Deck 17: Sound Waves

Question 1

The variation in the pressure of helium gas,measured from its equilibrium value,is given by ΔP = 2.9 × 10−⁵ cos (6.20x − 3 000t)where x and t have units m and s.Determine the speed (in m/s)of the wave.

A)1 515
B)153
C)484
D)828
E)101

Answer 1

484

Question 2

Determine the intensity (in W/m²)of a harmonic longitudinal wave with a pressure amplitude of 8.0 × 10−³ N/m² propagating down a tube filled with helium.(ρ = 0.179 kg/m³,v = 972 m/s. )

A)3.7 × 10−⁷
B)1.8 × 10−⁷
C)9.2 × 10−⁸
D)4.6 × 10−⁸
E)1.5 × 10−⁹

Answer 2

1.8 × 10−⁷

Question 3

It is possible to hear an approaching train before you can see it by listening to the sound wave through the track.If the elastic modulus is 2.0 × 10¹¹ N/m² and the density of steel is 7.8 × 10³ kg/m³,approximately how many times faster is the speed of sound in the track than in air? (v_air ≈ 340 m/s. )

A)20
B)5
C)10
D)15
E)25

Answer 3

15

Question 4

The variation in the pressure of helium gas,measured from its equilibrium value,is given by ΔP = 2.9 × 10−⁵ cos (6.20x − 3 000t)where x and t have units m and s,and ΔP is measured in N/m².Determine the frequency (in Hz)of the wave.

A)1 500
B)477
C)1.01
D)0.32
E)239

Question 5

A jet plane has a sound level of 150 dB.What is the intensity in W/m²?

A)1
B)100
C)10
D)1 000
E)10 000

Question 6

Calculate the pressure amplitude (in N/m²)of a 500 Hz sound wave in helium if the displacement amplitude is equal to 5.0 × 10−⁸ m.(ρ = 0.179 kg/m³,v = 972 m/s. )

A)3.5 × 10−²
B)1.6 × 10−²
C)2.7 × 10−²
D)4.2 × 10−²
E)2.0 × 10−²

Question 7

A car approaches a stationary police car at 36 m/s.The frequency of the siren (relative to the police car)is 500 Hz.What is the frequency (in Hz)heard by an observer in the moving car as he approaches the police car? (Assume the velocity of sound in air is 343 m/s. )

A)220
B)448
C)526
D)552
E)383

Question 8

A car moving at 36 m/s passes a stationary police car whose siren has a frequency of 500 hz.What is the change in the frequency (in Hz)heard by an observer in the moving car as he passes the police car? (The speed of sound in air is 343 m/s. )

A)416
B)208
C)105
D)52
E)552

Question 9

A truck moving at 36 m/s passes a police car moving at 45 m/s in the opposite direction.If the frequency of the siren is 500 Hz relative to the police car,what is the frequency heard by an observer in the truck after the police car passes the truck? (The speed of sound in air is 343 m/s. )

A)361
B)636
C)393
D)396
E)383

Question 10

The velocity of sound in sea water is 1.53 × 10³ m/s.Find the bulk modulus (in N/m²)of sea water if its density is 1.03 × 10³ kg/m³.

A)2.57 × 10⁹
B)2.19 × 10⁹
C)2.04 × 10⁹
D)2.41 × 10⁹
E)2.82 × 10⁹

Question 11

By what factor will an intensity change when the corresponding sound level increases by 3 dB?

A)3
B)0.5
C)2
D)4
E)0.3

Question 12

A point source emits sound waves with a power output of 100 watts.What is the sound level (in dB)at a distance of 10 m?

A)139
B)119
C)129
D)109
E)10

Question 13

A truck moving at 36 m/s passes a police car moving at 45 m/s in the opposite direction.If the frequency of the siren relative to the police car is 500 Hz,what is the frequency heard by an observer in the truck as the police car approaches the truck? (The speed of sound in air is 343 m/s. )

A)396
B)636
C)361
D)393
E)617

Question 14

The Young's modulus for aluminum is 7.02 × 10¹⁰ N/m².If the speed of sound in aluminum is measured to be 5.10 km/s,find its density (in kg/m³).

A)11.3 × 10³
B)7.80 × 10³
C)2.70 × 10³
D)29.3 × 10³
E)1.40 × 10³

Question 15

The variation in the pressure of helium gas,measured from its equilibrium value,is given by ΔP = 2.9 × 10−⁵ cos (6.20x − 3 000t)where x and t have units m and s,and ΔP is measured in N/m².Determine the wavelength (in m)of the wave.

A)1 500
B)0.32
C)477
D)1.01
E)0.50

Question 16

A point source emits sound with a power output of 100 watts.What is the intensity (in W/m²)at a distance of 10.0 m from the source?

A)7.96 × 10−²
B)7.96 × 10−¹
C)7.96 × 10⁰
D)7.96 × 10¹
E)7.96 × 10−³

Question 17

Calculate the intensity level in dB of a sound wave that has an intensity of 15 × 10−⁴ W/m².

A)20
B)200
C)92
D)9
E)10

Question 18

A sculptor strikes a piece of marble with a hammer.Find the speed of sound through the marble (in km/s).(The Young's modulus is 50 × 10⁹ N/m² and its density is 2.7 × 10³ kg/m³. )

A)5.1
B)4.3
C)3.5
D)1.3
E)1.8

Question 19

By what factor is the intensity of sound at a rock concert louder than that of a whisper when the two intensity levels are 120 dB and 20 dB respectively?

A)10¹²
B)10⁸
C)10⁶
D)10¹⁰
E)10¹¹

Question 20

Calculate the displacement amplitude (in m)of a 20 kHz sound wave in helium if it has a pressure amplitude of 8.0 × 10−³ N/m².(ρ = 0.179 kg/m³,v = 972 m/s. )

A)2.9 × 10−¹⁰
B)3.7 × 10−¹⁰
C)7.8 × 10−⁹
D)2.4 × 10−⁹
E)1.9 × 10−¹⁰

Question 21

A boy has climbed to the top of a 6.00 m tall tree.When he shouts,the sound waves have an intensity of
At a 1.00 m distance from the top of the tree.The ratio of intensity at the base of the tree to the intensity 1.00 m from the top of the tree is

A) .
B) .
C) .
D) .
E) .

Question 22

A stone is thrown into a quiet pool of water.With no fluid friction,the amplitude of the waves falls off with distance r from the impact point as

A)1/r³
B)1/r²
C)1/r^3/2
D)1/r^1/2
E)1/r

Question 23

A spherical wave has the form
)
The amplitude of the wave a distance r from the source is

A)0.002 cm.
B) .
C) .
D) .
E) .

Question 24

A friend hands you an equation sheet with the following equation for the Doppler effect:
)This version of the equation is correct with signs as given only if

A)the observer and source are approaching each other.
B)the observer is approaching the source while the source is moving away from the observer.
C)the observer is moving away from the source while the source is approaching the observer.
D)the observer and source are moving away from each other.
E)the observer and source are moving in perpendicular directions.

Question 25

A spherical wave has the form
)
The velocity of the wave (in m/s)is

A)0.005 88.
B)16.0
C)340.
D)1 360.
E)2 720.

Question 26

A spherical wave has the form
)
The wavelength of the wave is

A)0.25 m.
B)0.50 m.
C)4.0 m.
D)8.0 m.
E)4.0π m.

Question 27

To decrease the intensity of the sound you are hearing from your speaker system by a factor of 36,you can

A)reduce the amplitude by a factor of 12 and increase your distance from the speaker by a factor of 3.
B)reduce the amplitude by a factor of 4 and increase your distance from the speaker by a factor of 3.
C)reduce the amplitude by a factor of 2 and increase your distance from the speaker by a factor of 3.
D)reduce the amplitude by a factor of 3 and increase your distance from the speaker by a factor of 4.
E)reduce the amplitude by a factor of 3 and increase your distance from the speaker by a factor of 12.

Question 28

While you are sounding a tone on a toy whistle,you notice a friend running toward you.If you want her to hear the same frequency that you hear even though she is approaching,you must

A)stay put.
B)run towards her at the same speed.
C)run away from her at the same speed.
D)stay put and play a note of higher frequency.
E)run towards her and play a note of higher frequency.

Question 29

A boat sounds a fog horn on a day when both the sea water and the air temperature are 25.0° C.The speed of sound in sea water is 1 533 m/s.How much earlier (in s)does a dolphin 1 000 m from the source hear the sound than a person in a boat that is also 1 000 m distant? (Ignore the time it takes the sound to reach the water surface. )

A)0.652
B)2.12
C)2.24
D)2.77
E)2.90

Question 30

The fundamental frequency of a above middle C on the piano is 440 Hz.This is the tenor high A,but a convenient note in the mid-range of women's voices.When we calculate the wavelength,we find that it is

A)much shorter than the length of either a man's or woman's lips.
B)shorter than the length of a man's lips,but about the length of a woman's lips.
C)longer than a woman's lips,but about the length of a man's lips.
D)much longer than the length of either a man's or a woman's lips.
E)about the same length as either a man's or woman's lips.

Question 31

On a day when the speed of sound in the upper air is 320 m/s,you fly coast to coast in the United States,a distance of about 4 850 km,in about one hour,if the Mach number for the speed of your airplane is about

A)1.
B)2
C)3
D)4
E)5.

Question 32

Which of the following does not have a precise definition in terms of the physical properties of sound waves?

A)frequency
B)intensity
C)loudness
D)sound level
E)wavelength

Question 33

(Do not try the following: it could kill you.This question is only about a hypothetical possibility. )If you were standing below an object emitting a fixed frequency sound falling at terminal velocity,as it approached you,you would hear the sound

A)drop in frequency.
B)stay at the same frequency.
C)increase in frequency.
D)decrease in loudness.
E)stay at the same loudness.

Question 34

A source of sound waves is placed at the center of a very large sound-reflecting wall.The source emits 0.500 W of power.Two meters from the source the intensity in W/m² is

A)9.95 × 10−³.
B)1.99 × 10−².
C)0.313.
D)0.399.
E)0.625.

Question 35

A person standing in the street is unaware of a bird dropping that is falling from a point directly above him with increasing velocity.If the dropping were producing sound of a fixed frequency,as it approaches the person would hear the sound

A)drop in frequency.
B)stay at the same frequency.
C)increase in frequency.
D)decrease in loudness.
E)stay at the same loudness.

Question 36

A spherical wave has the form
)
The frequency of the wave (in Hz)is

A)3.68 × 10−⁴.
B)7.35 × 10−⁴.
C)1 360.
D)2 720.
E)2 720π.

Question 37

A wave generated in a medium is a longitudinal wave when

A)there is a net transport of matter by the wave.
B)the molecules of the medium are unable to exert forces on each other.
C)molecular displacements are parallel to the wave velocity.
D)molecular displacements are perpendicular to the wave velocity.
E)the density of the medium is less than the density of water.

Question 38

A truck moving at 36 m/s passes a police car moving at 45 m/s in the opposite direction.If the frequency of the siren is 500 Hz relative to the police car,what is the change in frequency (in Hz)heard by an observer in the truck as the two vehicles pass each other? (The speed of sound in air is 343 m/s. )

A)242
B)238
C)240
D)236
E)234

Question 39

How fast (in m/s)is the Concorde moving if it reaches Mach 1.5? (The speed of sound in air is 344 m/s. )

A)229
B)516
C)416
D)728
E)858

Question 40

When you hear the horn of a car that is approaching you,the frequency that you hear is larger than that heard by a person in the car because for the sound you hear

A)wave crests are farther apart by the distance the car travels in one period.
B)wave crests are closer together by the distance the car travels in one period.
C)the car gets ahead of each wave crest before it emits the next one.
D)the speed of sound in air is increased by the speed of the car.
E)a speeding car emits more wavecrests in each period.

Question 41

A speaker is producing 30 W of sound.At which of the following frequencies would it sound the loudest?

A)50 Hz
B)100 Hz
C)500 Hz
D)1 000 Hz
E)10 000 Hz

Question 42

When a workman strikes a steel pipeline with a hammer,he generates both longitudinal and transverse waves.The two types of reflected waves return 2.4 s apart.How far away is the reflection point? (For steel,v_L = 6.2 km/s,v_T = 3.2 km/s)

Question 43

If a plane is flying at a speed so that the apex half-angle of its shockwave is 30°,what is its speed if the speed of sound is 320 m/s?

A)960 m/s
B)640 m/s
C)320 m/s
D)160 m/s
E)Choose this answer if a shock wave cannot form in this situation.

Question 44

An airplane traveling at half the speed of sound emits sound at a frequency of 5 000 Hz.At what frequency does a stationary listener hear the sound as the plane approaches,and after it passes by? Assume the airplane is not flying very high.

Question 45

The intensity level of an orchestra is 85 dB.A single violin achieves a level of 70 dB.How does the intensity of the sound of the full orchestra compare with that of the violin?

Question 46

A microphone in the ocean is sensitive to the sounds emitted by porpoises.To produce a usable signal,sound waves striking the microphone must have an intensity of 1.02 × 10−¹¹ W/m².If porpoises emit sounds with a power of 0.049 9 W,how far away can a porpoise be and still be heard? Disregard absorption of sound waves by the water.

Question 47

If a supersonic plane is flying at increasing speed as the temperature decreases,what happens to the Mach number in this instance?

A)It stays the same.
B)It increases.
C)It decreases.
D)It can do any of the above.
E)It can do either (a)or (b)but not (c).

Question 48

A bat,flying at 5.00 m/s,emits a chirp at 40.0 kHz.If this sound pulse is reflected by a wall,what is the frequency of the echo received by the bat? (v_sound = 340 m/s. )