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14 Sound Waves

14 Sound Waves. character of sound waves speed of sound sound intensity resonance Homework: 2, 3, 4, 10, 17, 41, 45, 81, 91. Sound Waves. pressure/density waves compressions rarefactions reflects refracts (similar to light) diffracts (bends around corners). Sound Spectrum.

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14 Sound Waves

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  1. 14 Sound Waves • character of sound waves • speed of sound • sound intensity • resonance • Homework: • 2, 3, 4, 10, 17, 41, 45, 81, 91.

  2. Sound Waves • pressure/density waves • compressions • rarefactions • reflects • refracts (similar to light) • diffracts (bends around corners)

  3. Sound Spectrum • ultrasonic (f > 20kHz) • (human) audible (20Hz < f < 20kHz) • infrasonic (f < 20Hz) • dogs, cats (50Hz < f < 45kHz) • bats up to 120kHz • elephants as low as 5Hz

  4. Sound Speed • in air: v = 331 + 0.6TC m/s • in solids: ~ 1800 to 5000 m/s • in liquids: ~ 1100 to 1500 m/s • in gases: ~ 300 to 1200 m/s

  5. Sound Intensity • intensity, I: power/area [watt/m2] • decibel: b = 10log(I/Io) • Io =10-12 watt/m2. • Example: 10-9 watt/m2 = 30 decibels

  6. Example: Intensity • point source of sound, 0.010 watts • I at 10 meters: = power/area = 0.010watts/(4p102m2) = 7.96x10-6 watt/m2. • b = 10log(7.96x10-6/10-12) = 69dB

  7. Resonance in Tubes • Due to constructive interference of waves within the tube • open at both ends: all harmonics • closed at one end: odd harmonics only

  8. Doppler Effect • f’ increase on approach • F’ decrease on separation • Eq. 14.14a page 487

  9. demos • Vernier microphone, logger pro, physics with computers, voice program • measure voice, chilandi plates, tuning forks, anyone with perfect pitch, meter stick? • Open/closed tubes • Take data on chilandi plates, compare to sand patterns with wave driver applied.

  10. End

  11. Example of wavelength distortion due to source motion:

  12. Nature of Sound Waves • Longitudinal • Oscillations are: • Condensations (higher pressure areas) and • Rarefactions (lower pressure areas) • Sound travels at about 343m/s at room temperature and normal atmospheric pressure

  13. Doppler Effect • Frequency received is different than the Source frequency due to: • Source Motion, • Receiver Motion or, • a combination of Source and Receiver motions.

  14. values of “A” and “f”?

  15. Decibels • intensity levelwhere Io = 1.0x10-12 W/m2. • Example: Intensity of sound is 4.0x10-5 W/m2. Intensity level is

  16. Sound Intensity (I) • Intensity = power/area = P/A [watt/meter2] • Spherical Radiation I = P/4pr2. • Example: Small speaker emits 1.0W of sound in all directions. Intensity 10m from the speaker is 1.0/(4p102) W/m2.

  17. Frequency of Sound • Audible Range: 20Hz to 20,000Hz • Infrasonic: f < 20Hz • Ultrasonic: f > 20,000Hz

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