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.
- Implosive or explosive pressure changes produces sound pulses.
- Vortex shedding is another source of periodic sound waves.
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 a medium 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 (typically ears or antennae) or
  - electromechanical detectors (typically 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.

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
	Δt

Where …

s	is the distance from the observer to the reflecting surface (note that this value is doubled since the sound has to go out and come back),
v_sound	is the speed of sound in the intervening medium, and
Δt	is the 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).

The Physics Hypertextbook

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