Deck 14: Periodic Motion

Question 1

Intensity: A transverse wave traveling along a string transports energy at a rate r. If we want to double this rate, we could

A) increase the amplitude of the wave by a factor of 8.
B) increase the amplitude of the wave by a factor of 4.
C) increase the amplitude of the wave by a factor of 2.
D) increase the amplitude by a factor of
E) increase the amplitude by a factor of

Answer 1

increase the amplitude by a factor of

Question 2

Standing sound waves: A pipe that is 120 cm long resonates to produce sound of wavelengths 480 cm, 160 cm, and 96 cm but does not resonate at any wavelengths longer than these. This pipe is

A) open at both ends.
B) open at one end and closed at the other end.
C) closed at both ends.
D) We cannot tell because we do not know the frequency of the sound.

Answer 2

open at one end and closed at the other end.

Question 3

Waves on a string: You are generating traveling waves on a stretched string by wiggling one end. If you suddenly begin to wiggle more rapidly without appreciably affecting the tension, you will cause the waves to move down the string

A) faster than before.
B) slower than before.
C) at the same speed as before.

Answer 3

at the same speed as before.

Question 4

Sound: What characteristic of sound determines the "pitch" of a musical note?

A) amplitude
B) wavelength
C) frequency
D) phase
E) intensity

Question 5

Mathematics of traveling waves: For the wave shown in the figure, the wavelength is

A) 8 m.
B) 4 m.
C) 2 m.
D) 1 m.
E) unable to be determined from the given information.

Question 6

Standing waves on a string: A wave pulse traveling to the right along a thin cord reaches a discontinuity where the rope becomes thicker and heavier. What is the orientation of the reflected and transmitted pulses?

A) Both pulses are right side up.
B) The reflected pulse returns right side up while the transmitted pulse is inverted.
C) The reflected pulse returns inverted while the transmitted pulse is right side up.
D) Both pulses are inverted.

Question 7

Standing waves on a string: A guitar string is fixed at both ends. If you tighten it to increase its tension

A) the frequencies of its vibrational modes will increase but its wavelengths will not be affected.
B) the wavelength increases but the frequency is not affected.
C) both the frequency and the wavelength increase.

Question 8

Mathematics of traveling waves: Four traveling waves are described by the following equations, where all quantities are measured in SI units and y represents the displacement. I: y = 0.12 cos(3x - 21t)
II: y = 0.15 sin(6x + 42t)
III: y = 0.13 cos(6x + 21t)
IV: y = -0.27 sin(3x - 42t)
Which of these waves have the same period?

A) I and III, and also II and IV
B) I and IV, and also II and III
C) I and II, and also III and IV
D) All of them have the same period.
E) They all have different periods.

Question 9

Standing waves on a string: A wave pulse traveling to the right along a thin cord reaches a discontinuity where the rope becomes thinner and lighter. What is the orientation of the reflected and transmitted pulses?

A) Both pulses are right side up.
B) The reflected pulse returns right side up while the transmitted pulse is inverted.
C) The reflected pulse returns inverted while the transmitted pulse is right side up.
D) Both pulses are inverted.

Question 10

Waves on a string: When a weight W is hanging from a light vertical string, the speed of pulses on the string is V. If a second weight W is added without stretching the string, the speed of pulses on this string will now become

A) 2V.
B) V
C) V.
D) V/
E) V/2

Question 11

Mathematics of traveling waves: The figure shows the displacement y of a traveling wave at a given position as a function of time and the displacement of the same wave at a given time as a function of position. Determine the wavelength of the wave.

A) 2.0 cm
B) 3.0 m
C) 2.0 m
D) 4.0 m
E) 1.5 m

Question 12

Standing waves on a string: Consider the waves on a vibrating guitar string and the sound waves the guitar produces in the surrounding air. The string waves and the sound waves must have the same

A) wavelength.
B) velocity.
C) frequency.
D) amplitude.
E) More than one of the above is true.

Question 13

Mathematics of traveling waves: Four traveling waves are described by the following equations, where all quantities are measured in SI units and y represents the displacement. I: y = 0.12 cos(3x + 2t)
II: y = 0.15 sin(6x - 3t)
III: y = 0.23 cos(3x + 6t)
IV: y = -0.29 sin(1.5x - t)
Which of these waves have the same speed?

A) I and III
B) I and IV
C) II and III
D) I and II
E) III and IV

Question 14

Standing sound waves: Consider a pipe of length L that is open at both ends. What are the wavelengths of the three lowest-pitch tones produced by this pipe?

A) 4L, 2L, L
B) 2L, L, L/2
C) 2L, L, 2L/3
D) 4L, 4L/3, 4L/5
E) 4L, 2L, L/2

Question 15

Sound level: Which one of the following statements is true?

A) Both the intensity level (in dB) and the sound intensity can never be negative.
B) The intensity level (in dB) obeys an inverse-square distance law, but the sound intensity does not.
C) Both intensity level (in dB) and sound intensity obey inverse-square distance laws.
D) The sound intensity can never be negative, but the intensity level (in dB) can be negative.
E) Both the intensity level (in dB) and the sound intensity can be negative.

Question 16

Mathematics of traveling waves: The figure shows the displacement y of a wave at a given position as a function of time and the displacement of the same wave at a given time as a function of position. Determine the frequency of the wave.

A) 4.0 Hz
B) 0.50 Hz
C) 3.0 Hz
D) 0.33 Hz
E) 0.25 Hz

Question 17

Standing sound waves: The lowest-pitch tone to resonate in a pipe of length L that is open at both ends is 200 Hz. Which one of the following frequencies will NOT resonate in the same pipe?

A) 400 Hz
B) 600 Hz
C) 800 Hz
D) 900 Hz
E) 1000 Hz

Question 18

Standing sound waves: In a resonating pipe that is open at both ends, there

A) are displacement nodes at each end.
B) are displacement antinodes at each end.
C) is a displacement node at one end and a displacement antinode at the other end.
D) None of the above are possible.

Question 19

Waves on a string: In the figure, which of the curves best represents the variation of wave speed as a function of tension for transverse waves on a stretched string?

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

Question 20

Mathematics of traveling waves: For the wave shown in the figure, the frequency is

A) 0.5 Hz.
B) 1 Hz.
C) 2 Hz.
D) 4 Hz.
E) unable to be determined from the given information.

Question 21

Mathematics of traveling waves: The figure shows the displacement y of a traveling wave at a given position as a function of time and the displacement of the same wave at a given time as a function of position. How fast is the wave traveling?

A) 3.0 m/s
B) 0.75 m/s
C) 0.66 m/s
D) 1.5 m/s
E) 2.0 m/s

Question 22

Doppler effect: When a rocket is traveling toward a mountain at 100 m/s, the sound waves from this rocket's engine approach the mountain at speed V. If the rocket doubles its speed to 200 m/s, the sound waves from the engine will now approach the mountain at speed

A) 4V.
B) 2V.
C) V.
D) V.

Question 23

Waves on a string: A heavy stone of mass m is hung from the ceiling by a thin 8.25-g wire that is 65.0 cm long. When you gently pluck the upper end of the wire, a pulse travels down the wire and returns 7.84 ms later, having reflected off the lower end. The stone is heavy enough to prevent the lower end of the wire from moving. What is the mass m of the stone?

A) 8.90 kg
B) 23.1 kg
C) 35.6 kg
D) 227 kg
E) 349 kg

Question 24

Waves on a string: A 8.0-m long wire with a mass of 10 g is under tension. A transverse wave for which the frequency is 570 Hz, the wavelength is 0.10 m, and the amplitude is 3.7 mm is propagating on the wire. The maximum transverse acceleration of a point on a wire is closest to

A) 47,000 m/s².
B) 41,000 m/s².
C) 35,000 m/s².
D) 29,000 m/s².
E) 53,000 m/s².

Question 25

Waves on a string: A 6.00-m long rope is under a tension of 600 N. Waves travel along this rope at 40.0 m/s. What is the mass of the rope?

A) 1.00 kg
B) 1.25 kg
C) 2.25 kg
D) 2.50 kg
E) 1.12 kg

Question 26

Standing sound waves: The lowest-pitch tone to resonate in a pipe of length L that is closed at one end and open at the other end is 200 Hz. Which one of the following frequencies will NOT resonate in the same pipe?

A) 400 Hz
B) 600 Hz
C) 1000 Hz
D) 1400 Hz
E) 1800 Hz

Question 27

Shock waves: Shock waves occur when

A) the frequency of the waves is the resonant frequency of the system.
B) the amplitude of waves exceeds the critical shock value.
C) two waves from different sources collide with each other.
D) the wave source is traveling at a speed greater than the wave speed.
E) the period of the waves matches the lifetime of the waves.

Question 28

Mathematics of traveling waves: Ocean tides are waves that have a period of 12 hours, an amplitude (in some places) of 1.50 m, and a speed of 750 km/hr. What is the distance between adjacent crests of these waves?

A) 9000 km
B) 32,400 km
C) 9000 m
D) 32,400 m
E) 2500 m

Question 29

Mathematics of traveling waves: A transverse wave is traveling on a string stretched along the horizontal x-axis. The equation for the vertical displacement y of the string is given by y = 0.0020 cos[π(15x - 52t)], where all quantities are in SI units. The maximum speed of a particle of the string is closest to

A) 0.33 m/s.
B) 0.43 m/s.
C) 0.53 m/s.
D) 0.64 m/s.
E) 0.74 m/s.

Question 30

Beats: Two pure tones are sounded together and a particular beat frequency is heard. What happens to the beat frequency if the frequency of one of the tones is increased?

A) It increases.
B) It decreases.
C) It does not change.
D) It becomes zero.
E) We cannot tell from the information given.

Question 31

Waves on a string: A 6.0-m wire with a mass of 50 g, is under tension. A transverse wave, for which the frequency is 810 Hz, the wavelength is 0.40 m, and the amplitude is 4.0 mm, is propagating on the wire. The time for a crest of this wave to travel the length of the wire is closest to

A) 19 ms.
B) 16 ms.
C) 21 ms.
D) 23 ms.
E) 25 ms.

Question 32

Mathematics of traveling waves: The vertical displacement y(x,t) of a string stretched along the horizontal x-axis is given by y(x,t) = (6.00 mm) cos[(3.25 m-1)x - (7.22 rad/s)t].
(a) What is the minimum time for each complete cycle of the wave?
(b) What is the distance between adjacent crests of the wave?
(c) How fast does this wave travel

Question 33

Waves on a string: The density of aluminum is 2700 kg/m³. If transverse waves propagate at in a aluminum wire, what is the tension on the wire?

A) 52 N
B) 31 N
C) 42 N
D) 62 N

Question 34

Doppler effect: A plane flies toward a stationary siren at 1/4 the speed of sound. Then the plane stands still on the ground and the siren is driven toward it at 1/4 the speed of sound. In both cases, a person sitting in the plane will hear the same frequency of sound from the siren.

Question 35

Standing sound waves: A tube open at one end and closed at the other end produces sound having a fundamental frequency of 350 Hz. If you now open the closed end, the fundamental frequency becomes

A) 87.5 Hz.
B) 175 Hz.
C) 350 Hz.
D) 700 Hz.
E) 1400 Hz.

Question 36

Mathematics of traveling waves: You and your surfing buddy are waiting to catch a wave a few hundred meters off the beach. The waves are conveniently sinusoidal, and you notice that when you're on the top of one wave and moving toward your friend, she is exactly halfway between you and the trough of the wave. 1.50 seconds later, your friend is at the top of the wave. You estimate the horizontal distance between you and your friend at 8.00 m.
(a) What is the frequency of the waves?
(b) Find the speed of the waves.

Question 37

Doppler effect: A stationary siren emits sound of frequency 1000 Hz and wavelength 0.343 m. An observer who is moving toward the siren will measure a frequency f and wavelength λ for this sound such that

A) f > 1000 Hz and λ > 0.343 m.
B) f > 1000 Hz and λ = 0.343 m.
C) f > 1000 Hz and λ < 0.343 m.
D) f = 1000 Hz and λ < 0.343 m.

Question 38

Standing sound waves: In a resonating pipe that is open at one end and closed at the other end, there

A) are displacement nodes at each end.
B) are displacement antinodes at each end.
C) is a displacement node at the open end and a displacement antinode at the closed end.
D) is a displacement node at the closed end and a displacement antinode at the open end.

Question 39

Mathematics of traveling waves: Find the speed of an ocean wave whose vertical displacement y as a function of time t is given by y(x,t) = 3.7 cos(2.2x - 5.6t), where all quantities are in SI units.

A) 2.5 m/s
B) 1.9 m/s
C) 3.5 m/s
D) 4.5 m/s

Question 40

Waves on a string: Waves travel along a 100-m length of string which has a mass of 55 g and is held taut with a tension of 75 N. What is the speed of the waves?

A) 3700 m/s
B) 370 m/s
C) 37 m/s
D) 0.37 m/s
E) 3.7 m/s

Question 41

Standing waves on a string: A platinum wire that is 1.20 m long has a radius of 0.500 mm and is fixed at both ends. In its third harmonic it vibrates at 512 Hz. The density of platinum is 21.4 × 10³ kg/m³. What is the tension in the wire?

A) 4.00 kN
B) 2.00 kN
C) 2.82 kN
D) 1.41 kN
E) 1.00 kN

Question 42

Standing waves on a string: A 2.0-m string is fixed at both ends and tightened until the wave speed is What is the frequency of the standing wave shown in the figure?

A) 120 Hz
B) 230 Hz
C) 350 Hz
D) 470 Hz

Question 43

Intensity: Two people are talking at a distance of 3.0 m from where you are and you measure the sound intensity as 1.1 × 10^-7 W/m². Another student is 4.0 m away from the talkers. What sound intensity does the other student measure? Assume that the sound spreads out uniformly and undergoes no significant reflections or absorption.

A) 6.2 × 10^-8 W/m²
B) 1.5 × 10^-7 W/m²
C) 8.3 × 10^-8 W/m²
D) 7.8 × 10^-7 W/m²
E) 2.5 × 10^-8 W/m²

Question 44

Intensity: The intensity of sunlight falling on the earth is about 1.4 kW/m² (before any gets absorbed by our atmosphere). At what rate does the sun emit light energy? (The earth-sun distance = 1.5 × 10⁸ km and the earth's radius = 6.4 × 10³ km.)

A) 4.0 × 10²⁶ W
B) 3.2 × 10²² W
C) 7.2 × 10¹⁴ W
D) 7.6 × 10⁸ W

Question 45

Speed of sound: A pipe that is 20.0 m long and 10.0 cm in diameter contains olive oil. The density of the olive oil is 890 kg/m³ and the bulk modulus is 1.3 × 10⁹ Pa. A 3.4-Hz longitudinal wave is transmitted in the oil. How many milliseconds does it take for the wave to travel the length of the pipe in the oil?

A) 17 ms
B) 15 ms
C) 14 ms
D) 13 ms
E) 12 ms

Question 46

Standing waves on a string: A thin taut string is fixed at both ends and stretched along the horizontal x-axis with its left end at x = 0. It is vibrating in its third OVERTONE, and the equation for the vertical displacement of any point on the string is y(x,t) = (1.22 cm) sin[(14.4 m-1)x] cos[(166 rad/s)t].
(a) What are the frequency and wavelength of the fundamental mode of this string?
(b) How long is the string?
(c) How fast do waves travel on this string?

Question 47

Standing waves of frequency 57 Hz are produced on a string that has mass per unit length 0.0160 kg/m. With what tension must the string be stretched between two supports if adjacent nodes in the standing wave are to be 0.71 meters apart?

Question 48

Speed of sound: The density of aluminum is 2.7 × 10³ kg/m³. The speed of longitudinal waves in an aluminum rod is measured to be 5.1 × 10³ m/s. What is the value of Young's modulus for this aluminum?

A) 7.0 × 10¹⁰ N/m²
B) 7.0 × 10⁷ N/m²
C) 2.2 × 10¹¹ N/m²
D) 2.2 × 10⁹ N/m²
E) 1.4 × 10⁷ N/m²

Question 49

Standing waves on a string: A 1.0-g string that is 0.64 m long is fixed at both ends and is under tension. This string produces a 100-Hz tone when it vibrates in the third harmonic. The speed of sound in air is 344 m/s. The tension in the string, in is closest to

A) 2.8 N.
B) 2.3 N.
C) 1.8 N.
D) 3.4 N.
E) 3.9 N.

Question 50

Standing waves on a string: A guitar string 0.650 m long has a tension of 61.0 N and a mass per unit length of 3.00 g/m.
(a) What is the speed of waves on the string when it is plucked?
(b) What is the string's fundamental frequency of vibration when plucked?

Question 51

Standing waves on a string: A 2.00-m long piano wire with a mass per unit length of 12.0 g/m is under a tension of 8.00 kN. What is the frequency of the fundamental mode of vibration of this wire?

A) 204 Hz
B) 102 Hz
C) 408 Hz
D) 510 Hz
E) 153 Hz

Question 52

Intensity: Observer A is 3.0 m from a tiny light bulb and observer B is 12.0 m from the same bulb. Assume that the light spreads out uniformly and undergoes no significant reflections or absorption. If observer B sees a light of intensity I, the light intensity that A sees is

A) I.
B) 9I.
C) 16I.
D) 36I.
E) 144I.

Question 53

Standing waves on a string: A standing wave is oscillating at 690 Hz on a string, as shown in the figure. What is the speed of traveling waves on this string?

A) 280 m/s
B) 410 m/s
C) 210 m/s
D) 140 m/s

Question 54

Speed of sound: The speed of sound in an alcohol at a temperature of 20°C is 1260 m/s. The density of the alcohol at that temperature is At a temperature of 20°C, find the bulk modulus of the alcohol.

A) 1030 MPa
B) 1080 MPa
C) 1140 MPa
D) 1190 MPa
E) 1240 MPa

Question 55

Standing waves on a string: A heavy stone of mass m is hung from the ceiling by a thin 8.25-g wire that is 65.0 cm long. When you gently pluck the upper end of the wire, a pulse travels down the wire and returns 7.84 ms later, having reflected off the lower end. The speed of sound in the room is 344 m/s, and the stone is heavy enough to prevent the lower end of the wire from moving. If the wire is vibrating in its second overtone, what is the frequency of the sound it will produce?

A) 128 Hz
B) 191 Hz
C) 255 Hz
D) 383 Hz
E) 765 Hz

Question 56

Standing waves on a string: Two violinists are trying to tune their instruments in an orchestra. One is producing the desired frequency of 440.0 Hz. The other is producing a frequency of 448.4 Hz. By what percentage should the out-of-tune musician change the tension in his string to bring his instrument into tune at 440.0 Hz?

A) +1.9%
B) -1.9%
C) +3.7%
D) -3.7%
E) +8.4%

Question 57

Standing waves on a string: A heavy stone of mass m is hung from the ceiling by a thin 8.25-g wire that is 65.0 cm long. When you gently pluck the upper end of the wire, a pulse travels down the wire and returns 7.84 ms later, having reflected off the lower end. The speed of sound in the room is 344 m/s, and the stone is heavy enough to prevent the lower end of the wire from moving. If the wire is vibrating in its second overtone, what is the wavelength of the sound it will produce?

A) 0.217 m
B) 0.433 m
C) 0.650 m
D) 0.899 m
E) 1.35 m

Question 58

Intensity: A tiny vibrating source sends waves uniformly in all directions. An area of 3.25 cm² on a sphere of radius 2.50 m centered on the source receives energy at a rate of 4.20 J/s.
(a) What is the intensity of the waves at 2.50 m from the source and at 10.0 m from the source?
(b) At what rate is energy leaving the vibrating source of the waves?

Question 59

Intensity: Calculate the light intensity 1.51 m from a light bulb that emits 100 W of visible light, assuming that the light radiates uniformly in all directions.

A) 3.49 W/m²
B) 4.01 W/m²
C) 43.9 W/m²
D) 50.5 W/m²

Question 60

Standing waves on a string: A thin 2.00-m string of mass 50.0 g is fixed at both ends and under a tension of 70.0 N. If it is set into small-amplitude oscillation, what is the frequency of the first harmonic mode?

A) 6.61 Hz
B) 13.2 Hz
C) 26.5 Hz
D) 52.9 Hz

Question 61

Sound level: A sound source emits 20.0 W of acoustical power spread equally in all directions. The threshold of hearing is 1.0 × 10^-12 W/m². What is the sound intensity level 30.0 m from the source?

A) 92.5 dB
B) 81.5 dB
C) 1.77 × 10^-3 dB
D) 1.77 × 10^-3 W
E) -27.5 dB

Question 62

Standing sound waves: An air column, open at one end and closed at the other, is being designed so that its second lowest resonant frequency is 440 Hz. What should be the length of the column if the speed of sound in air is 340 m/s?

A) 0.386 m
B) 0.772 m
C) 1.16 m
D) 0.193 m
E) 0.580 m

Question 63

Standing sound waves: A pipe is 0.90 m long and is open at one end but closed at the other end. If it resonates with a tone whose wavelength is 0.72 m, what is the wavelength of the next higher overtone in this pipe?

A) 0.36 m
B) 0.40 m
C) 0.45 m
D) 0.51 m
E) 0.58 m

Question 64

Sound level: A certain source of sound waves radiates uniformly in all directions. At a distance of 20 m from the source the intensity level is 51 db. What is the total acoustic power output of the source, in watts? (Note: The reference intensity I₀ is 1.0 × 10^-12 W/m².)

Question 65

Standing sound waves: A violin with string length 32 cm and string density resonates with the first overtone from a 2.0-m long organ pipe with one end closed and the other end open. What is the tension in the string?

A) 1000 N
B) 110 N
C) 450 N
D) 4100 N
E) 56 N

Question 66

Standing sound waves: The speed of sound in the air inside a 0.640-m long gas column is 340 m/s. What is the fundamental resonant frequency of this air column if it is
(a) open at one end and closed at the other end?
(b) open at both ends?

Question 67

Sound level: The sound level at 1.0 m from a certain talking person talking is 60 dB. You are surrounded by five such people, all 1.0 m from you and all talking equally loud at the same time. The threshold of hearing is 1.0 × 10^-12 W/m². What sound level are you being exposed to? You can neglect any absorption, reflection, or interference of the sound. The threshold of hearing is 1.0 × 10^-12 W/m².

A) 300 dB
B) 60 dB
C) 74 dB
D) 67 dB
E) 81 dB

Question 68

Sound level: A howler monkey is the loudest land animal and, under some circumstances, can be heard up to a distance of 5.0 km. Assume the acoustic output of a howler to be uniform in all directions and that the threshold of hearing is 1.0 × 10^-12 W/m². The acoustic power emitted by the howler is closest to

A) 0.31 mW.
B) 0.11 mW.
C) 1.1 mW.
D) 3.2 mW.
E) 11 mW.

Question 69

Sound level: If the intensity level at distance d of one trombone is 70 dB, what is the intensity level of 76 identical trombones, all at distance d?

A) 146 dB
B) 89 dB
C) 5320 dB
D) 76 dB
E) 82 dB

Question 70

Sound level: A certain crying baby emits sound with an intensity of 8.0 × 10^-8 W/m². What is the intensity level due to a set of five such crying babies, all crying with the same intensity? You can neglect any absorption, reflection, or interference of the sound. The lowest detectable intensity is 1.0 × 10^-12 W/m².

A) 79 dB
B) 69 dB
C) 56 dB
D) 49 dB
E) 36 dB

Question 71

Standing sound waves: A 0.25-m string, vibrating in its sixth harmonic, excites a 0.96-m pipe that is open at both ends into its second overtone resonance. The speed of sound in air is 345 m/s. The common resonant frequency of the string and the pipe is closest to

A) 540 Hz.
B) 360 Hz.
C) 450 Hz.
D) 630 Hz.
E) 700 Hz.

Question 72

Sound level: At a distance of 2.00 m from a point source of sound, the intensity level is 80.0 dB. What will be the intensity level at a distance of 4.00 m from this source? The lowest detectable intensity is 1.0 × 10^-12 W/m².

A) 77.0 dB
B) 74.0 dB
C) 60.0 dB
D) 40.0 dB
E) 20.0 dB

Question 73

Standing sound waves: A pipe that is 0.46 m long and open at both ends vibrates in the second overtone with a frequency of In this situation, the distance from the center of the pipe to the nearest antinode is closest to

A) 7.7 cm.
B) 3.8 cm.
C) 12 cm.
D) 15 cm.
E) zero.

Question 74

Standing sound waves: A 1.30-m long gas column that is open at one end and closed at the other end has a fundamental resonant frequency 80.0 Hz. What is the speed of sound in this gas?

A) 104 m/s
B) 61.5 m/s
C) 26.0 m/s
D) 246 m/s
E) 416 m/s

Question 75

Sound level: An enclosed chamber with sound absorbing walls has a 2.0 m × 1.0 m opening for an outside window. A loudspeaker is located outdoors, 84 m away and facing the window. The intensity level of the sound entering the window space from the loudspeaker is 56 dB. Assume the acoustic output of the loudspeaker is uniform in all directions and that acoustic energy incident upon the ground is completely absorbed and therefore is not reflected into the window. The threshold of hearing is 1.0 × 10^-12 W/m². The acoustic power output of the loudspeaker is closest to

A) 0.035 W.
B) 0.070 W.
C) 0.18 W.
D) 0.35 W.
E) 0.70 W.

Question 76

Sound level: An enclosed chamber with sound absorbing walls has a 2.0 m × 1.0 m opening for an outside window. A loudspeaker is located outdoors, 78 m away and facing the window. The intensity level of the sound entering the window space from the loudspeaker is 79 dB. Assume the acoustic output of the loudspeaker is uniform in all directions and that the acoustic energy incident upon the ground is completely absorbed and therefore is not reflected into the window. The threshold of hearing is 1.0 × 10^-12 W/m². The acoustic power entering through the window space is closest to

A) 160 µW.
B) 79 µW.
C) 320 µW.
D) 790 µW.
E) 1600 µW.

Question 77

Sound level: The intensity level of a "Super-Silent" power lawn mower at a distance of 1.0 m is 100 dB. You wake up one morning to find that four of your neighbors are all mowing their lawns using identical "Super-Silent" mowers. When they are each 20 m from your open bedroom window, what is the intensity level of the sound in your bedroom? You can neglect any absorption, reflection, or interference of the sound. The lowest detectable intensity is 1.0 × 10^-12 W/m².

A) 80 dB
B) 104 dB
C) 400 dB
D) 50 dB
E) 40 dB

Question 78

Standing sound waves: One of the harmonics of a column of air open at one end and closed at the other has a frequency of 448 Hz and the next higher harmonic has a frequency of 576 Hz. What is the fundamental frequency of the air column?

A) 32 Hz
B) 64 Hz
C) 88 Hz
D) 128 Hz
E) 256 Hz

Question 79

Sound level: The howler monkey is the loudest land animal and, under some circumstances, can be heard up to a distance of 8.9 km. Assume the acoustic output of a howler to be uniform in all directions and that the threshold of hearing is 1.0 × 10^-12 W/m². A juvenile howler monkey has an acoustic output of What is the ratio of the acoustic intensity produced by the juvenile howler to the reference intensity I₀, at a distance of 210 m?

A) 110
B) 230
C) 76
D) 170
E) 300

Question 80

Standing sound waves: A string, 0.28 m long and vibrating in its third harmonic, excites an open pipe that is 0.82 m long into its second overtone resonance. The speed of sound in air is 345 m/s. The speed of transverse waves on the string is closest to

A) 120 m/s.
B) 110 m/s.
C) 100 m/s.
D) 98 m/s.
E) 91 m/s.