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Chapter 14

Chapter 14 . Sound. 14.1 – Sound Waves & Beats. Sound Waves travel as compressions & expansions Alternating regions of compressed and expanded air These regions move away from source as longitudinal wave. Sound waves exhibit sine wave behavior. Speed of sound. Same for all frequencies

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Chapter 14

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  1. Chapter 14 Sound

  2. 14.1 – Sound Waves & Beats • Sound Waves travel as compressions & expansions • Alternating regions of compressed and expanded air • These regions move away from source as longitudinal wave

  3. Sound waves exhibit sine wave behavior

  4. Speed of sound • Same for all frequencies • Remember v=fλ …. And v only depends on medium, not f

  5. Speed of Sound • Normal atmospheric pressure and temperature speed of sound is 343 m/s. • Speed of sound is determined by properties of medium that it travels through • Speed of sound increases with temperature • More rigid/stiff an object is… the faster sound will travel through it • Steel – 5960 m/s • Plastic – 2680 m/s • Fresh Water @ 20° C – 1482 m/s • Air – 340 m/s

  6. Lightning/Thunder • Use speed of sound to calculate how far lightning is away from you

  7. Human Hearing • The pitch of a sound is simply the frequency of the sound wave • Higher frequency, higher the pitch • Humans can typically hear pitches ranging from 20 Hz (infrasonic) to 20,000 Hz (ultrasonic), but that range diminishes as you age… typically at the high end • Loudness of a sound is affected only by amplitude

  8. Beats • Two tones of slightly different frequency are sounded together. This produces a fluctuation in the loudness of combined sounds. • Amount of beats per second is equal to the difference in frequencies. • Fbeat = |f1-f2| • Two tuning forks…. One 340 Hz another 310 Hz are sounded together. Beats will occur at a frequency of 30 Hz

  9. 14.2 Standing Sound Waves • Guitar strings, flute • Blowing air across the open end of a bottle • In general a standing wave in a bottle has a node at bottom and antinode at the top

  10. Harmonics for pipe open at one end • First Harmonic w node at bottom and antinode at top would be ¼ of a wavelength fitting in the bottle • So λ= ¼ L and f1 = v/4L • In general,

  11. Harmonics for pipes open at both ends • Must have antinode at each open end

  12. 14.3 – The Doppler Effect • The frequency of a sound wave will change do to the perceived motion of the sound source • Waves bunched when approaching (high F, low λ) • Waves spread out when receding (low F, high λ)

  13. Doppler Effect • Waves From an object that is approaching have short wavelength, which means they have a higher freq. And a higher pitch • waves coming from an object that is moving away from you have a longer wavelength and therefore a lower freq, and lower pitch • Cars approaching and leaving

  14. Bow shock/sonic Boom • Sometimes the source of a wave can travel faster than the waves it is creating • http://www.youtube.com/watch?v=6o0zmafxTmE&safe=active • Can be easily seen with a speed boat • http://www.youtube.com/watch?v=2Sv4o4Kktm4&safe=active • Sonic boom created when a object or plane breaks through the sound barrier

  15. Human Perception of Sound • Loudness is determined by Intensity

  16. Doubling loudness corresponds to increasing the intensity by a factor of 10. • 2x as loud means a 10 dB increase in Intensity

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