sound - Speed of sound, Sound pressure, Sound pressure level, Measurement of sound

A wave motion comprising a sequence of pressure pulses passing through some medium, typically air. The source of sound is a mechanical oscillator in the medium, such as a vibrating guitar string or a loudspeaker cone in air. It can be detected aurally, or using microphones or transducers. The speed of sound in air is 332 m/s; in fresh water at 20°C, it is 1482 m/s.

Humans and many animals use their ears to hear sound, but loud sounds and low-frequency sounds can be perceived by other parts of the body through the sense of touch. The quietest sounds that humans can hear have an amplitude of approximately 20 µPa (micropascals) or a sound pressure level (SPL) of 0 dB re 20 µPa (often incorrectly abbreviated as 0 dB SPL).

Speed of sound

The speed at which sound travels depends on the medium through which the waves are passing, and is often quoted as a fundamental property of the material.

Sound pressure

Sound pressure is the pressure deviation from the local ambient pressure caused by a sound wave. The instantaneous sound pressure is the deviation from the local ambient pressure caused by a sound wave at a given location and given instant in time. The effective sound pressure is the root mean square of the instantaneous sound pressure over a given interval of time. The local instantaneous sound intensity is the product of the sound pressure and the acoustic particle velocity and is, therefore, a vector quantity.

Sound pressure level

As the human ear can detect sounds with a very wide range of amplitudes, sound pressure is often measured as a level on a logarithmic decibel scale.

The sound pressure level (SPL) or Lp is defined as

where p is the root-mean-square sound pressure and p0 is a reference sound pressure. (When using sound pressure levels, it may be important to quote the reference sound pressure used.) Commonly used reference sound pressures, defined in the standard ANSI S1.1-1994, are 20 µPa in air and 1 µPa in water.

Examples of sound pressure and sound pressure levels

Source of sound	sound pressure	sound pressure level
	pascal	dB re 20 µPa
threshold of pain	100	134
hearing damage during short-term effect	20	approx. 120
jet, 100 m distant	6 - 200	110 - 140
jack hammer, 1 m distant / discotheque	2	approx. 90
major road, 10 m distant	2×10	80 - 90
passenger car, 10 m distant	2×10	60 - 80
TV set at home level, 1 m distant	2×10−2	ca. 60
normal talking, 1 m distant	2×10	40 - 60
very calm room	2×10	20 - 30
leaves noise, calm breathing	6×10−5	10
auditory threshold at 2 kHz	2×10−5	0

Measurement of sound

Decibel, sone, mel, phon sound pressure level Particle velocity, acoustic velocity, sound velocity Particle displacement, particle amplitude, particle acceleration Sound power, acoustic power, sound power level Sound intensity, acoustic intensity, sound intensity level Acoustic impedance, sound impedance, characteristic impedance Speed of sound, amplitude Sound energy flux See also Template:Sound measurements