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Sound pressure ''p'' (or acoustic pressure) is the measurement in Pascal s 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 unit is pascal (symbol: Pa) and that is equal to a force ''F'' of one newton (1 N) applied over an area ''A'' of one square metre (1 m2).

The amplitude of sound pressure from a point source decreases in the free field (direct field) proportional to the inverse of the distance ''r'' from that source. That is 1/''r'' and really not squared!

Sound Pressure Level is a Decibel scale based on a reference sound pressure of 20 µPa (micropascals) {Link without Title} , calculated in DB as:
:
L_p=20\, \log_{10}\left( rac{p_1}{p_0} ight)\mathrm{dB}


This is written "dBSPL".
:Reference sound pressure ''p''0 = 2 × 10-5 Pa = 20 µPa {Link without Title}

Sound pressure ''p'' in N/m2 or Pa is:
:
p = Zv = rac{I}{v} = \sqrt{IZ}


: ''Z'': Acoustic Impedance , Sound Impedance , or Characteristic Impedance ; Pa·s/m
: ''v'': Particle Velocity ; m/s
: ''I'': Acoustic Intensity or Sound Intensity ; W/m2

Sound pressure ''p'' is connected to Particle Displacement (or particle amplitude) ξ m, by:
:
\xi = rac{v}{2 \pi f} = rac{v}{\omega} = rac{p}{Z \omega} = rac{p}{ 2 \pi f Z}


Sound pressure ''p'':
:
p = ho c \omega \xi = Z \omega \xi = { 2 \pi f \xi Z} = rac{a Z}{\omega} = c \sqrt{ ho E} = \sqrt{ rac{P_{ac} Z}{A}}

normally in units of pascals.

where:

The distance law for the sound pressure ''p'' is inverse-proportional to the distance ''r'' of a punctual sound source. This is not like Sound Intensity which follows the Inverse-square Law .
:
p \propto rac{1}{r}
(proportional)
:
rac{p_1} {p_2} = rac{r_2}{r_1}

:
p_1 = p_{2} \cdot r_{2} \cdot rac{1}{r_1}


''Note: The often used term "intensity of sound pressure" is nonsensical. 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.''


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