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Chapter 13

Chapter 13. Waves and Sound. 13.1 Vibrations. Wave- vibration through space, time Through solid, liquid, or gas Need matter to vibrate to make sound Parts of a wave Wavelength Amplitude Crest Trough Frequency Period See pictures on board. 13.2 Wave motion.

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Chapter 13

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  1. Chapter 13 Waves and Sound

  2. 13.1 Vibrations • Wave- vibration through space, time • Through solid, liquid, or gas • Need matter to vibrate to make sound • Parts of a wave • Wavelength • Amplitude • Crest • Trough • Frequency • Period • See pictures on board

  3. 13.2 Wave motion • Air itself does not travel at speed of sound (334 m/s), or else we’d feel wind when speaking • Wave speed = frequency x wavelength (lambda x nu) • See problems on board • C = 3x10^8 m/s for light, 334 m/s for sound

  4. 13.3 Types of Waves • Transverse- direction of wave perpendicular to source of vibration • Longitudinal- direction of wave along direction of source of vibration (compression and rarefaction) • Examples

  5. 13.4 Sound • Longitudinal waves • Compress and rarefact, like many ping pong balls on paddle • Many vibrations • Frequency = pitch (high = high frequency) • We hear 20 to 20,000 Hz (range shrinks with age), below is infrasonic, above is ultrasonic • Solids and liquids conduct sound better than air • Vibrations off surface- ear drum, speaker • Slower than speed of light, but speed of sound increases 0.6 m/s per degree rise

  6. 13.5 Sound Reflected • Echo- sound reflection • Need smooth surface (not carpet) • Multiple reflections-reverberations • Study of sound- acoustics • Why is the auditorium have good acoustics?

  7. 13.6 Sound Refracted • Sound bends when at different speeds/temperatures • Bend-refraction • Bend away from ground • “Blind spots” in water • Echolocation • Ultrasound

  8. 13.7 Forced Vibrations • Vibrate, then hold close to something to force vibrations on larger surface to force air into motion more than before • Natural frequency- vibrate at certain frequency (aluminum bat vs. wrench) • Examples • All things have springiness (vibrate)

  9. 13.8 Resonance • Forced vibration matches natural frequency- resonance • If same frequency, strike one, other sounds, sympathetic vibration • 1831- cavalry troops destroy Manchester, England bridge when marching in rhythm to bridge’s natural frequency (1940 Washington state bridge broken by wind interference)

  10. 13.9 Interference • Interference- add two waves together • Note: some are out of phase • Constructive- both build, or amplify • Destructive- cancel (anti-noise) • See pictures on board • Example from Crichton’s Timeline • Beat- sound interference- loudness flux in periods (tune instrument) • Standing wave- pass through waves (nodes and antinodes)- stringed instrument

  11. 13.10 Doppler Effect • Waves if stationary vibration is concentric • Waves if move less than wave speed looks different (see board) • Center is where vibration was • B observes higher than A (Christian Doppler) • Car horn sounds different in passing, blue shift- light approaches, red shift- light leaves (things moving to and away from Earth)

  12. 13.11 Wave barriers • Can’t keep up with waves • Overlap to get barrier- wave pool • Why so difficult to fly supersonic- then easy • V-wave = bow wave- outrun wave produced

  13. 13.12 Shock Wave • Shock wave- 3 D overlapping spheres that form cone • Crack of shock wave- sonic boom • Only hear if faster than sound, overlap in one burst • Overpressure then underpressure • Can be silent, but speed makes sound (whip is silent, but speed makes the familiar noise)

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