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Physics Chp 13

Physics Chp 13. SOUND. Compression vs Rarefaction Pitch – how we perceive the frequency Speed depends on medium (air, water, ice) 3D – goes in all directions. Doppler Effect As you or the source moves the sound changes. Towards – higher Away - lower. Sound Intensity I = P/A

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Physics Chp 13

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  1. Physics Chp 13 SOUND

  2. Compression vs Rarefaction • Pitch – how we perceive the frequency • Speed depends on medium (air, water, ice) • 3D – goes in all directions

  3. Doppler Effect • As you or the source moves the sound changes. • Towards – higher • Away - lower

  4. Sound Intensity • I = P/A • I = P/ 4πr2 • Threshold of hearing Io = 1x10-12 W/m2

  5. Audible Sounds • Depends on intensity as well as frequency • Resonance is when something vibrates at a natural frequency

  6. Our Ear converts the sound wave into electrical waves so our brain can sense the sound.

  7. Standing waves • Simplest is the fundamental. • On a string it has two nodes and one antinode. • λ1=2L

  8. For a string the first standing wave is half a wavelength and then goes up by whole intergers

  9. Then it increases by additional antinodes • λ2=L • λ3= 2/3L • λ4=1/2L

  10. Since v=fλ and the speed remains const • fn = n v/2L n= 1,2,3….. Harmonic series • f1 is the fundamental

  11. BUT a closed on one end is different • Then the node has to hit the closed end to reflect back with no loss of the wave • fn = n(v/4L) n= 1,3,5……

  12. If you use a mid C (261.6 hz) over a closed tube and get a resonant sound at 0.983 m and 1.64 m, which harmonics are present and what is the likely length for the first harmonic? Assume v = 343 m/s

  13. fn = n(v/4L) n= 1,3,5…… • 261.6 hz = n (343 m/s / 4(0.983m) ) n = 3 • 261.6 hz = n (343 m/s / 4(1.64m) ) n = 5 • So if n is 1 then 261.6 hz = 1(343m/s / 4L) • L = 0.328 m

  14. When two frequencies are slightly off they interfere with each other and create a pattern of changing intensities. The difference in frequencies gives the beat frequency. • f1 = 256hz and f2= 259 hz beat freq = 3hz

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