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Sound pressure


 
 

Sound pressure is the local pressurePressure

Pressure is the force per unit area applied on a surface in a direction perpendicular to that surface....
 deviation from the ambient (average, or equilibrium) pressure caused by a soundSound

Sound is a disturbance of mechanical energy that propagates through matter as a wave....
 waveWave

A wave is a disturbance that propagates through space or spacetime, often transferring energy....
. Sound pressure can be measured using a microphoneMicrophone

A microphone, sometimes referred to as a mike or mic , is an acoustic to electric transducer that converts sound...
 in air and a hydrophoneHydrophone

A hydrophone is a sound-to-electricity transducer for use in water or other liquids, analogous to a microphone for air....
 in water. The SI unit for sound pressure is the pascalPascal (unit)

The pascal is the SI derived unit of pressure or stress ....
 (symbol: Pa). The instantaneous sound pressure is the deviation from the local ambient pressure p0 caused by a soundFacts About Sound

Sound is a disturbance of mechanical energy that propagates through matter as a wave....
 wave at a given location and given instant in time. The effective sound pressure is the root mean squareRoot mean square

In mathematics, root mean square , also known as the quadratic mean, is a statistical measure of the magnitude of a va...
 of the instantaneous sound pressure over a given interval of time (or space). In a sound wave, the complementary variable to sound pressure is the acoustic particle velocityParticle velocity

Particle velocity is the velocity v of a particle in a medium as it transmits a wave....
. For small amplitudes, sound pressure and particle velocity are linearly related and their ratio is the acoustic impedanceAcoustic impedance

The acoustic impedance Z is the ratio of sound pressure p to particle velocity v in a medium or acoustic component....
. The acoustic impedance depends on both the characteristics of the wave and the mediumTransmission medium

A transmission medium is any material substance which can propagate waves or energy....
. The local instantaneous sound intensitySound intensity

The sound intensity, I, is defined as the sound power Pac per unit area A....
 is the product of the sound pressure and the acoustic particle velocity and is, therefore, a vector quantity.

The sound pressure deviation p is

where
F = force,
A = area.


The entire pressure ptotal is

where
p0 = local ambient pressure,
p = sound pressure deviation.

Examples of sound pressure and sound pressure levels

Sound pressure in air:
Source of sound Sound pressure Sound pressure level
  pascalPascal (unit)

The pascal is the SI derived unit of pressure or stress ....
 
dBDecibel

The decibel is a measure of the ratio between two quantities, and is used in a wide variety of measurements in acoustics, p...
 re 20 µPa
Theoretical limit for undistorted sound at
1 atmosphereAtmosphere (unit)

Standard atmosphere is a unit of pressure....
 environmental pressurePressure

Pressure is the force per unit area applied on a surface in a direction perpendicular to that surface....
 
101,325 Pa 191 dB
KrakatoaKrakatoa Overview

Krakatoa is a volcanic island in the Sunda Strait between Java and Sumatra in Indonesia....
 explosion at 100 miles (160 km) in air
20,000 Pa 180 dB
Simple open-ended thermoacoustic device  12,000 Pa 176 dB
M1 GarandM1 Garand

The M1 Garand was the first semi-automatic rifle in the world to be generally issued to infantry....
 being fired at 1 m
5,000 Pa 168 dB
Jet engineJet engine

A jet engine is an engine that discharges a fast moving jet of fluid to generate thrust in accordance with Newton's third la...
 at 30 mMetre

The metre, or meter , is a measure of length....
 
630 Pa 150 dB
RifleRifle

A rifle is a firearm with a stock and a barrel that has a spiral groove or grooves cut into its interior....
 being fired at 1 m
200 Pa 140 dB
Threshold of painThreshold of pain

In hearing, the threshold of pain is the sound pressure or sound pressure level beyond which sound becomes unbearable for a ...
 
100 Pa 130 dB
126 dB
Hearing damage (due to short-term exposure) 20 Pa approx. 120 dB
JetJet Overview

JET, Jet or Jets all have a wide variety of meanings:...
 at 100 m
6 – 200 Pa 110 – 140 dB
Jack hammerJack Hammer

Jack Hammer may mean* Jackhammer, a pneumatic drill construction tool...
 at 1 m
2 Pa approx. 100 dB
Hearing damage (due to long-term exposure) 6×10-1 Pa approx. 85 dB
Major road at 10 m 2×10-1 – 6×10-1 Pa 80 – 90 dB
Passenger carAutomobile

An automobile is a wheeled passenger vehicle that carries its own motor....
 at 10 m
2×10-2 – 2×10-1 Pa 60 – 80 dB
TV (set at home level) at 1 m 2×10-2 Pa approx. 60 dB
Normal talking at 1 m 2×10-3 – 2×10-2 Pa 40 – 60 dB
Very calm room 2×10-4 – 6×10-4 Pa 20 – 30 dB
Leaves rustling, calm breathing 6×10-5 Pa 10 dB
Auditory threshold at 2 kHz 2×10-5 Pa 0 dB


Sound pressure in water:

Source of sound Sound pressure Sound pressure level
  pascalPascal (unit)

The pascal is the SI derived unit of pressure or stress ....
 
dBDecibel

The decibel is a measure of the ratio between two quantities, and is used in a wide variety of measurements in acoustics, p...
 re 1 µPa
Auditory threshold of a diver at 1 kHz 2.2 · 10-3 Pa 67 dB


The formula for the sum of the sound pressure levels of n incoherent radiating sources is

From the formula of the sound pressure level we find

This inserted in the formula for the sound pressure level to calculate the sum level shows

Beyond 191 dB

As sound pressure levels approach 191 dB in air at sea level, their waveforms become distortedDistortion

A distortion is the alteration of the original shape of an object, image, sound, waveform or other form of information or r...
; the exact level at which this happens varies with the barometric pressure. Sound waves are made up of rarefactionRarefaction

Rarefaction is the reduction of a medium's density, or the opposite of compression....
 and compressionPhysical compression

Physical compression is the result of the subjection of a material to compressive stress, resulting in reduction of volume....
 cycles, but when the compression half of the wave cycle is double atmospheric pressure, the rarefaction half of the cycle approaches a perfect vacuum (no further air molecules to remove). At this point, the only possible increase in sound level that could be achieved is on the compression side of the waveform. (The rarefaction half of a sine wave would be clippedClipping (audio)

Clipping is a form of distortion that occurs when an amplifier is overdriven, which happens when it attempts to increase vol...
 at any level above about 191 dB.) Any wave approaching these intensities is no longer considered sound, but a shock waveShock wave

A shock wave is a type of propagating disturbance....
. Examples of such an occurrence are large-scale manned rocket launchRocket launch

Rockets can be launched from the following:...
es, sonic boomSonic boom

A sonic boom is the audible component of a shock wave in air....
s, munitions explosionExplosion Summary

An explosion is a sudden increase in volume and release of energy in a violent manner, usually with the generation of high t...
s, thunderThunder

Thunder is, even today, not completely understood by modern science....
, earthquakeEarthquake

An earthquake is a phenomenon that results from and is powered by the sudden release of stored energy that radiates seismic ...
s and volcanic explosions.

See also

  • DecibelDecibel Summary

    The decibel is a measure of the ratio between two quantities, and is used in a wide variety of measurements in acoustics, p...
    , especially the Acoustics sectionDecibel

    The decibel is a measure of the ratio between two quantities, and is used in a wide variety of measurements in acoustics, p...
  • SoneFacts About Sone

    The sone is a unit of perceived loudness N after a proposal of S....
  • LoudnessLoudness

    Loudness is the quality of a sound that is the primary psychological correlate of physical intensity....
  • Weber-Fechner law (The case of Sound)
  • Stevens' power lawStevens' power law Overview

    Stevens' power law is a proposed relationship between the magnitude of a physical stimulus and its perceived intensity or st...
  • Sound power levelFacts About Sound power level

    Sound power level or acoustic power level is a logarithmic measure of the sound power in comparison to a specified ref...
  • Sound level meterFacts About Sound level meter

    A sound level meter is used to measure sound levels, usually in terms of subjective loudness....
  • AmplitudeAmplitude

    Amplitude is a nonnegative scalar measure of a wave's magnitude of oscillation, that is, magnitude of the maximum disturbanc...
  • AcousticsAcoustics

    Acoustics is a branch of physics that studies sound, namely mechanical waves in gases, liquids, and solids....


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