Editing Decibel (section)

=== Acoustics ===
The decibel is commonly used in [[acoustics]] as a unit of [[sound power level]] or [[sound pressure level]]. The reference pressure for sound in air is set at the typical threshold of perception of an average human and there are [[Sound pressure#Examples of sound pressure|common comparisons used to illustrate different levels of sound pressure]]. As sound pressure is a root-power quantity, the appropriate version of the unit definition is used:
: <math>
L_p = 20 \log_{10}\!\left(\frac{p_{\text{rms}}}{p_{\text{ref}}}\right)\,\text{dB},
</math>
where ''p''<sub>rms</sub> is the [[root mean square]] of the measured sound pressure and ''p''<sub>ref</sub> is the standard reference sound pressure of 20 [[micropascal]]s in air or 1 micropascal in water.<ref>ISO 1683:2015</ref>

Use of the decibel in underwater acoustics leads to confusion, in part because of this difference in reference value.<ref>Chapman, D. M., & Ellis, D. D. (1998). Elusive decibel: Thoughts on sonars and marine mammals. Canadian Acoustics, 26(2), 29-31.</ref><ref>C. S. Clay (1999), Underwater sound transmission and SI units, J Acoust Soc Am 106, 3047</ref>

[[Sound intensity#Sound intensity level|Sound intensity]] is proportional to the square of sound pressure. Therefore, the sound intensity level can also be defined as:
: <math>
L_p = 10 \log_{10}\!\left(\frac{I}{I_{\text{ref}}}\right)\,\text{dB},
</math>

The human ear has a large [[dynamic range]] in sound reception. The ratio of the sound intensity that causes permanent damage during short exposure to that of the quietest sound that the ear can hear is equal to or greater than 1 trillion (10<sup>12</sup>).<ref>{{cite web |title=Loud Noise Can Cause Hearing Loss |url=https://www.cdc.gov/nceh/hearing_loss/what_noises_cause_hearing_loss.html |website=cdc.gov |date=7 October 2019 |publisher=Centers for Disease Control and Prevention |access-date=30 July 2020}}</ref> Such large measurement ranges are conveniently expressed in logarithmic scale: the base-10 logarithm of 10<sup>12</sup> is 12, which is expressed as a sound intensity level of 120&nbsp;dB re 1 pW/m<sup>2</sup>. The reference values of I and p in air have been chosen such that this corresponds approximately to a sound pressure level of 120&nbsp;dB re 20&nbsp;[[Pascal (unit)|μPa]].

Since the human ear is not equally sensitive to all sound frequencies, the acoustic power spectrum is modified by [[frequency weighting]] ([[A-weighting]] being the most common standard) to get the weighted acoustic power before converting to a sound level or noise level in decibels.<ref name=Pierre>{{citation |url= http://storeycountywindfarms.org/ref3_Impact_Sound_Pressure.pdf |archive-url=https://web.archive.org/web/20151222153918/http://storeycountywindfarms.org/ref3_Impact_Sound_Pressure.pdf |archive-date=2015-12-22 |url-status=live |author=Richard L. St. Pierre, Jr. and Daniel J. Maguire |title=The Impact of A-weighting Sound Pressure Level Measurements during the Evaluation of Noise Exposure |date=July 2004 |access-date=2011-09-13}}</ref>