Chapter 20 Sound
The physicist usually takes the
objective position and defines
sound as a form of energy that
exists whether or not it is heard
and goes on from there to
investigate its nature.
Dr. Jie Zou
PHY 1071
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Main topics
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Origin of sound
Nature of sound in air
Speed of sound in air
Natural frequency
Resonance
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PHY 1071
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Origin of sound
• Most sounds are waves produced by the vibrations of
material objects. For example,
– In a piano, violin, and guitar, the sound is produced by the vibrating
strings.
– Your voice results from the vibration of your vocal cords.
• In each of these cases,
– The original vibrations stimulates the vibration of something larger or
more massive, such as the sounding board of a stringed instrument, or the
air in the throat and mouth of a singer.
– This vibrating material then sends a disturbance through the surrounding
medium, usually air, in the form of longitudinal waves.
• Under ordinary conditions, the frequency of the vibrating
source and the frequency of the sound waves produced are
the same.
Dr. Jie Zou
PHY 1071
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Pitch and frequency of sound
• Pitch: we describe our subjective impression
about frequency of sound by the word pitch.
• Frequency corresponds to pitch:
– A high-pitched sound like that form a piccolo has a
high frequency of vibration.
– A low-pitched sound like that form a fog horn has
a low frequency of vibration.
A picture of piccolo
• The human ear of a young person can normally
hear pitches corresponding to the range of
frequencies between 20 and 20,000 hertz.
– Infrasonic: frequencies below 20 hertz.
– Ultrasonic: frequencies above 20,000 hertz.
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PHY 1071
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Nature of sound in air
• Compression and rarefaction
• When the door is opened, a compression travels
across the room.
• When the door is closed, a rarefaction travels
across the room.
• It is not the medium itself that travels across the
room, but the energy-carrying pulse. The pulse
(disturbance) travels from the door to the curtain.
• A continual swing of the door open and closed in
a periodic fashion will set up a wave of periodic
compressions and rarefactions that will make the
curtain swing in and out of the window.
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PHY 1071
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Nature of sound in air (cont.)
• When the prong of the tuning fork next to the tube moves
toward the tube, a compression enters the tube.
• When the prong swings away in the opposite direction, a
rarefaction follows the compression.
• As the source (the prongs of the tuning fork) vibrates,
compressions and rarefaction travel in the same direction
from the tuning fork through the air.
• The frequency of the vibrating source and the frequency of
the wave it produces are the same.
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PHY 1071
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Speed of sound in the air
• Thunder is heard after a flash of lightning is seen.
• Sound requires a recognizable time to travel from one
place to another.
• The speed of sound does not depend on the loudness or
frequency of the sound.
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PHY 1071
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Natural frequency and resonance
• Natural frequency: any object
composed of an elastic material will
vibrate when disturbed at its own
special set f frequencies, which
together form its special sound.
• Resonance: when the frequency of
forced vibrations on an object
matches the object’s natural
frequency, a dramatic increase in
amplitude occurs.
Dr. Jie Zou
PHY 1071
Pumping a swing in rhythm
with its natural frequency
produces a large amplitude.
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The effect of resonance
• Resonance is not restricted to wave motion. It occurs whenever
successive impulses are applied to a vibrating object in rhythm with its
natural frequency.
• In 1940, four months after being completed, the Tacoma Narrows
Bridge in the state of Washington was destroyed by wind-generated
resonance.
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PHY 1071
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