetic waves can travel in vacuum.

d The speed of sound is the same in all media.

e The power transmitted by a sinusoidal wave on a string

decreases with increasing frequency of the wave.

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Question 9

If an observer’s distance from a point source is doubled,

the sound intensity level will be

a increased by 6 dB.

b decreased by 6 dB.

c decreased by 4 dB.

d increased by 36 dB.

e decreased by 36 dB.

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Question 10

Two small identical speakers are in phase(see figure 2).

The speakers are 3.0 m apart. An observer stands at point

X, 4.0 m in front of one of the speakers.

The sound he hears will be a maximum if the wavelength is

a 3.0 m.

b 1.0 m.

c 2.5 m.

d 1.5 m.

e 2.0 m.

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Question 11

Tube A (open at both ends) is twice as long as

tube B (open at one end) as shown in figure 3.

The ratio of their fundamental frequencies (f1A/f1B) is:

a 0.5

b 2

c 1

d 4

e 0.25

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Question 12

The displacement of a sound wave in air is given by:

s (x,t) = (7.00*10**-6)*[cos(5.23*x - 1800*t)]

where x and s are in meters and t is in seconds.

What is the pressure amplitude of this wave ?

a 8.62 Pa

b 1.32 Pa

c 9.54 Pa

d 5.20 Pa

e 4.31 Pa

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Question 13

A listener hears two sound waves from two loud-speakers that

are in phase. At the listener’s location a phase difference

of 450 degrees is detected. What is the path difference

if the wavelength of the waves is 4 m.

a 1 m.

b 10 m.

c zero.

d 99 m.

e 5 m.

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Question 14

In an air pipe, closed at one end, the three successive

resonance frequencies are 425 Hz, 595 Hz, and 765 Hz. If

the speed of sound in air is 340 m/s, the length of the

pipe is:

a 1.0 m.

b 0.5 m.

c 1.5 m.

d 2.5 m.

e 2.0 m.

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Question 15

A bat is moving toward a wall with a velocity of 30 m/s.

The bat is emitting a sound with frequency 40.0 kHz. The

frequency of the reflected sound as heard by the bat is:

[take the speed of sound in air = 340 m/s]

a 43.5 kHz.

b 40.0 kHz.

c 33.5 kHz.

d 43.9 kHz.

e 47.7 kHz.

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Answers

1 d

2 d

3 d

4 b

5 d

6 c

7 e

8 c

9 b

10 b

11 c

12 d

13 e

14 a

15 e