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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

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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.