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APHY20104/21/23 1
12.1 Characteristics of Sound
Range of human hearing ~ 20 – 20,000 Hz
Infrasonic (< 20 Hz) Earthquakes, nuclear test detection,
machines
Ultrasonic (> 20,000 Hz) Medicine (fetal development, tumor
removal, etc.), camera ranger, jewelry cleaning, animal sonar/echolocation
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12.1 Characteristics of Sound The speed of sound in a
material depends on the material’s elastic and inertial properties and its temperature.
inertial
elasticv
m/s )6.0331( Tvair
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12.1 Characteristics of Sound When the pressure is high, the
displacement is low.
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12.2 Intensity of Sound For humans: threshold of hearing (I ~
10-12 W/m2) and pain (I ~ 1 W/m2)
We perceive differences in loudness as logarithmic
For humans: threshold of hearing (β = 0 dB) and pain (β = 120 dB)
oI
Ilog10
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12.4 Sources of Sound The harmonics are determined by whether the
column is open at both ends or closed at one end.
3,... 2, 1,n 2
L
vnfn ... 5, 3, 1,n
4
L
vnfn
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12.5 Quality of Sound
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12.6 Interference of Sound Waves
The speakers emit at same frequency and are in phase
The waves at C constructively interfere (path difference = nλ) and at D destructively interfere (path difference = {n+½} λ)
What do you hear if the speakers emit different frequencies?
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12.7 Doppler Effect Apparent change in frequency due to
the motion of the sound source and/or the observer.
sourcesnd
obssnd
vv
vvff
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In class: Other problems ↓
13. (a) From Table 12-2, the intensity in normal conversation, when about 50 cm from the speaker, is about 3x10-6 W/m2.
(b) The number of people needed to produce 100 W of sound is
APHY20104/21/23 10
obs
snd
30.0 m s 1 1 1550 Hz 1690 Hz
343m s
vf f
v
obs
snd
30.0 m s 1 1 1550 Hz 1410 Hz
343m s
vf f
v
49. (a) Observer moving towards stationary source.
(b) Observer moving away from stationary source.
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