APHY20104/22/23 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

APHY20104/22/23 2

12.1 Characteristics of Sound The speed of sound in a

material depends on the material’s elastic and inertial properties and its temperature.

inertialelastic

v

m/s )6.0331( Tvair

APHY20104/22/23 3

12.1 Characteristics of Sound When the pressure is high, the

displacement is low.

APHY20104/22/23 4

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)

oIIlog10

APHY20104/22/23 5

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

Lvnfn ... 5, 3, 1,n

4

Lvnfn

APHY20104/22/23 6

12.5 Quality of Sound

APHY20104/22/23 7

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?

APHY20104/22/23 8

12.7 Doppler Effect Apparent change in frequency due to

the motion of the sound source and/or the observer.

sourcesnd

obssnd

vvvvff

APHY20104/22/23 9

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

APHY20104/22/23 10

obs

snd

30.0 m s 1 1 1550 Hz 1690 Hz

343m sv

f fv

obs

snd

30.0 m s 1 1 1550 Hz 1410 Hz

343m sv

f fv

49. (a) Observer moving towards stationary source.

(b) Observer moving away from stationary source.