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What is sound?

What is sound?. Sound is a longitudinal wave produced by a vibrating source Examples of sources: tuning fork, vocal cords, lips or reed on a musical instrument The vibration produces COMPRESSIONS and RAREFACTIONS in the air

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What is sound?

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  1. What is sound? • Sound is a longitudinal wave produced by a vibrating source • Examples of sources: tuning fork, vocal cords, lips or reed on a musical instrument • The vibration produces COMPRESSIONS and RAREFACTIONS in the air • Must have a medium to travel through, will not travel through a vacuum http://dev.physicslab.org/Document.aspx?doctype=3&filename=WavesSound_IntroductionWaves.xml

  2. Sound waves propagate in three dimensions • Compressions and Rarefactions produce wave fronts that become a spherical wave • Energy radiates outward from sound source in a direction perpendicular to the wave front.

  3. What determines pitch? • Pitch is the audible sound our ears perceive, and it depends upon FREQUENCY • Higher frequency = higher pitch • If sound source is moving, the perceived frequency of the hearer will change, so the pitch will change • This is called the DOPPLER EFFECT

  4. Doppler Effect Misconception • the frequency does not change as the moving source approaches or recedes • the frequency only changes as the moving source passes you ( the perceived frequency)

  5. Resonance • Resonance occurs when the frequency of a force applied to a system matches the natural frequency of the system • Result: large amplitudes of vibration • Example: breaking glass with your voice http://www.youtube.com/watch?v=ofESdVdX-fY

  6. OR a bridge collapsing

  7. Beats

  8. Beats

  9. Beats

  10. Beats • The beat frequency (how often the beat is heard) is determined by the difference in the source frequencies • Example: A tuning fork at 440 Hz and 448 Hz are struck at the same time. What is the beat frequency produced by the two sound waves? • As the sound source frequencies are farther and farther apart…. • Beat frequency is created by superposition/interference- there is constructive & destructive interference that creates the beat

  11. What determines volume? • Energy of a wave is determined by amplitude • Volume of sound is also determined by amplitude • How do we measure volume? • On the Decibel (dB) scale • Measure of sound intensity

  12. Intensity • rate at which energy flows through a unit area perpendicular to the direction of wave motion • intensity = ΔE / (Δt • area) = P / area • Since the area of a sphere is equal to 4πr2, • Intensity = P / 4πr2 (units: W/m2) • E: energy (J) • t: time (s) • P: power (W) • r: radius from source (m)

  13. Decibel (dB) • describes the ratio of two intensities of sound; the threshold of hearing is commonly used as the reference intensity • Ratio = Intensity of source/threshold of hearing • threshold of hearing = 1.0 x 10-12 W/m2 • logarithmic scale • Means that an increase of 10 dB is power of ten increase (ten fold increase) in intensity

  14. Human hearing

  15. Audiogram

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