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
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
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
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.
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
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
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
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.
Question 9
If an observerís 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.
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.
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
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
Question 13
A listener hears two sound waves from two loud-speakers that
are in phase. At the listenerís 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.
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.
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.

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