- Sound intensity
The sound intensity, "I", (acoustic intensity) is defined as the
sound power "Pac" per unit area "A". The usual context is the noise measurement of soundintensity in the air at a listener's location. For instantaneousacoustic pressure "p"inst("t") andparticle velocity "v"("t") the average acoustic intensity during time "T" is given by:I = frac{1}{T} int_{0}^{T}p_{inst}(t) cdot v(t),dtNotice that both v("t") and "I" are vectors, which means that both have a "direction" as well as a magnitude. The direction of the intensity is the average direction in which the energy is flowing.The
SI units of intensity are W/m2 (watt s persquare metre ).For a spherical sound source, the intensity in the radial direction as a function of distance "r" from the centre of the source is::I_r = frac{P_{ac{A} = frac{P_{ac{4 pi r^2} ,
Here "P"ac (upper case) is the sound power and "A" the surface area of a sphere of radius "r". Thus the sound intensity decreases with 1/"r"2 the distance from an acoustic point source, while the sound pressure decreases only with 1/"r" from the distance from an acoustic point source after the 1/"r"-distance law.:I sim {p^2} sim dfrac{1}{r^2} ,:dfrac{I_1}{I_2} = dfracr_2}^2}r_1}^2} ,:I_1 = I_{2} cdot {r_{2}^2} cdot dfrac{1}r_1}^2} ,
where "p" (lower case) is the
RMS sound pressure (acoustic pressure).Hence:p sim dfrac{1}{r} ,
The sound intensity "I" in W/m2 of a plane progressive wave is::I = frac{p^2}{Z} = Z cdot v^2 = xi^2 cdot omega^2 cdot Z = frac{a^2 cdot Z}{omega^2} = E cdot c = frac{P_{ac{A}
where:
Sound intensity level , "LI", is the magnitude of sound intensity, expressed in logarithmic units (decibel s).:L_I=10 log_{10} frac {I_o} (dB-SIL),where "Io" is the reference intensity, 10-12 W/m2Note 1: The term "
intensity " is used exclusively for the measurement of sound in watts per unit area.
To describe the strength of sound in terms other than strict intensity, one can use "magnitude" "", "amplitude ", or "level " instead.Sound intensity is not the same physical quantity as
sound pressure . Hearing is directly sensitive to sound pressure which is [http://www.sengpielaudio.com/calculator-soundlevel.htm related to sound intensity] . In stereo the level differences have been called "intensity" differences, but sound intensity is a specifically defined quantity and cannot be sensed by a simple microphone, nor would it be valuable in music recording if it could.See also
*
Intensity External links
* [http://ccrma.stanford.edu/~jos/pasp/Acoustic_Intensity.html Acoustic Intensity]
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