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20SoundEncoding - CS1315 Introduction to Media Computation...

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CS1315: Introduction to Media Computation Sound Encoding and Manipulation
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How sound works: Acoustics, the physics of sound Sounds are waves of air pressure Sound comes in cycles The frequency of a wave is the number of cycles per second (cps), or Hertz (Complex sounds have more than one frequency in them.) The amplitude is the maximum height of the wave
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Volume and pitch: Psychoacoustics, the psychology of sound Our perception of pitch is related (logarithmically) to frequency Higher frequencies are perceived as higher pitches We can hear between 5 Hz and 20,000 Hz (20 kHz) A above middle C is 440 Hz Our perception of volume is related (logarithmically) to changes in amplitude If the amplitude doubles, it’s about a 3 decibel (dB) change
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“Logarithmically?” It’s strange, but our hearing works on ratios not differences , e.g., for pitch. We hear the difference between 200 Hz and 400 Hz, as the same as 500 Hz and 1000 Hz Similarly, 200 Hz to 600 Hz, and 1000 Hz to 3000 Hz Intensity (volume) is measured as watts per meter squared A change from 0.1W/m 2 to 0.01 W/m 2 , sounds the same to us as 0.001W/m 2 to 0.0001W/m 2
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Decibel is a logarithmic measure A decibel is a ratio between two intensities: 10 * log 10 (I 1 /I 2 ) As an absolute measure, it’s in comparison to threshold of audibility 0 dB can’t be heard.
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