Monday, September 14, 2009

Element of sound

  • band: 20-20k Hz. 0~20Hz – infrasonic, > 20k Hz – Ultrasonic (supersonic)
  • sound pressure level (SPL): variation above and below normal atmospheric pressure
  • sound depends on the production of changes in atmospheric pressure
  • acoustic masking: one sound can mask another sound, making it inaudible.
  • If specify too high sample rate when replaying sound, the audio will sound squeaky. If the sample rate is too low, it will sound slow.
  • Famous Note: (ref from class note)
    Middle C = 261.626 Hz (standard tuning)
    Concert A (A above middle C) = 440 Hz
    Middle C = 256 Hz (Scientific tuning)
    Lowest note on piano A=27.5 Hz
    Highest note on piano C= 4186.009
  • Golden ears: can hear above 20kHz, dogs can hear 44kHz, cats up to 80kHz.
  • The maximum frequency that can be heard gets lower with age and hearing damage.
  • We hear best in the range of 200Hz to 4000Hz, close to the range of human's voice.
  • Video has two spatial dimensions: width and height. Audio has one: loudness. Audio is stored as a series of measurements of loudness. In analog world, when sound is transmitted via speaker cables, those changes are changes in the voltage on the wire. [the last 3 items were added at 12/28/2009]

Storage - Stereo audio, 16 bits, 44k Hz sampling rate, PCM encoding in 3 minutes:
3 minutes x 60 sec/min x 44000 samples/sec x 16 bits/sample x 2 channel / (8 bits/byte) = 32MB

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