1. Chapter 14 Sound

2. Characteristics of sound 2 A special and important type of mechanical wave Speed of sound: Loudness: related to the energy of sound depends on the medium Audible range: 20Hz ~ 20000Hz Pitch: related to the frequency of sound Infrasonic & ultrasonic

3. Intensity of sound 3 Sound level: using a logarithmic scale Unit: decibel (dB) Loudness: represented by the intensity Range of ear: 10 -12 W/m 2 ~ 10W/m 2 (painful) I 0 = 10 -12 W/m 2 Notice: logarithmic scale 50dB ~ 20dB?

4. Distance & sound level 4 Example1: Sound from a loudspeaker is measured to be 60dB at a distance of 2m. To obtain a level of 40dB, how can we choose the distance? Solution: Sound level For a spherical wave: Distance we choose:

5. Doppler effect 5 The change in pitch: speeding car or train Source moves toward observer → higher Source moves away from observer → lower Doppler effect: occurs for all types of waves For light: red shift → big bang theory

6. Physical analysis 6 Choose the frame of medium, observer and source move in a straight line, v o and v s are relative to medium (air), v is the wave speed in the air, and: f : emitted frequency by the source f ’ : detected frequency by the observer 1) Observer and source both stay at rest:

7. Moving case A 7 2) Observer moving, source stationary: vovo Extra v o / S v/ v O toward source: away from source:

8. Moving case B 8 3) Source moving, observer stationary: vsvs S v O toward observer: away from observer:

9. General case 9 Thinking: Why case A is different from case B? Change in both wave speed & wavelength: Upper sign: moving toward each other Lower sigh: moving away from each other Doppler effect for light:

10. Sound of moving car 10 Example2: When a car passes, an observer hears the sound of car changing from 1200Hz to 1000Hz. If the speed of sound is 330m/s, what is the speed of car and the original frequency? Solution: Toward observer Away from observer: Solving equations:

11. Sound echo 11 Thinking: Someone carrying a loudspeaker moves at speed u to a wall, and hears a sound echo. If the emitted frequency is f, what is the frequency f ’? (Suppose the speed of sound is v) uu

12. Shock wave 12 Source in a supersonic speed vΔt vΔt S vsvs ··· vs Δtvs Δt shock wave v s / v : Mach number