
Sound pressure
June 03, 2005
Sound pressure p (or acoustic pressure) is the measurement in pascals of the root mean square (RMS) pressure deviation (from atmospheric pressure) caused by a sound wave passing through a fixed point. The symbol for pressure is the lower case p. (The upper case P is the symbol for power. This is often misprinted.)
The amplitude of sound pressure from a point source decreases in the free field (direct field) proportional to the inverse of the distance from that source. Sound pressure level is a decibel scale based on a reference sound pressure of 20 µPa (micropascals), calculated in dB as:
This is written "dB (SPL)".
p_{0}: Reference sound pressure of 2 × 10^{5} Pa = 20 µPa
Sound pressure p in N/m^{2} or Pa is:
Z: acoustic impedance, sound impedance, or characteristic impedance, in Pa·s/m
v: particle velocity in m/s
J: acoustic intensity or sound intensity, in W/m^{2}
Sound pressure p is connected to particle displacement (or particle amplitude) ξ, in m, by:
Sound pressure p:
normally in units of N/m^{2} = Pa.
where:
p: sound pressure, in N/m^{2} = Pa
f: frequency, in Hz
ρ: density of air, in kg/m^{3}
c: speed of sound, in m/s
v: sound velocity, in m/s
ω: angular frequency = 2π·f
ξ: particle displacement (particle amplitude), in m
Z: acoustic impedance (characteristic impedance) = c · ρ, in Pa·s/m
a: particle acceleration, in m/s^{2}
E or w sound energy density, in J/m
P_{ac} sound power or acoustic power, in W
A area, in m^{2}
Note: The often used term "intensity of sound pressure" is not correct. Use "magnitude", "strength", "amplitude", or "level" instead. "Sound intensity" is sound power per unit area, while "pressure" is a measure of force per unit area. Intensity is not equivalent to pressure.
Sound measurements

Sound power

Sound power level (SWL)

Sound pressure

Sound pressure level (SPL)

Sound intensity

Sound intensity level (SIL)

Particle velocity

Particle velocity level (sometimes called Sound velocity level) (SVL)

Sound energy flux

Acoustic impedance

Speed of sound
[This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article: Sound Pressure.]
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