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14 Sound Waves

Explore the world of sound waves, from their characteristics to how they behave in different mediums. Learn about speed, intensity, resonance, and the Doppler effect. Discover the fascinating nature of sound waves and their 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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