Chapter 3 Acoustics

Chapter 3 Acoustics - Speech and Hearing Science 420...

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Speech and Hearing Science 420 Introduction to Acoustics These powerpoint slides support a lecture on Acoustics (you need to read and understand Chapter 3 in Raphael, Borden and Harris). I expect that some of this (perhaps even a large portion of this) lecture will be a review of material covered in SpHrng 340 … but that’s probably not a bad thing.
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Acoustics: the study of sound Sound: Is a disturbance in a medium (like air) Produces a wave that travels through space The wave can be simple or complex Simple Wave: one frequency only--a pure tone Complex: multiple frequencies—includes all other sounds, including speech Understanding simple waves provides a foundation for describing complex sounds
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Simple harmonic motion Pure tones result from simple harmonic motion (SHM) Characteristics of SHM: The pattern of vibration repeats itself (it’s periodic) Each cycle takes the same amount of time (the period is constant) Frequency (determined by period) is constant The graphic representation is a sine wave Tuning forks and pendulums (including swings) move in SHM
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One cycle of SHM in tuning fork Initial impact starts movement (displacement) away from rest Elasticity in the fork allows displacement, but also generates a restoring force that stops the movement Restoring forces push the fork back to resting position Inertia carries the tines past resting position (overshoot) Restoring force builds up in the other direction Fork returns to rest position again (one full cycle) Overshoots . . . builds up restoring force . . . the pattern repeats
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More on forces in SHM Inertia and restoring forces (RF) vary continuously during cycle: RF is stronger when inertia is weak (when tines are more displaced) Inertia is strong when RF is weak (around rest position) Interplay between the two forces lets vibration persist
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Compression and Rarefactions in SHM Compressions and rarefactions of air molecules resulting from the vibrating tines of a tuning fork. It is the disturbance that moves. Think about putting a slinky on the table and starting it moving. Note compressions and rarefactions.
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Motion of a Swing is SHM Given one push, a swing will move back and forth because the earth's gravity is pulling down while the rope is making the swing move in a partial circle. When the swing has returned to the starting point it still has momentum to use up, so it moves beyond the center and up on the other side until gravity slows it down and pulls it back to the center,
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Chapter 3 Acoustics - Speech and Hearing Science 420...

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