1. Energy flux at your eardrums
Sound Intensity
Energy flux at your eardrums
§ 14.5 1

2. Sound Intensity
I = (Power output)/(Area)
Units: W/m2 2

3. Intensity and Distance
Inverse-square law:
Source transmits power P
At distance R, energy is spread over area A = 4pR2
Intensity at distance R is P/A
IR = P
4pR2 3

4. Sound Intensity Level Decibel scale I b = (10 dB) log10 I0
Reference intensity I0 = 10–12 W/m2
Audibility threshold at 1000 Hz
A 10-dB increase in intensity level represents a factor-of-10 increase in sound intensity 4

5. Decibel Differences I b = (10 dB) log10 I0 I2 b2 – b1 = (10 dB) log105

6. Example Problem
30 m from an outdoor concert stage, the sound intensity level is 80 dB. What is the sound intensity level 40 m from the stage?
b2 – b1 = (20 dB) log10(R1/R2)
b2 = b1 + (20 dB) log10(R1/R2)
= 80 dB + (20 dB) log10(30/40)
= 80 dB + (20 dB) (–0.125)
= 80 dB – 2.50 dB = 77.5 dB 6

7. Doppler Effect
Distance and time
§ 14.6

8. Moving Source or Detector
Source: successive wave fronts do not emanate from the same place
Detector: successive wave fronts are not detected at the same place
Simulation:

9. Moving Source or DetectoruS v uD vO 1 2 3 D
Equations of Motion:
Source xS = uSt
Observer xO = D + uDt
nth wavefront xn = uS(nT) + v (t – nT)

10. Differing Formulas u – uD Mine: fD = fS u – uS 1 + vD/v
Book’s: fD = fS
1 – vS/v
1 + vD/v
Difference: Book uses |v|, propagation direction is –