Sound intensity, also known as acoustic intensity, is defined as the power carried by sound waves per unit area in a direction perpendicular to that area. The SI unit of intensity, which includes sound intensity, is the watt per square meter (W/m2). One application is the noise measurement of sound intensity in the air at a listener's location as a sound energy quantity.
Sound intensity is not the same physical quantity as sound pressure. Human hearing is sensitive to sound pressure which is related to sound intensity. In consumer audio electronics, the level differences are called "intensity" differences, but sound intensity is a specifically defined quantity and cannot be sensed by a simple microphone.
The commonly used reference sound intensity in air is
being approximately the lowest sound intensity hearable by an undamaged human ear under room conditions.
The proper notations for sound intensity level using this reference are LI /(1 pW/m2) or LI (re 1 pW/m2), but the notations dB SIL, dB(SIL), dBSIL, or dBSIL are very common, even if they are not accepted by the SI.
The reference sound intensity I0 is defined such that a progressive plane wave has the same value of sound intensity level (SIL) and sound pressure level (SPL), since
The equality of SIL and SPL requires that
where p0 = 20 μPa is the reference sound pressure.
In air at ambient temperature, z0 = 410 Pa·s/m, hence the reference value I0 = 1 pW/m2.
In an anechoic chamber which approximates a free field (no reflection) with a single source, measurements in the far field in SPL can be considered to be equal to measurements in SIL. This fact is exploited to measure sound power in anechoic conditions.
Sound intensity is defined as the time averaged product of sound pressure and acoustic particle velocity. Both quantities can be directly measured by using a sound intensity p-u probe comprising a microphone and a particle velocity sensor, or estimated indirectly by using a p-p probe that approximates the particle velocity by integrating the pressure gradient between two closely spaced microphones.
Pressure-based measurement methods are widely used in anechoic conditions for noise quantification purposes. The bias error introduced by a p-p probe can be approximated by
where is the “true” intensity (unaffected by calibration errors), is the biased estimate obtained using a p-p probe, is the root-mean-squared value of the sound pressure, is the wave number, is the density of air, is the speed of sound and is the spacing between the two microphones. This expression shows that phase calibration errors are inversely proportional to frequency and microphone spacing and directly proportional to the ratio of the mean square sound pressure to the sound intensity. If the pressure-to-intensity ratio is large then even a small phase mismatch will lead to significant bias errors. In practice, sound intensity measurements cannot be performed accurately when the pressure-intensity index is high, which limits the use of p-p intensity probes in environments with high levels of background noise or reflections.
On the other hand, the bias error introduced by a p-u probe can be approximated by
where is the biased estimate obtained using a p-u probe, and are the Fourier transform of sound pressure and particle velocity, is the reactive intensity and is the p-u phase mismatch introduced by calibration errors. Therefore, the phase calibration is critical when measurements are carried out under near field conditions, but not so relevant if the measurements are performed out in the far field. The “reactivity” (the ratio of the reactive to the active intensity) indicates whether this source of error is of concern or not. Compared to pressure-based probes, p-u intensity probes are unaffected by the pressure-to-intensity index, enabling the estimation of propagating acoustic energy in unfavorable testing environments provided that the distance to the sound source is sufficient.
^"Letter symbols to be used in electrical technology – Part 3: Logarithmic and related quantities, and their units", IEC 60027-3 Ed. 3.0, International Electrotechnical Commission, 19 July 2002.
^Ross Roeser, Michael Valente, Audiology: Diagnosis (Thieme 2007), p. 240.
^Thompson, A. and Taylor, B. N. sec 8.7, "Logarithmic quantities and units: level, neper, bel", Guide for the Use of the International System of Units (SI) 2008 Edition, NIST Special Publication 811, 2nd printing (November 2008), SP811 PDF
^Sound Power Measurements, Hewlett Packard Application Note 1230, 1992.
^Fahy, Frank (2017). Sound Intensity. CRC Press. ISBN 978-1138474192. OCLC 1008875245.
^Jacobsen, Finn (2013-07-29). Fundamentals of general linear acoustics. ISBN 9781118346419. OCLC 857650768.
^ abcJacobsen, Finn; de Bree, Hans-Elias (2005-09-01). "A comparison of two different sound intensity measurement principles" (PDF). The Journal of the Acoustical Society of America. 118 (3): 1510–1517. Bibcode:2005ASAJ..118.1510J. doi:10.1121/1.1984860. ISSN 0001-4966. S2CID 56449985.
Relationships of Acoustic Quantities Associated with a Plane Progressive Acoustic Sound Wave
Table of Sound Levels. Corresponding Sound Intensity and Sound Pressure