General Physics L07_Sound.ppt Energy flux at your eardrums

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Presentation transcript:

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

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

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

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

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

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

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

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: http://www.astro.ubc.ca/~scharein/a311/Sim/doppler/Doppler.html

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)

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

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

Instructive Cases vS vD 0.1 v 0 -0.1 v 0 0 0.1 v 0 -0.1 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