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Copyright © 2012 Pearson Education Inc.

PowerPoint® Lectures for

University Physics, Thirteenth Edition

– Hugh D. Young and Roger A. Freedman

Lectures by Wayne Anderson

Chapter 16

Sound and Hearing


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


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 displace-ment 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?


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.


Different ways to describe a sound wave

• Sound can be described by a graph of displace-ment versus position, or by a drawing showing the displacements of individual particles, or by a graph of the pres-sure fluctuation versus position.

• The pressure amplitude is pmax = BkA.


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.

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.

A16.1


Amplitude of a sound wave

• Follow Examples 16.1 and 16.2 using Figure 16.4 below.


Perception of sound waves

• The harmonic content greatly affects our perception of sound.


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:

(fluid)

(solid rod)

(ideal gas)

Bv

Yv

RTvM


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.


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.


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.

Q16.2


Increasing the pressure amplitude of a sound wave by a

factor of 4 (while leaving the frequency unchanged)





E. The answer depends on the frequency of the sound

wave.

A16.2


Increasing the frequency of a sound wave by a factor of 4

(while leaving the pressure amplitude unchanged)





E. The answer depends on the frequency of the sound wave.

Q16.3


Increasing the frequency of a sound wave by a factor of 4

(while leaving the pressure amplitude unchanged)





E. The answer depends on the frequency of the sound wave.

A16.3


The decibel scale

• The sound intensity level is = (10 dB) log(I/I0).

• Table 16.2 shows examples for some common sounds.


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.


Standing sound waves and normal modes

• The bottom figure shows displacement

nodes and antinodes.

• A pressure node is always a displace-

ment antinode, and a pressure antinode

is always a displacement node, as

shown in the figure at the right.


The sound of silence

• Follow Conceptual Example 16.10, using Figure 16.14 below, in

which a loudspeaker is directed at a wall.


Organ pipes

• Organ pipes of different sizes produce tones with different frequencies (bottom figure).

• The figure at the right shows displacement nodes in two cross-sections of an organ pipe at two instants that are one-half period apart. The blue shading shows pressure variation.


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.


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.


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.

Q16.4




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.


A16.4




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.


Q16.5




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.


A16.5


A. 110 Hz.

B. 220 Hz.

C. 440 Hz.

D. 880 Hz.

E. 1760 Hz.

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


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.

A16.6


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.


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.


Loudspeaker interference

• Follow Example 16.13 using Figure 16.23 below.


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.


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.

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.

A16.7


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.


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.


The Doppler effect and frequencies


how the frequency of the sound is affected.


A moving listener


how the motion of the listener affects the frequency of

the sound.


A moving source and a moving listener


how the motion of both the listener and the source

affects the frequency of the sound.


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.

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






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


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





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





A16.9


A double Doppler shift



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.


A supersonic airplane


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