1. Dr. Jie ZouPHY 13711 Chapter 17 Sound Waves (cont.)

2. Dr. Jie ZouPHY 13712 Outline Intensity of periodic sound waves The Doppler effect Shock wave

3. Dr. Jie ZouPHY 13713 Intensity of periodic sound waves Intensity I of a wave: The power per unit area, or the rate at which the energy being transported by the wave flows through a unit area A perpendicular to the direction of travel of the wave, or I = P/A. P: Rate of energy transfer or power. The amount of energy that passes by a given point during one period of oscillation can be shown to be E = (1/2)  A(  s max ) 2. Power P = E /T= (1/2)  Av(  s max ) 2. Intensity I = P/A = (1/2)  v(  s max ) 2. Using  P max =  v  s max, I = (  P max ) 2 /(2  v). The intensity of a periodic sound wave  (  P max ) 2.

4. Dr. Jie ZouPHY 13714 Example 17.3 The faintest sounds the human ear can detect at a frequency of 1000 Hz corresponds to an intensity of about 1.00 x 10 -12 W/m 2 - the so-called threshold of hearing. The loudest sounds the ear can tolerate at this frequency corresponds to an intensity of about 1.00 W/m 2 - threshold of pain. Find the pressure amplitude and displacement amplitude associated with these two limits.

5. Dr. Jie ZouPHY 13715 The Doppler effect Doppler effect (sound waves): The change in pitch (frequency) of the sound, due to the relative motion between a source of sound and the receiver. Stationary source, moving observer: Stationary observer, moving source: Both source and observer are in motion:

6. Dr. Jie ZouPHY 13716 Example 17.6 As an ambulance travels east down a highway at a speed of 33.5 m/s (75 mi/h), its siren emits sound at a frequency of 400 Hz. What frequency is heard by a person in a car traveling west at 24.6 m/s (55 mi/h) (a) as the car approaches the ambulance and (b) as the car moves away from the ambulance?

7. Dr. Jie ZouPHY 13717 Shock Waves When the speed v s of the source > the wave speed v. The envelope of the waves fronts is a cone whose apex half-angle  is given by: sin  = vt/v s t = v/v s. Mach number: The ratio v s /v is called the Mach number. Shock wave: The conical wave front produced when v s > v (supersonic speeds) si known as a shock wave.

8. Dr. Jie ZouPHY 13718 Homework Ch. 17, P. 537, Problems: #18, #37.