Honors Physics
Chapter 14 Sound
Semester Review
Terms
frequency
audible
infrasonic
ultrasonic
sound intensity
decibel
beats
Doppler effect
mach speed
bow wave
sonic boom
shock wave cone
threshold of hearing
threshold of pain
concepts
Characteristics of sound wave
Intensity, power/area
Decibels
Wave interference
Doppler effect
o Approaching source
o Departing source
Mach speed
Angle of shock wave
Formula
Speed of sound in various substances
Problems
A cricket releases a chirp with 1.5 x 10-4 watts of power.
a. What is the intensity of this chirp 9.0 m from the cricket?(1.47 x 10-7W/m2)
b. What is the intensity of this chirp in decibels? (51.7)
Sound waves and the speed of sound
1. MC A sound wave with a frequency of 15 Hz is in what region of the sound spectrum: (a) audible, (b)
infrasonic, (c) ultrasonic, or (d) supersonic?
2. MC A sound wave in air (a) is longitudinal, (b) is transverse, (c) has longitudinal and transverse
components, (d) travels faster than a sound wave through a liquid.
3. MC The speed of sound is generally greatest in (a) solids, (b) liquids, (c) gases, (d) a vacuum.
4. MC The speed of sound in air (a) is about 1/3 km/s, (b) is about 1 5 mi s, (c) depends on temperature, (d)
all of the preceding.
5. CQ Suggest a possible explanation of why some flying insects produce buzzing sounds and some do not.
6. CQ Explain why sound travels faster in warmer than in colder air.
7. CQ Two sounds that differ in frequency are emitted from a single loudspeaker. Which sound will reach
your ear first, the one with the lower or the higher frequency?
8. CQ The speed of sound in air depends on temperature. What effect, if any, should humidity have?
10.  What is the speed of sound in air at (a) 10°C and (b) 20°C? (a) 337 m s (b) 343 m s
11.  The speed of sound in air on a summer day is 350 m s. What is the air temperature? 32°C
14.  A 1.00-m-long string with a mass of 15.0 g is placed under a tension of 35.0 N. It is plucked so it
vibrates in its fundamental standing-wave mode. What are the (a) frequency and (b) wavelength of the
sound? Assume normal room temperature. (a) 24
15. IE  A tuning fork vibrates at a frequency of 256 Hz. (a) When the air temperature increases, the
wavelength of the sound from the tuning fork (1) increases, (2) remains the same, (3) decreases. Why? (b)
If the temperature rises from 0°C to 20°C, what is the change in the wavelength? (a) (1) increases (b)
0.047 m Hz (b) 14.2 m
27.  Sound propagating through air at 30°C passes through a vertical cold front into air that is 4.0°C. If
the sound has a frequency of 2500 Hz, by what percentage does its wavelength change in crossing the
boundary? 4.5%
Sound Intensity
28. MC If the air temperature increases, would the sound intensity from a constant-output point source (a)
increase, (b) decrease, or (c) remain unchanged?
29. MC The decibel scale is referenced to a standard intensity of (a) 1.0 W m2 , (b) 10 12 W m2 , (c) normal
conversation, (d) the threshold of pain.
30. MC If the intensity level of a sound at 20 dB is increased to 40 dB, the intensity would increase by a factor
of (a) 10, (b) 20, (c) 40, (d) 100.
31. CQ Where is the intensity greater and by what factor? (1) At a point a distance R from a power source P,
or (2) at a point a distance 2R from a power source of 2P. Explain.
32. CQ The Richter scale, used to measure the intensity level of earthquakes, is a logarithmic scale, as is the
decibel scale. Why are such scales used?
33. CQ Can there be negative decibel levels, such as 10 dB? If so, what would these mean?
34.  Calculate the intensity generated by a 1.0-W point source of sound at a location (a) 3.0 m and (b) 6.0 m
from it. (a) 8.8 10 3 W m2 (b) 2.2 10 3 W m2
35. IE  (a) If the distance from a point sound source triples, the sound intensity will be (1) 3, (2) 1 3, (3) 9,
(4) 1 9 times the original value. Why? (b) By how much must the distance from a point source be
increased to reduce the sound intensity by half?
39.  Find the intensity levels in decibels for sounds with intensities of (a) 10 2 W m2 , (b) 10 6 W m2 , and (c)
2
10 15 W m . (a) 100 dB (b) 60 dB (c) 30 dB
48. IE  A dog’s bark has a sound intensity level of 40 dB. (a) If two of the same dogs are barking, the
intensity level is (1) less than 40 dB, (2) between 40 dB and 80 dB, (3) 80 dB. (b) What would be the
intensity level? (a) (2) between 40 dB and 80 dB (b) 43 dB
Sound Phenomena and The Doppler Effect
56. MC Constructive and destructive interference of sound waves depends on (a) the speed of sound, (b)
diffraction, (c) phase difference, (d) all of the preceding.
57. MC Beats are the direct result of (a) interference, (b) refraction, (c) diffraction, (d) the Doppler effect.
58. MC Police radar makes use of (a) refraction, (b) the Doppler effect, (c) interference, (d) sonic boom.
59. CQ Do interference beats have anything to do with the “beat” of music? Explain. no,
60. CQ (a) Is there a Doppler effect if a sound source and an observer are moving with the same velocity? (b)
What would be the effect if a moving source accelerated toward a stationary observer?
65. IE  A violinist tuning her instrument to a piano note of 264 Hz detects three beats per second. (a) The
frequency of the violin could be (1) less than 264 Hz, (2) equal to 264 Hz, (3) greater than 264 Hz, (4) both
(1) and (3). Why? (b) What are the possible frequencies of the violin tone?
68.  While standing near a railroad crossing, you hear a train horn. The frequency emitted by the horn is
400 Hz. If the train is traveling at 90.0 km h and the air temperature is 25°C, what is the frequency you hear
(a) when the train is approaching and (b) after it has passed? (a) 431 Hz (b) 373 Hz
72. The frequency of an ambulance siren is 700 Hz. What are the frequencies heard by a stationary
pedestrian as the ambulance approaches and moves away from her at a speed of 90.0 km h . (Assume that
the air temperature is 20°C.) 755 Hz approaching, 652 Hz moving away
74. IE  A fighter jet flies at a speed of Mach 1.5. (a) If the jet were to fly faster than Mach 1.5, the halfangle of the conical shock wave would (1) increase, (2) remain the same, or (3) decrease. Why? (b) What
is the half-angle of the conical shock wave formed by the jet plane at Mach 1.5? (a) (3) decrease (b) 42°
75.  The half-angle of the conical shock wave formed by a supersonic jet is 30°. What are (a) the Mach
number of the aircraft and (b) the actual speed of the aircraft if the air temperature is 20 C? (a) 2.0 (b)
638 m s
80.  A supersonic jet flies directly overhead relative to an observer, at an altitude of 2.0 km (Fig. )
When the observer hears the first sonic boom, the plane has flown a horizontal distance of 2.5 km at a
constant speed. (a) What is the angle of the shock wave cone? (b) At what Mach number is the plane
flying? (Assume that the speed of sound is at an average constant temperature of 15°C.) (a) 39° (b) 1.6