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The sound intensity, I, (acoustic intensity) is defined as the sound power P_{ac} per unit area A. The usual context is the noise measurement of sound intensity in the air at a listener's location. For instantaneous acoustic pressure p_{inst}(t) and particle velocity v(t) the average acoustic intensity during time T is given by

_{o} is the reference intensity, 10^{-12} W/m^{2}## See also

## External links

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Notice 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/m^{2} (watts per square 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:

- $$

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.

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where p (lower case) is the RMS sound pressure (acoustic pressure).

Hence

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The sound intensity I in W/m^{2} of a plane progressive wave is:

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where:

Symbol | Units | Meaning |
---|---|---|

p | pascals | RMS sound pressure |

f | hertz | frequency |

ξ | m, metres | particle displacement |

c | m/s | speed of sound |

v | m/s | particle velocity |

ω = 2πf | radians/s | angular frequency |

ρ | kg/m^{3}
| density of air |

Z = c · ρ | N·s/m³ | characteristic acoustic impedance |

a | m/s² | particle acceleration |

I | W/m² | sound intensity |

E | W·s/m³ | sound energy density |

P_{ac}
| W, watts | sound power or acoustic power |

A | m² | area |

Sound intensity level, L_{I}, is the magnitude of sound intensity, expressed in logarithmic units (decibels).

- $L\_I=10\; log\_\{10\}\; frac$
>{I_o} (dB-SIL),

Note 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" "strength", "amplitude", or "level" instead.

Sound intensity is not the same physical quantity as sound pressure. Hearing is directly sensitive to sound pressure which is 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.

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Last updated on Wednesday October 08, 2008 at 00:11:27 PDT (GMT -0700)

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This article is licensed under the GNU Free Documentation License.

Last updated on Wednesday October 08, 2008 at 00:11:27 PDT (GMT -0700)

View this article at Wikipedia.org - Edit this article at Wikipedia.org - Donate to the Wikimedia Foundation

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