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Sound Intensity & Resonance
12-2 Sound Intensity & Resonance
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Sound waves carry energy.
The power of the wave is the energy transported per second, measured in J/S or Watts
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Sound Intensity: The rate at which energy flows through a unit area perpendicular to the direction of wave motion (pressure amplitude) Sound Intensity is given by: I=P/A measured in [W/M2] P=Power, A=Area intercepting the sound (4πr²for a spherical wave)
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Audibility The human ear picks up sound only if it is within the audible range AND has sufficient intensity The lowest intensity we can hear is for a 1 kHz sound at 1 x W/m2 I° = human threshold of hearing
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To recap: ~frequency of sound determines the pitch ~intensity sound determines loudness ~I° = 1x10-12 W/m² threshold of hearing ~Ip = 1.0 W/m² threshold of pain
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Relative intensity is found by comparing the intensity of two sounds
Ip/Io = 1 x 1012 (a trillion times the intensity) Due to the huge range we compare sounds on a logarithmic The relative intensity is also called decibel level
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The sound level b is defined as:
decibel 10-12 W/m2, human hearing threshold Note: this is not an intensity (units aren’t W/m2) this does measure the relative loudness of a sound.
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Ranking of common sounds
Scale starts at 0dB Each 10 dB increase in sound level, means 10 x the intensity of the sound.
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Forced Vibrations When a vibration of one object causes another to vibrate. Examples: 1) sounding board of a musical instrument 2) the box of a music box
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When the frequency of a forced vibration (driving frequency) matches the object’s natural frequency, a dramatic increase in amplitude occurs. This is known as RESONANCE.
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Tacoma Narrows Bridge
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