Chapter 15: Sound
A longitudinal wave of
compressions and
rarefactions
Sound Waves
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A pulse of sound occurs because air
pressure is disturbed in a series of high
and low pressure regions.
The pressure variation transmits the
pulse through the matter.
Describing Sound
The frequency of the sound wave is
the number of oscillations in
pressure each second.
 The wavelength of the wave is the
distance between successive regions
of high or low pressure.
(Compressions or Rarefactions)
 The motion of the wave is parallel to
the medium.
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Speed of sound
Sound requires a medium to travel
through.
 Sound travels fastest in solids, then
liquids, and slowest in gases in general.
 The speed of sound in air varies with
temperature.
 The warmer the temperature, the faster
the sound travels.
 331m/s at 0 C, 343m/s at 20 C (about
room temp), +0.6m/s per 1 C above 0.
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Sound Behavior
Sound waves can reflect off hard
objects like walls.
 Bats use echoes and sonar to “find”
objects they can’t “see”.
 Sound waves can also be diffracted
spreading out after passing through
narrow openings.
 Sound waves can interfere and produce
“dead spots” at nodes.
 The speed of a sound wave is v = fλ.
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Loudness
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Amplitude is the measure of pressure variations
along the wave.
The loudness of a sound depends on the amplitude
of the wave.
Sound level is measured in decibels (dB).
This is the ratio of a given sound wave pressure
variation to the most faintly heard sound’s pressure
variation
(0dB = 2 x 10-5pa) and 10x as much pressure is 2 x
10-4pa or 20 dB.
A sound about twice as loud is a 10 dB increase.
Loudness cont’d
If 0 dB is the most quiet sound a
human ear can hear, then as amplitude
increases, loudness increases
exponentially.
 The human threshold of pain is 110 dB.
 The pressure on the human eardrum is
10x greater for each increase of 20 dB,
so 40 dB would be 100 x greater than
0 dB.
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Pitch
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Pitch, the highness or lowness of a sound,
is caused by the FREQUENCY of a wave.
The average human ear is sensitive to
frequencies between 20 Hz and 16,000 Hz
Low frequency is associated with low pitch
and vice versa.
Older people lose sensitivity to higher
frequencies as they age.
Long exposure to 100dB or louder can
cause hearing damage.
The ear’s sensitivity depends on BOTH
pitch AND amplitude.
Doppler Shift
The apparent change of pitch due to
movement of the source or observer
is called Doppler shift.
 Sirens, trains, planes, and light can
all “change” frequency as they move
relative to you.
 This movement creates a change of
pitch.
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Doppler explained
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It is important to remember that though a
source is moving in a medium, the speed
of the wave does not change.
Because the waves move, they begin to
“stack up” and the wavelength is
shortened.
This creates a change in frequency and
happens if the observer is moving and the
source is stationary as well.
Happens for all types of waves.
Example Problem
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Find the frequency of a sound wave
moving in air at room temperature
with a wavelength of 0.667m.
Recall v = f x λ, so f = v / λ
 f = 343 m/s / 0.667 m
 f = 514 Hz
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Your turn to Practice 15.1
Please
352
 Please
 Please
 Please
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do Practice Problems # 2-3 p
do Ch 15 Rev. p 368 #s 4,5,6
do Ch 15 Rev p 369 #s 24-28
do Ch 15 Rev p 370 #s 29-34