Physics – Chapter 26
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Sound is a form of energy that spreads
out through space
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What is SOUND?
 The source of all sound is a vibration
 Vibrating objects transfer energy into the medium (usually
the air) in the form of longitudinal waves
 The air undergoes compression & rarefaction
3
Sound & Frequency
 Higher frequency = higher pitch
 Human range of hearing = 20 Hz - 20,000 Hz
 Sound below 20 Hz = infrasonic
 Sound above 20,000 Hz = ultrasonic
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Sound Transmission
 Sound travels in solids, liquids, and gases
 In general, sound is transmitted faster in liquids than in
gases, and still faster in solids
Example:
boat motors sound louder (and closer to you) under
water
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Sound Transmission
 Sound cannot travel in a vacuum
 Sound waves require a medium (stuff to travel through)
 NO SOUND IN SPACE!
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Speed of Sound
 Speed of sound in a gas depends on:
1.
Temperature of the gas
 At 0°c = 330 m/s
 Increases by 0.6 m/s per degree
2. Mass of the particles in the gas

Hydrogen molecules and helium atoms move faster
and transmit sound much more quickly than heavier
gases (like oxygen & nitrogen)
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Speed of Sound
 Speed of sound in a solid depends on the material’s
elasticity (ability to return to original shape after a force is
applied)


Steel is very elastic
Putty (modeling clay) is inelastic
 Sound travels 15X faster in steel and 4X faster in water than
in air
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Think!
Why is there a delay in time between seeing lightning and
hearing thunder?
Answer:
Sound waves travel slower than light waves
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Sound Intensity & Loudness:
 Intensity is measured in units called Decibels (dB)
 Loudness is subjective (determined by the listener) but
related to intensity
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Sound Intensity & Loudness:
 Every increase in 10 dB = 10X increase in intensity
Examples:
 A sound of 10 dB = 10X as intense as sound of 0 dB
 20 dB is not twice but 10 times as intense as 10 dB, or
100X as intense as 0 dB
 A 60 dB sound is 100X as intense as a 40 dB sound
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Sound Intensity & Loudness:
Examples:
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Resonance:
 Occurs when an object vibrates at its
natural frequency
 The Tacoma Narrows Bridge collapse
was caused by resonance.
 Wind produced a force that
resonated with the natural frequency
of the bridge. Amplitude increased
steadily over several hours until the
bridge collapsed.
Physics - Ch.26 SOUND
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Resonance:
Lexus LFA!
The Glass Harp!
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Interference:
 Both transverse and longitudinal waves display wave
interference when they are superimposed.
 Noise Canceling Earphones use destructive interference
15
Interference:
 “Dead spots” can occur in poorly designed theaters
 Reflected sound waves interfere with un-reflected waves
to form zones of low amplitude.
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Beats:
 When two tones of slightly different frequency are sounded
together the sound is loud, then faint, then loud, then
faint, and so on
 This periodic variation in the loudness of sound is called
beats
Physics - Ch.26 SOUND
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