Chapter 16
Sound and Hearing
PowerPoint® Lectures for
University Physics, Thirteenth Edition
– Hugh D. Young and Roger A. Freedman
Lectures by Wayne Anderson
Copyright © 2012 Pearson Education Inc.
Goals for Chapter 16
• To describe sound waves in terms of particle displacements or
pressure variations
• To calculate the speed of sound in different materials
• To calculate sound intensity
• To find what determines the frequencies of sound from a pipe
• To study resonance in musical instruments
• To see what happens when sound waves overlap
• To investigate the interference of sound waves of slightly
different frequencies
• To learn why motion affects pitch
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Introduction
• Most people prefer listening
to music instead of noise. But
what is the physical difference
between the two?
• We can think of a sound wave
either in terms of the displacement of the particles or of the
pressure it exerts.
• How do humans actually
perceive sound?
• Why is the frequency of
sound from a moving source
different from that of a
stationary source?
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Sound waves
• Sound is simply any
longitudinal wave in a
medium.
• The audible range of
frequency for humans is
between about 20 Hz and
20,000 Hz.
• Ultrasonic sound waves have
frequencies above human
hearing and infrasonic waves
are below.
• Figure 16.1 at the right shows
sinusoidal longitudinal wave.
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Different ways to describe a sound wave
•
Sound can be
described by a
graph of displacement versus
position, or by a
drawing showing
the displacements
of individual
particles, or by a
graph of the pressure fluctuation
versus position.
•
The pressure
amplitude is
pmax = BkA.
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Q16.1
At a compression in a sound wave,
A. particles are displaced by the maximum distance in the
same direction as the wave is moving.
B. particles are displaced by the maximum distance in the
direction opposite to the direction the wave is moving.
C. particles are displaced by the maximum distance in the
direction perpendicular to the direction the wave is moving.
D. the particle displacement is zero.
Copyright © 2012 Pearson Education Inc.
A16.1
At a compression in a sound wave,
A. particles are displaced by the maximum distance in the
same direction as the wave is moving.
B. particles are displaced by the maximum distance in the
direction opposite to the direction the wave is moving.
C. particles are displaced by the maximum distance in the
direction perpendicular to the direction the wave is moving.
D. the particle displacement is zero.
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Amplitude of a sound wave
• Follow Examples 16.1 and 16.2 using Figure 16.4 below.
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Perception of sound waves
• The harmonic content greatly affects our perception of sound.
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Speed of sound waves
• The speed of sound
depends on the
characteristics of the
medium. Table 16.1 gives
some examples.
• The speed of sound:
v B

(fluid)
v  Y
(solid rod)
v   RT
M
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(ideal gas)
The speed of sound in water and air
• Follow Example 16.3 for the speed of sound in water,
using Figure 16.8 below.
• Follow Example 16.4 for the speed of sound in air.
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Sound intensity
• The intensity of a sinusoidal sound wave is proportional
to the square of the amplitude, the square of the
frequency, and the square of the pressure amplitude.
• Study Problem-Solving Strategy 16.1.
• Follow Examples 16.5, 16.6, and 16.7.
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Q16.2
Increasing the pressure amplitude of a sound wave by a
factor of 4 (while leaving the frequency unchanged)
A. causes the intensity to increase by a factor of 16.
B. causes the intensity to increase by a factor of 4.
C. causes the intensity to increase by a factor of 2.
D. has no effect on the wave intensity.
E. The answer depends on the frequency of the sound
wave.
Copyright © 2012 Pearson Education Inc.
A16.2
Increasing the pressure amplitude of a sound wave by a
factor of 4 (while leaving the frequency unchanged)
A. causes the intensity to increase by a factor of 16.
B. causes the intensity to increase by a factor of 4.
C. causes the intensity to increase by a factor of 2.
D. has no effect on the wave intensity.
E. The answer depends on the frequency of the sound
wave.
Copyright © 2012 Pearson Education Inc.
Q16.3
Increasing the frequency of a sound wave by a factor of 4
(while leaving the pressure amplitude unchanged)
A. causes the intensity to increase by a factor of 16.
B. causes the intensity to increase by a factor of 4.
C. causes the intensity to increase by a factor of 2.
D. has no effect on the wave intensity.
E. The answer depends on the frequency of the sound wave.
Copyright © 2012 Pearson Education Inc.
A16.3
Increasing the frequency of a sound wave by a factor of 4
(while leaving the pressure amplitude unchanged)
A. causes the intensity to increase by a factor of 16.
B. causes the intensity to increase by a factor of 4.
C. causes the intensity to increase by a factor of 2.
D. has no effect on the wave intensity.
E. The answer depends on the frequency of the sound wave.
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The decibel scale
• The sound intensity level  is  = (10 dB) log(I/I0).
• Table 16.2 shows examples for some common sounds.
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Examples using decibels
• Follow Example 16.8, which deals with hearing loss due to loud
sounds.
• Follow Example 16.9, using Figure 16.11 below, which
investigates how sound intensity level depends on distance.
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Standing sound waves and normal modes
• The bottom figure shows displacement
nodes and antinodes.
• A pressure node is always a displacement antinode, and a pressure antinode
is always a displacement node, as
shown in the figure at the right.
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The sound of silence
• Follow Conceptual Example 16.10, using Figure 16.14 below, in
which a loudspeaker is directed at a wall.
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Organ pipes
•
Organ pipes of different sizes
produce tones with different
frequencies (bottom figure).
•
The figure at the right shows
displacement nodes in two crosssections of an organ pipe at two
instants that are one-half period
apart. The blue shading shows
pressure variation.
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Harmonics in an open pipe
• An open pipe is open at both ends.
• For an open pipe n = 2L/n and fn = nv/2L (n = 1, 2, 3, …).
• Figure 16.17 below shows some harmonics in an open pipe.
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Harmonics in a closed pipe
• A closed pipe is open at one end and closed at the other end.
• For a closed pipe n = 4L/n and fn = nv/4L (n = 1, 3, 5, …).
• Figure 16.18 below shows some harmonics in a closed pipe.
• Follow Example 16.11.
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Q16.4
The air in an organ pipe is replaced by helium (which has a
lower molar mass than air) at the same temperature. How
does this affect the normal-mode wavelengths of the pipe?
A. The normal-mode wavelengths are unaffected.
B. The normal-mode wavelengths increase.
C. The normal-mode wavelengths decrease.
D. The answer depends on whether the pipe is open or closed.
Copyright © 2012 Pearson Education Inc.
A16.4
The air in an organ pipe is replaced by helium (which has a
lower molar mass than air) at the same temperature. How
does this affect the normal-mode wavelengths of the pipe?
A. The normal-mode wavelengths are unaffected.
B. The normal-mode wavelengths increase.
C. The normal-mode wavelengths decrease.
D. The answer depends on whether the pipe is open or closed.
Copyright © 2012 Pearson Education Inc.
Q16.5
The air in an organ pipe is replaced by helium (which has a
lower molar mass than air) at the same temperature. How
does this affect the normal-mode frequencies of the pipe?
A. The normal-mode frequencies are unaffected.
B. The normal-mode frequencies increase.
C. The normal-mode frequencies decrease.
D. The answer depends on whether the pipe is open or closed.
Copyright © 2012 Pearson Education Inc.
A16.5
The air in an organ pipe is replaced by helium (which has a
lower molar mass than air) at the same temperature. How
does this affect the normal-mode frequencies of the pipe?
A. The normal-mode frequencies are unaffected.
B. The normal-mode frequencies increase.
C. The normal-mode frequencies decrease.
D. The answer depends on whether the pipe is open or closed.
Copyright © 2012 Pearson Education Inc.
Q16.6
When you blow air into an open organ pipe, it produces a
sound with a fundamental frequency of 440 Hz.
If you close one end of this pipe, the new fundamental
frequency of the sound that emerges from the pipe is
A. 110 Hz.
B. 220 Hz.
C. 440 Hz.
D. 880 Hz.
E. 1760 Hz.
Copyright © 2012 Pearson Education Inc.
A16.6
When you blow air into an open organ pipe, it produces a
sound with a fundamental frequency of 440 Hz.
If you close one end of this pipe, the new fundamental
frequency of the sound that emerges from the pipe is
A. 110 Hz.
B. 220 Hz.
C. 440 Hz.
D. 880 Hz.
E. 1760 Hz.
Copyright © 2012 Pearson Education Inc.
Resonance and sound
• In Figure 16.19(a) at the
right, the loudspeaker
provides the driving force
for the air in the pipe. Part
(b) shows the resulting
resonance curve of the
pipe.
• Follow Example 16.12.
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Interference
•
The difference in the lengths of the paths traveled by the sound
determines whether the sound from two sources interferes
constructively or destructively, as shown in the figures below.
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Loudspeaker interference
• Follow Example 16.13 using Figure 16.23 below.
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Beats
• Beats are heard when two tones of slightly different frequency (fa
and fb) are sounded together. (See Figure 16.24 below.)
• The beat frequency is fbeat = fa – fb.
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Q16.7
You hear a sound with a frequency of 256 Hz. The
amplitude of the sound increases and decreases
periodically: it takes 2 seconds for the sound to go from
loud to soft and back to loud. This sound can be thought of
as a sum of two waves with frequencies
A. 256 Hz and 2 Hz.
B. 254 Hz and 258 Hz.
C. 255 Hz and 257 Hz.
D. 255.5 Hz and 256.5 Hz.
E. 255.75 Hz and 256.25 Hz.
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A16.7
You hear a sound with a frequency of 256 Hz. The
amplitude of the sound increases and decreases
periodically: it takes 2 seconds for the sound to go from
loud to soft and back to loud. This sound can be thought of
as a sum of two waves with frequencies
A. 256 Hz and 2 Hz.
B. 254 Hz and 258 Hz.
C. 255 Hz and 257 Hz.
D. 255.5 Hz and 256.5 Hz.
E. 255.75 Hz and 256.25 Hz.
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The Doppler effect
• The Doppler effect for sound is the shift in frequency when there is
motion of the source of sound, the listener, or both.
• Use Figure 16.27 below to follow the derivation of the frequency the
listener receives.
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The Doppler effect and wavelengths
• Study Problem-Solving Strategy 16.2.
• Follow Example 16.14 using Figure 16.29 below to see
how the wavelength of the sound is affected.
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The Doppler effect and frequencies
• Follow Example 16.15 using Figure 16.30 below to see
how the frequency of the sound is affected.
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A moving listener
• Follow Example 16.16 using Figure 16.31 below to see
how the motion of the listener affects the frequency of
the sound.
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A moving source and a moving listener
• Follow Example 16.17 using Figure 16.32 below to see
how the motion of both the listener and the source
affects the frequency of the sound.
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Q16.8
On a day when there is no wind, you are moving toward a
stationary source of sound waves. Compared to what you
would hear if you were not moving, the sound that you
hear has
A. a higher frequency and a shorter wavelength.
B. the same frequency and a shorter wavelength.
C. a higher frequency and the same wavelength.
D. the same frequency and the same wavelength.
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A16.8
On a day when there is no wind, you are moving toward a
stationary source of sound waves. Compared to what you
would hear if you were not moving, the sound that you
hear has
A. a higher frequency and a shorter wavelength.
B. the same frequency and a shorter wavelength.
C. a higher frequency and the same wavelength.
D. the same frequency and the same wavelength.
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Q16.9
On a day when there is no wind, you are at rest and a
source of sound waves is moving toward you. Compared to
what you would hear if the source were not moving, the
sound that you hear has
A. a higher frequency and a shorter wavelength.
B. the same frequency and a shorter wavelength.
C. a higher frequency and the same wavelength.
D. the same frequency and the same wavelength.
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A16.9
On a day when there is no wind, you are at rest and a
source of sound waves is moving toward you. Compared to
what you would hear if the source were not moving, the
sound that you hear has
A. a higher frequency and a shorter wavelength.
B. the same frequency and a shorter wavelength.
C. a higher frequency and the same wavelength.
D. the same frequency and the same wavelength.
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A double Doppler shift
• Follow Example 16.18 using Figure 16.33 below.
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Shock waves
• A “sonic boom” occurs if the source is supersonic.
• Figure 16.35 below shows how shock waves are generated.
• The angle  is given by sin = v/vS, where v/vS is called the
Mach number.
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A supersonic airplane
• Follow Example 16.19 using Figure 16.37 below.
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