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
4/21/2017
APHY201

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.
4/21/2017
APHY201

3. 12.1 Characteristics of Sound
When the pressure is high, the displacement is low.
4/21/2017
APHY201

4. 12.2 Intensity of Sound
For humans: threshold of hearing (I ~ 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)
4/21/2017
APHY201

5. 12.4 Sources of Sound
The harmonics are determined by whether the column is open at both ends or closed at one end.
4/21/2017
APHY201

6. 12.5 Quality of Sound
4/21/2017
APHY201

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?
4/21/2017
APHY201

8. 12.7 Doppler Effect
Apparent change in frequency due to the motion of the sound source and/or the observer.
4/21/2017
APHY201

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
4/21/2017
APHY201

10. 49. (a) Observer moving towards stationary source.
(b) Observer moving away from stationary source.
4/21/2017
APHY201