Deck 17: Sound

Question 1

A (point-source) loudspeaker produces an average intensity of 56.8 W/m $2$ at two meters away. The output power of the loudspeaker is

A) 2900 W.
B) 1200 W.
C) 710 W.
D) 580 W.

Answer 1

2900 W.

Question 2

A listener records a sound intensity of 12 W/m $2$ from a 250-W loudspeaker. The loudspeaker emits sound uniformly in all directions. The distance from the loudspeaker to the listener is

A) 2.6 m.
B) 5.2 m.
C) 1.3 m.
D) 4.1 m.

Answer 2

1.3 m.

Question 3

The sound intensity of a 100-W point-source loudspeaker at a distance of one meter is

A) 32 W/m $2$
B) 18 W/m $2$
C) 26 W/m $2$
D) 8.0 W/m $2$

Answer 3

8.0 W/m $2$

Question 4

The intensity level of a 0.5-W/m $2$ signal is

A) 270 dB.
B) 150 dB.
C) 117 dB.
D) 88 dB.

Question 5

Consider a 55-dB intensity level. The corresponding intensity in W/m $2$ is

A) $5.44 \times 10 ^ { - 7 }$ W/m $2$
B) $3.16 \times 10 ^ { - 7 }$ W/m $2$
C) $7.14 \times 10 ^ { - 7 }$ W/m $2$
D) $2.71 \times 10 ^ { - 7 }$ W/m $2$

Question 6

Consider a listener who is a distance $x$ from a loudspeaker. If the listener moves in so that her distance is now $x / 2$ from the loudspeaker, by what factor will the intensity of the sound increase?

A) 2
B) 3
C) 1.5
D) 4

Question 7

The intensity of sound (W/m $2$ ) varies as

A) the distance squared.
B) 1 over the distance.
C) 1 over the distance squared.
D) the distance.

Question 8

The intensity of sound (W/m $2$ ) varies as

A) the power squared.
B) 1 over the power.
C) 1 over the power squared.
D) the power.

Question 9

A person fires a pistol near the opening of a deep vertical well. Half a second later, the person hears the echo of the pistol firing. How deep is the well? (Use 344 m/s for the speed of sound.)

A) 168 m
B) 120 m
C) 98 m
D) 86 m

Question 10

The difference in intensity level (dB) between a single trumpet and two trumpets is

A) 3 dB.
B) 0 dB.
C) 2 dB.
D) none of the above.

Question 11

The wavelength of a 1000-Hz tone is (use 344 m/s for the speed of sound)

A) 3.1 m.
B) 4.5 m.
C) 34 cm.
D) 82 cm.

Question 12

A parked police car turns on its siren of 500 Hz. A thief in a car drives away from the parked police car at 31 m/s. The frequency of the siren as heard by the thief is (use 344 m/s for the speed of sound)

A) 455 Hz.
B) 465 Hz.
C) 530 Hz.
D) 545 Hz.

Question 13

Two cars travel in a straight line at the same speed; one car is ahead of the other. The car in the front blares its horn. The frequency as heard by the driver in the car behind

A) is higher.
B) is the same.
C) is lower.
D) cannot be determined.

Question 14

A stationary siren blares at 200 Hz. A person traveling in a car at 40 m/s toward the siren hears the siren at a frequency of (use 344 m/s for the speed of sound)

A) 185 Hz.
B) 191 Hz.
C) 223 Hz.
D) 245 Hz.

Question 15

A truck travels toward you blaring its horn; the frequency you hear is 76 Hz. Moments later the truck passes you, and now you hear the horn at 65 Hz as it travels away. If the speed of sound is 344 m/s, the true (stationary) frequency of the horn is

A) 70 Hz.
B) 72 Hz.
C) 68 Hz.
D) 66 Hz.

Question 16

A police car sounds its siren, which has a true frequency of 600 Hz, as it leaves the police station. If the police car is traveling at 35 m/s away from the station, the frequency of the siren as heard by listeners back at the station is (use 344 m/s for the speed of sound)

A) 668 Hz.
B) 545 Hz.
C) 621 Hz.
D) 582 Hz.

Question 17

The Mach half-angle for a jet traveling at exactly Mach 1 ( $V _ { j e t } = v$ ) is

A) 90º.
B) 0º.
C) 45º.
D) 30º.

Question 18

The Mach half-angle for a jet traveling at exactly Mach 2 ( $V _ { \text {jet } } = 2 v$ ) is

A) 90º.
B) 0º.
C) 45º.
D) 30º.

Question 19

As the velocity of a jet approaches infinity, the Mach half-angle becomes

A) 90º.
B) 0º.
C) 45º.
D) 30º.

Question 20

Two cars approach each other at the same speed. One driver hears the 500-Hz horn from the other car at a frequency of 600 Hz. The speed of the cars is

A) 27 m/s.
B) 31 m/s.
C) 29 m/s.
D) 33 m/s.

Question 21

The intensity of sound when integrated over any Gaussian surface ( $\int I d A$ ) that encloses the source yields

A) a conserved quantity.
B) the pressure.
C) a nonconserved quantity.
D) none of the above.

Question 22

The intensity of sound when integrated over any Gaussian surface ( $\int I d A$ ) that encloses the source yields

A) the energy.
B) the pressure.
C) the power.
D) none of the above.

Question 23

You are in a dark cave; suddenly you hear screeching sounds that are increasing in pitch. From this you discern that something is

A) coming toward you.
B) going away from you.
C) remaining stationary.
D) none of the above.

Question 24

If the pitch of the sound is increasing, then the wavelength

A) is increasing.
B) is decreasing.
C) is remaining constant.
D) cannot be determined.

Question 25

For any sound wave traveling in a medium, as the frequency is increased, the following remains constant:

A) Wavelength
B) Pitch
C) Velocity
D) None of the above

Question 26

The speed of sound in air v depends on the air pressure p and the air density as

A) $\nu \propto \sqrt { p \rho }$
B) $v \propto \sqrt { \frac { 1 } { p \rho } }$
C) $v \propto \sqrt { \frac { p } { \rho } }$
D) $v \propto \sqrt { \frac { \rho } { p } }$

Question 27

The speed of water waves in deep water depends on the two parameters

A) gravity and wavelength.
B) gravity and water depth.
C) wavelength and water depth.
D) none of the above.

Question 28

The speed of water waves in shallow water depends on the two parameters

A) gravity and wavelength.
B) gravity and water depth.
C) wavelength and water depth.
D) none of the above.

Question 29

Audible sound waves have wavelengths in air on the order of

A) a few microns to a few millimeters.
B) a few inches to a few feet.
C) a few tenths of miles to a few miles.
D) a few hundred miles to a few thousand miles.

Question 30

The speed of sound in air (in meters per second) is on the order of

A) 0003.
B) 0030.
C) 0300
D) 3000

Question 31

Diffraction is observed around an obstacle of size $\approx$ 20 $\lambda$ . If the obstacle is reduced in size to $\approx$ 2 $\lambda$ , the diffraction effect

A) increases.
B) remains unchanged.
C) decreases.
D) none of the above.

Question 32

All of the following "pairs" are measured or expressed in terms of a logarithmic scale except

A) intensity level-decibel.
B) musical scale-octave.
C) Richter scale-magnitude.
D) none of the above.

Question 33

A sonic boom can be heard by a listener

A) only at (or in the vicinity of) the time when the speed of the object reaches the speed of sound in the medium.
B) The previous answer is basically correct but must be modified to account for the time it takes the sound to reach the listener.
C) whenever the speed of the object is greater than or equal to the speed of sound in the medium.
D) The previous answer is basically correct but must be qualified by stipulating that the listener be at a location where the shock wave passes.

Question 34

Some musical (wind) instruments are open at both ends. The length L_both of the instrument required to achieve a specified fundamental frequency (compared to the length L_one of a musical instrument closed at only one end) is

A) L_both = 2 L_one.
B) 2 L_both = L_one.
C) L_both = 4 L_one.
D) 4 L_both = L_one.

Question 35

A musical (wind) instrument I_both is open at both ends. If it has the same fundamental frequency as an instrument I_one that is closed at one end, the numbers of possible audible harmonics for the two instruments are related by

A) n_both > n_one.
B) n_both = n_one.
C) n_both < n_one.
D) none of the above.

Question 36

A stationary emitter generates a sound at some particular frequency. The frequency detected by a receiver moving away from the emitter at speed v_r (< the speed of sound in still air) is roughly the same as if the

A) emitter were to move at the same speed v_r away from a stationary receiver.
B) emitter were to move at the same speed v_r toward a stationary receiver.
C) receiver were to move at the same speed v_r toward a stationary receiver.
D) none of the above.

Question 37

A stationary emitter generates a sound of frequency $V$ . The frequency shift $\delta v _ { \gamma }$ if the receiver is moving at a speed that is not < the speed of sound in still air is related to the frequency shift $\delta v _ { e }$ if the emitter moves at precisely the same speed (in the appropriate direction) with respect to a stationary receiver by the relation

A) $V _ { y } > V _ { e }$
B) $V _ { y } = V _ { e }$
C) $V _ { y } < V _ { e }$
D) none of the above.

Question 38

Confidence in the correctness of the answer for the previous question can be increased by recognizing that

A) if the emitter moves toward the receiver with a speed equal to that of sound, the frequency received is very large indeed.
B) if the receiver moves away from the emitter with a speed equal to that of sound, the frequency received is very small indeed.
C) both of the first two answers are relevant to the choice of the correct answer to the previous question.
D) none of the above.

Question 39

The angle $\theta$ that the wave front of a Mach cone makes with the direction of travel of a supersonic aircraft is related to the speed of sound v and the speed of the supersonic aircraft (emitter) V_E by the formula

A) V_E = v cos $\theta$
B) v = V_E cos $\theta$
C) V_E = v sin $\theta$
D) v = V_E sin $\theta$

Question 40

The intensity I of a wave is related to the distance the wave travels by the expression I ? 18 1/r². The assumptions on which this formula depend include all of the following except

A) the wave energy travels outward into three-dimensional space.
B) the medium is homogeneous.
C) the medium is nonabsorptive.
D) none of the above.

Question 41

An example of a longitudinal wave associated with earthquakes is

A) an S-wave.
B) a tsunami.
C) a P-wave.
D) a surface wave.

Question 42

Generally speaking, the speed of sound in solids (v_s) is related to the speed of sound in air (v_a) by the expression

A) v_s > v_a .
B) v_s $\approx$ v_a .
C) v_s < v_a .
D) none of the above.

Question 43

The primary reason for the correct answer to the previous question is that

A) the restoring force for solids is > the restoring force for air at atmospheric pressure.
B) the density of solids is > the density of air.
C) the effect of the first answer to this question predominates over the effect of the second answer.
D) the effect of the second answer to this question predominates over the effect of the first answer.

Question 44

An astute physics student is near a train track. As a train speeds by, the perceived frequency of the whistle changes from middle C to the A below middle C. If the initial perceived frequency of the whistle had been an octave above middle C, as the train whizzed by the frequency would have changed to

A) the A above middle C.
B) a frequency > the A above middle C.
C) a frequency < the A above middle C.
D) none of the above.

Question 45

The speed of earthquake waves near the surface of the Earth is on the order of 10 times the speed of sound in air. Periods are on the order of a second. Therefore, the distance between "crests" for these waves is on the order of

A) a fraction of an inch.
B) a foot.
C) several hundred feet.
D) a mile.

Question 46

The wavelength of tsunamis in midocean is on the order of several hundred miles. The speed of tsunami waves near the middle of the ocean is on the order of the speed of sound in air. Therefore, the time between successive crests for a ship in midocean that encounters a tsunami is on the order of

A) a day.
B) an hour.
C) a minute.
D) a second.

Question 47

The wavelength of tsunamis in midocean is on the order of several hundred miles. The speed of tsunami waves where the ocean is a few miles deep is on the order of the speed of sound in air. As the wave approaches land, its amplitude must

A) increase modestly.
B) increase dramatically.
C) decrease modestly.
D) decrease dramatically.

Question 48

The explanation for the correct answer to the previous question is that

A) the frequency of the wave changes.
B) the energy of the wave changes.
C) the speed of the wave changes.
D) none of the above.

Question 49

Under ideal conditions, if a receiver and an emitter are both at rest with respect to the ground and a wind is blowing from receiver to emitter, the received frequency $V _ { y }$ will be related to the emitted frequency $V _ { E }$ by the expression

A) $V _ { y } > V _ { E }$
B) $V _ { y } = V _ { E }$
C) $V _ { y } < V _ { E }$
D) none of the above.

Question 50

Under ideal conditions for sound waves, if a receiver is moving perpendicular to the line separating the receiver from an emitter, the received frequency $V _ { y }$ will be related to the emitted frequency $V _ { E }$ by the expression

A) $V _ { y } > V _ { E }$
B) $V _ { y } = V _ { E }$
C) $V _ { y } < V _ { E }$
D) none of the above.