chapter 12

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
12.1 Characteristics of Sound

Range of human hearing ~ 20 – 20,000 Hz

Infrasonic (< 20 Hz)


Ultrasonic (> 20,000 Hz)

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Earthquakes, nuclear test detection, machines
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.
elastic
v
inertial
vair  (331 0.6T ) m/s
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12.1 Characteristics of Sound

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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
 I 
  10 log  
 Io 

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For humans: threshold of hearing (β = 0 dB) and
pain (β = 120 dB)
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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.
vn
fn 
2L
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vn
fn 
4L
n  1, 2, 3,...
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n  1, 3, 5, ...
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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.
 vsnd  vobs
f   f 
 vsnd  vsource
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

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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.
I
P
A

 P  IA  I 4 r
2
   3 10
6

W m 4  0.50 m   9.425  10 6 W 
2
2
(b) The number of people needed to produce 100 W of sound is
 1 person 
7
7
100 W 

1.06

10

1

10
people

6
 9.425 10 W 
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49. (a) Observer moving towards stationary source.
 vobs 
 30.0 m s 
f   1 
1550 Hz   1690 Hz
 f  1 

 343m s 
 vsnd 
(b) Observer moving away from stationary source.
 vobs 
 30.0 m s 
f   1 
1550 Hz   1410 Hz
 f  1 

 343m s 
 vsnd 
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