1. General Physics L07_Sound.ppt Energy flux at your eardrums
Sound Intensity
Energy flux at your eardrums
§ 16.7 1

2. General Physics L07_Sound.ppt
Sound Intensity
I = (Power output)/(Area)
Units: W/m2 2

3. Intensity and Distance
General Physics L07_Sound.ppt
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. General Physics L07_Sound.ppt
Sound Intensity Level
Decibel scale
b = (10 dB) log10 I0 I
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. General Physics L07_Sound.ppt
Change with Distance
b = (10 dB) log10 I0 I
b2 – b1 = (10 dB) log10 I1 I2
Changing from I to 1/R^2 because without reflection or absorption of the wave energy, the intensity is inversely proportional to distance from the source
= (10 dB) log10
R12
R22
= (20 dB) log10 R1 R2 5

6. General Physics L07_Sound.ppt
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. General Physics L07_Sound.ppt Distance and time
Doppler Effect
Distance and time
§ 16.9

8. Moving Source or Detector
General Physics L07_Sound.ppt
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 Detector
General Physics L07_Sound.ppt
Moving Source or Detector vS v vD 1 2 3 D
Nth wave front: first term is position of source when front originates. Second term is wave velocity times time elapsed since front originated at time nT.
Equations of Motion:
Source xS = vSt
Detector xD = D + vDt
nth wavefront xn = vS(nT) + v (t – nT)

10. Doppler Shift
nth wave front received at tn when xn = xD Find TD = tn+1 − tn Then fD = 1/TD

11. General Physics L07_Sound.ppt
Differing Formulas
v – vS
v – vD
Mine: fD = fS
Book’s: fD = fS
1 ± vD/v
Difference: Book uses |v|, wave direction convention is opposite detector direction

12. Instructive Cases
vS vD 0.1 v v v v 0.1 v 0.1 v -0.1 v -0.1 v
vS vD
0 v
v 0
0 -v
-v 0
-1.1 v 0