Acoustics

• see also:
  • waves
  • sound
  • music theory
  • recording technology
  • computers & sound

Acoustics:

• the production, transmission of sound & their relation to our sense of hearing
• the ear is able to analyse the sound waves coming to it, distinguishing between two or more sounds when the sounds differ in one or more of the characteristics:
  • pitch:
    • primarily determined by frequency of the sound wave
    • see music theory
  • timbre:
    • primarily determined by the complexity of the wave
      • every body that vibrates with more than one frequency sends out a complex wave
      • the complexity is controlled by the number & relative intensity of the harmonics that are present
      • a "pure tone" with no overtones, may not be so pleasing as the "rich" tone of a violin or the human voice which contains 10 or more harmonics
      • any complex wave can be resolved into a number of simple waves
  • loudness:
    • determined by:
      • the rate at which energy is transmitted to the ear ie. its intensity
        • Weber-Fechner law for pure tones: loudness = k log (intensity / intensity at a reference level)
        • loudness is usually measured in decibels (dB) which is a logarithmic scale such that a difference of 1 bel equates to a ten-fold difference in audible intensity (1dB difference = 26% change in intensity)
          • quiet home, average living room = 40dB
          • ordinary conversation at 1m = 70dB
          • street traffic = 80dB
          • threshold of pain = 120dB
      • the efficiency of the middle ear conduction system:
        • pressure changes hitting the eardrum are magnified 30-60 times by the middle ear conduction system
      • the sensitivity of the inner ear to that frequency:
        • waves below the audible threshold are called infrasonic, whilst those above it are called ultrasonic
        • satisfactory fidelity of speech requires frequency range of 100-8000Hz
        • satisfactory fidelity of orchestral music requires frequency range of 40-14000Hz
        • if intensity becomes excessive (120dB), then the sensation becomes one of pain rather than hearing
        • the audible intensity range for the normal ear at its peak sensitivity frequency of 2000-4000Hz is from 10^-16 watt/cm² (0dB) to 10^-4 watt/cm² (ie. 120dB)
        • the smallest difference in intensity an ear can detect is ~1dB.
• musical tones:
  • the sensation we describe as musical tone is produced by a regular succession of compression-decompressions that come to the ear.
  • if vibrations from a source are regularly spaced, they will produce a musical tone with a fixed frequency.
  • vibrating sources that do not maintain a single frequency create unpitched sound
  • a noise may be defined as any undesired sound
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