The Nature of Sound

Summary

• Sound is a mechanical, longitudinal wave.
  • As a mechanical wave, sound requires a medium.
    • Sound cannot propagate through a vacuum.
    • There is no sound in outer space.
  • As a longitudinal wave, sound is a rapid variation in pressure that propagates.
    • Regions of above normal pressure (regions under compression) are called compressions or condensations.
    • Regions of below normal pressure (regions under tension) are called rarefactions or dilations.
• Sound is produced by small and rapid pressure changes.
  • Vibrating objects produce periodic sound waves.
    • Sources include human vocal cords, musical instruments (strings, brass, reeds, drums, bells), speakers, buzzers, motors, etc.
  • Implosive or explosive pressure changes produces sound pulses.
    • Sources include plosive consonants (p, t, k), finger snapping, crunchy foods, electric sparks, thunder, whip cracking, gunshots, etc.
  • Vortex shedding can result in periodic sound waves.
    • Sources include fricative consonants (f, θ, s, ʃ), whistling, musical instruments (flutes, recorders, organs), etc.
• The speed of sound depends upon the medium and its state.
  • Sound usually travels fast in gases, faster in liquids, and fastest in solids.
  • The speed of sound in air increases with temperature.
    • There are several formulas for calculating the speed of sound in air as a function of temperature.
    • The speed of sound in air at 0 °C is 331 m/s
    • The speed of sound in air at 20 °C is 343 m/s
  • The speed of sound in air is largely independent of amplitude and frequency.
• The amplitude of a sound wave corresponds to its intensity or loudness.
  • The intensity of a sound is…
    • a measure of its power density
    • usually measured on a logarithmic scale
    • discussed in more detail in another section of this book
  • The loudness of a sound is its intensity as perceived by the human ear.
  • The volume knob on a television, radio, etc. should really be given a different name.
• The frequency of a sound wave corresponds to its pitch.
  • The upper frequency limit for human hearing is around 18,000 to 20,000 Hz.
    • Frequencies above the range of human hearing are ultrasonic.
  • The lower frequency limit for human hearing is around 18 to 20 Hz.
    • Frequencies below the range of human hearing are infrasonic.
  • The frequency of a sound wave does not change as the sound wave propagates.
• Wavelength is inversely proportional to frequency (λ ∝ 1/f).
  • Large objects generally produce long wavelength, low frequency sounds.
  • Small objects generally produce short wavelength, high frequency sounds.
• The ability of an animal or electronic sensor to identify the location or direction of origin of a sound is known as sound localization.
  • Sound localization requires two or more…
    • sense organs (ears or antennae) or
    • electromechanical detectors (microphones)
    devoted to hearing…
    • in different locations (left and right sides of the head, for example) or
    • with different orientations (facing to the left or to the right).
  • All methods of sound localization rely on the difference in some characteristic as perceived or measured by the two organs or detectors.
    • interaural level difference — loudness, intensity, or amplitude
    • interaural time differences — time of arrival
    • interaural phase difference — phase differences
• A reflected sound wave is known as an echo.
  • Echoes can be used to determine the distance to a reflecting surface.

    2∆s = v_sound∆t

    Where…

    ∆s =	distance from the observer to the reflecting surface (note that this value is doubled since the sound has to go out and come back),
    vsound =	speed of sound in the intervening medium, and
    ∆t =	time between when the pulse was transmitted and when the echo was received.

  • This method has applications in…
    • animal echolocation
    • sonar (an acronym for sound navigation and ranging)
    • medical ultrasonography (The images generated are called sonograms.)