View source for Sound pressure

{{Short description|Local pressure deviation caused by a sound wave}}
{{Distinguish|Sound energy density}}
{{Sound measurements}}

'''Sound pressure''' or '''acoustic pressure''' is the local [[pressure]] deviation from the ambient (average or equilibrium) [[atmospheric pressure]], caused by a [[sound wave]]. In air, sound pressure can be measured using a [[microphone]], and in water with a [[hydrophone]]. The [[International System of Units|SI unit]] of sound pressure is the [[Pascal (unit)|pascal]] (Pa).<ref name="engineering toolbox">{{cite web|title=Sound Pressure is the force of sound on a surface area perpendicular to the direction of the sound|url=http://www.engineeringtoolbox.com/sound-pressure-d_711.html|access-date=22 April 2015}}</ref>

==Mathematical definition==
[[File:Sound pressure diagram.svg|thumb|250px|Sound pressure diagram: {{ordered list |silence; |audible sound; |atmospheric pressure; |sound pressure}}]]

A sound wave in a [[transmission medium]] causes a deviation (sound pressure, a ''dynamic'' pressure) in the local ambient pressure, a ''static'' pressure.

Sound pressure, denoted ''p'', is defined by
<math display="block">p_\text{total} = p_\text{stat} + p,</math>
where
* ''p''<sub>total</sub> is the total pressure,
* ''p''<sub>stat</sub> is the static pressure.

==Sound measurements==

===Sound intensity===
{{main article|Sound intensity}}
In a sound wave, the complementary variable to sound pressure is the [[particle velocity]]. Together, they determine the sound intensity of the wave.

''Sound intensity'', denoted '''I''' and measured in [[Watt|W]]·[[Metre|m]]<sup>−2</sup> in SI units, is defined by
<math display="block">\mathbf I = p \mathbf v,</math>
where
* ''p'' is the sound pressure,
* '''v''' is the particle velocity.

===Acoustic impedance===
{{main article|Acoustic impedance}}
''Acoustic impedance'', denoted ''Z'' and measured in Pa·m<sup>−3</sup>·s in SI units, is defined by<ref name="Wolfe">{{cite web |last=Wolfe |first=J. |title=What is acoustic impedance and why is it important? |url=http://www.phys.unsw.edu.au/jw/z.html |publisher=University of New South Wales, Dept. of Physics, Music Acoustics |access-date=1 January 2014}}</ref>
<math display="block">Z(s) = \frac{\hat{p}(s)}{\hat{Q}(s)},</math>
where
* <math>\hat{p}(s)</math> is the [[Laplace transform]] of sound pressure{{citation needed|date=April 2015}},
* <math>\hat{Q}(s)</math> is the Laplace transform of sound volume flow rate.

''Specific acoustic impedance'', denoted ''z'' and measured in Pa·m<sup>−1</sup>·s in SI units, is defined by<ref name="Wolfe"/>
<math display="block">z(s) = \frac{\hat{p}(s)}{\hat{v}(s)},</math>
where
* <math>\hat{p}(s)</math> is the Laplace transform of sound pressure,
* <math>\hat{v}(s)</math> is the Laplace transform of particle velocity.

===Particle displacement===
{{main article|Particle displacement}}
The ''particle displacement'' of a ''progressive [[sine wave]]'' is given by
<math display="block">\delta(\mathbf{r}, t) = \delta_\text{m} \cos(\mathbf{k} \cdot \mathbf{r} - \omega t + \varphi_{\delta, 0}),</math>
where
* <math>\delta_\text{m}</math> is the [[amplitude]] of the particle displacement,
* <math>\varphi_{\delta, 0}</math> is the [[phase shift]] of the particle displacement,
* '''k''' is the [[angular wavevector]],
* ''ω'' is the [[angular frequency]].

It follows that the particle velocity and the sound pressure along the direction of propagation of the sound wave ''x'' are given by
<math display="block">v(\mathbf{r}, t) = \frac{\partial \delta}{\partial t} (\mathbf{r}, t) = \omega \delta_\text{m} \cos\left(\mathbf{k} \cdot \mathbf{r} - \omega t + \varphi_{\delta, 0} + \frac{\pi}{2}\right) = v_\text{m} \cos(\mathbf{k} \cdot \mathbf{r} - \omega t + \varphi_{v, 0}),</math>
<math display="block">p(\mathbf{r}, t) = -\rho c^2 \frac{\partial \delta}{\partial x} (\mathbf{r}, t) = \rho c^2 k_x \delta_\text{m} \cos\left(\mathbf{k} \cdot \mathbf{r} - \omega t + \varphi_{\delta, 0} + \frac{\pi}{2}\right) = p_\text{m} \cos(\mathbf{k} \cdot \mathbf{r} - \omega t + \varphi_{p, 0}),</math>
where
* ''v''<sub>m</sub> is the amplitude of the particle velocity,
* <math>\varphi_{v, 0}</math> is the phase shift of the particle velocity,
* ''p''<sub>m</sub> is the amplitude of the acoustic pressure,
* <math>\varphi_{p, 0}</math> is the phase shift of the acoustic pressure.

Taking the Laplace transforms of ''v'' and ''p'' with respect to time yields
<math display="block">\hat{v}(\mathbf{r}, s) = v_\text{m} \frac{s \cos \varphi_{v,0} - \omega \sin \varphi_{v,0}}{s^2 + \omega^2},</math>
<math display="block">\hat{p}(\mathbf{r}, s) = p_\text{m} \frac{s \cos \varphi_{p,0} - \omega \sin \varphi_{p,0}}{s^2 + \omega^2}.</math>

Since <math>\varphi_{v,0} = \varphi_{p,0}</math>, the amplitude of the specific acoustic impedance is given by
<math display="block">z_\text{m}(\mathbf{r}, s) = |z(\mathbf{r}, s)| = \left|\frac{\hat{p}(\mathbf{r}, s)}{\hat{v}(\mathbf{r}, s)}\right| = \frac{p_\text{m}}{v_\text{m}} = \frac{\rho c^2 k_x}{\omega}.</math>

Consequently, the amplitude of the particle displacement is related to that of the acoustic velocity and the sound pressure by
<math display="block">\delta_\text{m} = \frac{v_\text{m}}{\omega},</math>
<math display="block">\delta_\text{m} = \frac{p_\text{m}}{\omega  z_\text{m}(\mathbf{r}, s)}.</math>

==Inverse-proportional law==
{{Further|Inverse-square law}}
When measuring the sound pressure created by a sound source, it is important to measure the distance from the object as well, since the sound pressure of a ''spherical'' sound wave decreases as 1/''r'' from the centre of the sphere (and not as 1/''r''<sup>2</sup>, like the sound intensity):<ref>{{cite book |last=Longhurst |first=R. S. |title=Geometrical and Physical Optics |url=https://archive.org/details/geometricalphysi0000long |url-access=registration |year=1967 |publisher=Longmans |location=Norwich}}</ref>
<math display="block">p(r) \propto \frac{1}{r}.</math>

This relationship is an ''inverse-proportional law''.

If the sound pressure ''p''<sub>1</sub> is measured at a distance ''r''<sub>1</sub> from the centre of the sphere, the sound pressure ''p''<sub>2</sub> at another position ''r''<sub>2</sub> can be calculated:
<math display="block">p_2 = \frac{r_1}{r_2}\,p_1.</math>

The inverse-proportional law for sound pressure comes from the inverse-square law for sound intensity:
<math display="block">I(r) \propto \frac{1}{r^2}.</math>
Indeed,
<math display="block">I(r) = p(r) v(r) = p(r)\left[p * z^{-1}\right](r) \propto p^2(r),</math>
where
* <math>*</math> is the [[convolution]] operator,
* ''z''<sup>−1</sup> is the convolution inverse of the [[specific acoustic impedance]],
hence the inverse-proportional law:
<math display="block">p(r) \propto \frac{1}{r}.</math>

The sound pressure may vary in direction from the centre of the sphere as well, so measurements at different angles may be necessary, depending on the situation. An obvious example of a sound source whose spherical sound wave varies in level in different directions is a [[bullhorn]].{{citation needed|date=October 2012}}

==Sound pressure level==
{{Other uses|Sound level (disambiguation){{!}}Sound level}}
'''Sound pressure level''' ('''SPL''') or '''acoustic pressure level''' ('''APL''') is a [[level (logarithmic quantity)|logarithmic measure]] of the effective pressure of a sound relative to a reference value.

Sound pressure level, denoted ''L''<sub>''p''</sub> and measured in [[Decibel|dB]],<ref name=IEC60027-3>[http://webstore.iec.ch/webstore/webstore.nsf/artnum/028981 "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.</ref> is defined by:<ref>{{cite book |vauthors=Attenborough K, Postema M|title=A pocket-sized introduction to acoustics |date=2008 |publisher=The University of Hull|location=Kingston upon Hull |url=https://hal.archives-ouvertes.fr/hal-03188302/document| isbn=978-90-812588-2-1 |doi=10.5281/zenodo.7504060}}</ref>
<math display="block">L_p = \ln\left(\frac{p}{p_0}\right) ~ \text{Np} = 2 \log_{10}\left(\frac{p}{p_0}\right)~\text{B} = 20 \log_{10}\left(\frac{p}{p_0}\right)~\text{dB},</math>
where
* ''p'' is the [[root mean square]] sound pressure,<ref>Bies, David A., and Hansen, Colin. (2003). ''Engineering Noise Control''.</ref>
* ''p''<sub>0</sub> is a '''reference sound pressure''',
* {{nowrap|1=1 Np}} is the [[neper]],
* {{nowrap|1=1 B = ({{sfrac|2}} ln 10) Np}} is the [[Decibel|bel]],
* {{nowrap|1=1 dB = ({{sfrac|20}} ln 10) Np}} is the [[decibel]].

{{anchor|Reference}}The commonly used reference sound pressure in air is<ref>Ross Roeser, Michael Valente, ''Audiology: Diagnosis'' (Thieme 2007), p. 240.</ref>
{{block indent | em = 1.5 | text = ''p''<sub>0</sub> = 20 μPa,}}
which is often considered as the [[threshold of human hearing]] (roughly the sound of a mosquito flying 3&nbsp;m away). The proper notations for sound pressure level using this reference are {{nobreak|''L''<sub>''p''/(20 μPa)</sub>}} or {{nobreak|''L''<sub>''p''</sub> (re 20 μPa)}}, but the suffix notations {{nobreak|dB SPL}}, {{nobreak|dB(SPL)}}, dBSPL, or dB<sub>SPL</sub> are very common, even if they are not accepted by the SI.<ref name=NIST2008>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 [http://physics.nist.gov/cuu/pdf/sp811.pdf PDF].</ref>

Most sound-level measurements will be made relative to this reference, meaning {{nobreak|1 Pa}} will equal an SPL of {{nobreak|94 dB}}. In other media, such as [[underwater acoustics|underwater]], a reference level of {{nobreak|1 μPa}} is used.<ref name=Morfey>{{cite book |last=Morfey |first=Christopher L. |title=Dictionary of Acoustics |year=2001 |publisher=Academic Press |location=San Diego |isbn=978-0125069403}}</ref> These references are defined in [[American National Standards Institute|ANSI]] [[ANSI/ASA S1.1-2013|S1.1-2013]].<ref>{{cite web |url=http://www.memtechacoustical.com/noise-terms-glossary |title=Noise Terms Glossary |access-date=2012-10-14}}</ref>

The main instrument for measuring sound levels in the environment is the [[sound level meter]]. Most sound level meters provide readings in A, C, and Z-weighted decibels and must meet international standards such as [[IEC 61672|IEC 61672-2013]].

===Examples===
The lower limit of audibility is defined as SPL of {{nobreak|0 dB}}, but the upper limit is not as clearly defined. While {{nobreak|1 [[atmosphere (unit)|atm]]}} ({{nobreak|194 dB peak}} or {{nobreak|191 dB SPL}})<ref name=":0">{{Cite book| last=Self| first=Douglas| url=https://books.google.com/books?id=L2PdDwAAQBAJ&dq=194+dB+SPL&pg=PP36 | title=Small Signal Audio Design| date=2020-04-17| publisher=CRC Press| isbn=978-1-000-05044-8| language=en| quote=this limit is reached when the rarefaction creates a vacuum, because you can’t have a lower pressure than that. This corresponds to about +194 dB SPL.}}</ref><ref name=":1">{{Cite book| last1=Guignard| first1=J. C.| url=https://books.google.com/books?id=LvRJAQAAIAAJ&q=191+dB+SPL| title=Aeromedical Aspects of Vibration and Noise| last2=King|first2=P. F.| author3=North Atlantic Treaty Organization Advisory Group for Aerospace Research and Development Aerospace Medical Panel| date=1972| publisher=North Atlantic Treaty Organization, Advisory Group for Aerospace Research and Development| language=en| quote=In air at an assumed atmospheric pressure of 1 bar (100,000 N/m<sup>2</sup>) this occurs theoretically at approximately 191 dB SPL (working with rms values}}</ref> is the largest pressure variation an undistorted sound wave can have in [[Earth's atmosphere]] (i. e., if the thermodynamic properties of the air are disregarded; in reality, the sound waves become progressively non-linear starting over 150&nbsp;dB), larger sound waves can be present in other [[atmosphere]]s or other media, such as underwater or through the Earth.<ref name="Audio1">{{cite book |last=Winer |first=Ethan |date=2013 |title=The Audio Expert |location=New York and London |publisher=Focal Press |chapter=1 |isbn=978-0-240-82100-9}}</ref>

[[File:Lindos1.svg|thumb|right|300px|[[Equal-loudness contour]], showing sound-pressure-vs-frequency at different perceived loudness levels]]

Ears detect changes in sound pressure. Human hearing does not have a flat [[spectral sensitivity]] ([[frequency response]]) relative to frequency versus [[amplitude]]. Humans do not perceive low- and high-frequency sounds as well as they perceive sounds between 3,000 and 4,000&nbsp;Hz, as shown in the [[equal-loudness contour]]. Because the frequency response of human hearing changes with amplitude, three weightings have been established for measuring sound pressure: A, B and C. 

In order to distinguish the different sound measures, a suffix is used: A-weighted sound pressure level is written either as dB<sub>A</sub> or L<sub>A</sub>. B-weighted sound pressure level is written either as dB<sub>B</sub> or L<sub>B</sub>, and C-weighted sound pressure level is written either as dB<sub>C</sub> or L<sub>C</sub>. Unweighted sound pressure level is called "linear sound pressure level" and is often written as dB<sub>L</sub> or just L. Some sound measuring instruments use the letter "Z" as an indication of linear SPL.<ref name="Audio1"/>

===Distance===
The distance of the measuring microphone from a sound source is often omitted when SPL measurements are quoted, making the data useless, due to the inherent effect of the [[#Inverse-proportional_law|inverse proportional law]]. In the case of ambient environmental measurements of "background" noise, distance need not be quoted, as no single source is present, but when measuring the noise level of a specific piece of equipment, the distance should always be stated. A distance of one [[metre]] (1&nbsp;m) from the source is a frequently used standard distance. Because of the effects of reflected noise within a closed room, the use of an [[anechoic chamber]] allows sound to be comparable to measurements made in a free field environment.<ref name="Audio1"/>

According to the inverse proportional law, when sound level ''L''<sub>''p''<sub>1</sub></sub> is measured at a distance ''r''<sub>1</sub>, the sound level ''L''<sub>''p''<sub>2</sub></sub> at the distance ''r''<sub>2</sub> is
<math display="block">L_{p_2} = L_{p_1} + 20 \log_{10}\left( \frac{r_1}{r_2} \right)~\text{dB}.</math>

===Multiple sources===
The formula for the sum of the sound pressure levels of ''n'' incoherent radiating sources is
<math display="block">L_\Sigma = 10 \log_{10}\left(\frac{p_1^2 + p_2^2 + \dots + p_n^2}{p_0^2}\right)~\text{dB} =
10 \log_{10}\left[\left(\frac{p_1}{p_0}\right)^2 + \left(\frac{p_2}{p_0}\right)^2 + \dots + \left(\frac{p_n}{p_0}\right)^2\right]~\text{dB}.</math>

Inserting the formulas
<math display="block">\left(\frac{p_i}{p_0}\right)^2 = 10^{\frac{L_i}{10~\text{dB}}},\quad i = 1, 2, \ldots, n</math>
in the formula for the sum of the sound pressure levels yields
<math display="block">L_\Sigma = 10 \log_{10} \left(10^{\frac{L_1}{10~\text{dB}}} + 10^{\frac{L_2}{10~\text{dB}}} + \dots + 10^{\frac{L_n}{10~\text{dB}}} \right)~\text{dB}.</math>

==Examples of sound pressure==
<!-- This section is linked from [[sound]] -->
{| class="wikitable sortable"
|+ Examples of sound pressure in air at [[Standard atmosphere (unit)|standard atmospheric pressure]]
|-
! rowspan="2" | Source of sound
! rowspan="2" | Distance
! colspan="2" | Sound pressure level{{efn|All values listed are the effective sound pressure unless otherwise stated.}}
|-
! ([[pascal (unit)|Pa]])
! ([[Decibel|dB{{sub|SPL}}]])
|-
| [[Shock wave]] (distorted sound waves > 1 [[Atmosphere (unit)|atm]]; waveform valleys are clipped at zero pressure)<ref name=":0" /><ref name=":1" />
|
| align="right" | >1.01×10<sup>5</sup>
| align="right" | >191
|-
| Simple open-ended [[thermoacoustics|thermoacoustic]] device<ref>{{Cite journal |title = Performance of a Thermoacoustic Sound Wave Generator driven with Waste Heat of Automobile Gasoline Engine |url = http://ci.nii.ac.jp/naid/130004080803/ |journal = Transactions of the Japan Society of Mechanical Engineers Series B |date = 2004-01-01 |issn = 0387-5016 |pages = 292–299 |volume = 70 |issue = 689 |doi = 10.1299/kikaib.70.292 |first1 = Masayasu |last1 = HATAZAWA |first2 = Hiroshi |last2 = SUGITA |first3 = Takahiro |last3 = OGAWA |first4 = Yoshitoki |last4 = SEO |doi-access = free}}</ref>
| {{Clarify|date=January 2016}}
| align="right" | 1.26×10<sup>4</sup>
| align="right" | 176
|-
|[[1883 eruption of Krakatoa]]<ref>{{Cite web|title=Krakatoa Eruption - The Loudest Sound|url=https://www.bksv.com/en/knowledge/blog/perspectives/krakatoa-eruption-sound|access-date=2021-03-24|website=Brüel & Kjær|quote=160 km (99 miles) away from the source, registered a sound pressure level spike of more than 2½ inches of mercury (8.5 kPa), equivalent to 172 decibels.}}</ref><ref name="winchester">{{Cite book|last=Winchester|first=Simon|title=Krakatoa: The Day the World Exploded, August 27, 1883|title-link=Krakatoa: The Day the World Exploded|publisher=Penguin/Viking|year=2003|isbn=978-0-670-91430-2|pages=218|author-link=Simon Winchester}}</ref>
|165 km
|
|172
|-
| [[.30-06 Springfield|.30-06 rifle]] being fired
| 1&nbsp;[[meter|m]] to<br>shooter's side
| align="right" | 7.09×10<sup>3</sup>
| align="right" | 171
|-
|Firecracker<ref>{{Cite journal|last1=Flamme|first1=GregoryA|last2=Liebe|first2=Kevin|last3=Wong|first3=Adam|date=2009|title=Estimates of the auditory risk from outdoor impulse noise I: Firecrackers|journal=Noise and Health|volume=11|issue=45|pages=223–230|doi=10.4103/1463-1741.56216|pmid=19805932|issn=1463-1741|doi-access=free}}</ref>
|0.5 m
|align="right" |7.09×10<sup>3</sup>
|align="right" |171
|-
| [[Stun grenade]]<ref>{{Cite web |url = https://www.cdc.gov/niosh/hhe/reports/pdfs/2013-0124-3208.pdf |title = NIOSH HHE Report No. 2013-0124-3208. Health hazard evaluation report: measurement of exposure to impulsive noise at indoor and outdoor firing ranges during tactical training exercises |author=Brueck S. E., Kardous C. A., Oza A., Murphy W. J |place=Cincinnati, OH |publisher=U.S. Department of Health and Human Services, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health |date = 2014 }}</ref>
| Ambient
| align="right" | 1.60×10<sup>3</sup><br>...8.00×10<sup>3</sup>
| align="right" | 158–172
|-
| {{convert|9|in|cm|adj=on}} party balloon inflated to rupture<ref name="balloonpoploudness">{{cite web |title=Did You Know How Loud Balloons Can Be? |url=http://www.canadianaudiologist.ca/issue/volume-3-issue-6-2016/column/science-matters/ |access-date=8 June 2018}}</ref>
| At ear
| align="right" | 4.92×10<sup>3</sup>
| align="right" | 168
|-
| {{convert|9|in|cm|adj=on}} diameter balloon crushed to rupture<ref name="balloonpoploudness"/>
| At ear
| align="right" | 1.79×10<sup>3</sup>
| align="right" | 159
|-
|{{convert|9|in|cm|adj=on}} party balloon inflated to rupture<ref name="balloonpoploudness" />
| 0.5 m
|align="right" |1.42×10<sup>3</sup>
|align="right" |157
|-
| {{convert|9|in|cm|adj=on}} diameter balloon popped with a pin<ref name="balloonpoploudness" />
| At ear
| align="right" | 1.13×10<sup>3</sup>
| align="right" | 155
|-
| LRAD 1000Xi [[Long Range Acoustic Device]]<ref name="lradproductsoverview">{{cite web |title=LRAD Corporation Product Overview for LRAD 1000Xi |url=http://www.lradx.com/site/content/view/2016/110/ |access-date=29 May 2014 |archive-date=16 March 2014 |archive-url=https://web.archive.org/web/20140316054039/http://www.lradx.com/site/content/view/2016/110/ |url-status=dead }}</ref>
| 1 m
| align="right" | 8.93×10<sup>2</sup>
| align="right" | 153
|-
| {{convert|9|in|cm|adj=on}} party balloon inflated to rupture<ref name="balloonpoploudness"/>
| 1 m
| align="right" | 731
| align="right" | 151
|-
| [[Jet engine]]<ref name="Audio1"/>
| 1 m
| align="right" | 632
| align="right" | 150
|-
| {{convert|9|in|cm|adj=on}} diameter balloon crushed to rupture<ref name="balloonpoploudness"/>
| 0.95 m
| align="right" | 448
| align="right" | 147
|-
| {{convert|9|in|cm|adj=on}} diameter balloon popped with a pin<ref name="balloonpoploudness"/>
| 1 m
| align="right" | 282.5
| align="right" | 143
|-
| Loudest [[human voice]]<ref name=Shure />
| 1 inch
| align="right" | 110
| align="right" | 135
|-
| [[Trumpet]]<ref>[http://www.soundonsound.com/sos/jan99/articles/brass778.htm Recording Brass & Reeds].</ref>
| 0.5 m
| align="right" | 63.2
| align="right" | 130
|-
| [[Vuvuzela]] horn<ref>{{Cite journal |last1=Swanepoel |first1=De Wet |author-link=De Wet Swanepoel |last2=Hall III |first2=James W. |last3=Koekemoer |first3=Dirk |date=February 2010 |title=Vuvuzela – good for your team, bad for your ears |url=http://www.scielo.org.za/pdf/samj/v100n2/v100n2a15.pdf |journal=South African Medical Journal |volume=100 |issue=4 |pages=99–100 |doi=10.7196/samj.3697 |pmid=20459912 |doi-access=free}}</ref>
| 1 m
| align="right" | 20.0
| align="right" | 120
|-
| [[Threshold of pain]]<ref name=painthres>{{cite web |last=Nave |first=Carl R. |title=Threshold of Pain |work=HyperPhysics |publisher=SciLinks |year=2006 |url=http://hyperphysics.phy-astr.gsu.edu/Hbase/sound/intens.html |access-date=2009-06-16}}</ref><ref name=dictionary>{{Cite book |editor1-last=Franks |editor1-first=John R. |editor2-last=Stephenson |editor2-first=Mark R. |editor3-last=Merry |editor3-first =Carol J. |date=June 1996 |title=Preventing Occupational Hearing Loss – A Practical Guide |publisher=[[National Institute for Occupational Safety and Health]] |pages=88 |url=https://www.cdc.gov/niosh/docs/96-110/pdfs/96-110.pdf |access-date=2009-07-15}}</ref><ref name=Shure>[https://service.shure.com/s/article/can-a-dynamic-microphone-handle-really-loud-sounds-maximum-spl?language=en_US Realistic Maximum Sound Pressure Levels for Dynamic Microphones] – [[Shure]].</ref>
| At ear
| align="right" | 20–200
| align="right" | 120–140
|-
| Risk of instantaneous [[noise-induced hearing loss]]
| At ear
| align="right" | 20.0
| align="right" | 120
|-
| [[Jet engine]]
| 100–30 m
| align="right" | 6.32–200
| align="right" | 110–140
|-
| [[Two-stroke]] [[chainsaw]]<ref name="sengpielaudio">{{cite web |work=sengpielaudio |title=Decibel Table – SPL – Loudness Comparison Chart |url=http://www.sengpielaudio.com/TableOfSoundPressureLevels.htm |access-date=5 Mar 2012}}</ref>
| 1 m
| align="right" | 6.32
| align="right" | 110
|-
| [[Jackhammer]]
| 1 m
| align="right" | 2.00
| align="right" | 100
|-
| Traffic on a busy roadway
| 10 m
| align="right" | 0.20–0.63
| align="right" | 80–90
|-
| [[Hearing damage]] (over long-term exposure, need not be continuous)<ref name=Hamby/>
| At ear
| align="right" | 0.36
| align="right" | 85
|-
| [[Automobile|Passenger car]]
| 10 m
| align="right" | 0.02–0.20
| align="right" | 60–80
|-
| [[United States Environmental Protection Agency|EPA]]-identified maximum to protect against hearing loss and other disruptive effects from noise, such as sleep disturbance, stress, learning detriment, etc.<ref>{{cite press release |title=EPA Identifies Noise Levels Affecting Health and Welfare |url=https://archive.epa.gov/epa/aboutepa/epa-identifies-noise-levels-affecting-health-and-welfare.html |publisher=[[United States Environmental Protection Agency|Environmental Protection Agency]] |date=April 2, 1974 |access-date=March 27, 2017}}</ref>
| Ambient
| align="right" | 0.06
| align="right" | 70
|-
| TV (set at home level)
| 1 m
| align="right" | 0.02
| align="right" | 60
|-
| Normal conversation
| 1 m
| align="right" | 2×10<sup>−3</sup>–0.02
| align="right" | 40–60
|-
| Very calm room
| Ambient
| align="right" | 2.00×10<sup>−4</sup><br>...6.32×10<sup>−4</sup>
| align="right" | 20–30
|-
| Light leaf rustling, calm breathing<ref name="Audio1"/>
| Ambient
| align="right" | 6.32×10<sup>−5</sup>
| align="right" | 10
|-
| [[Auditory threshold]] at 1&nbsp;kHz<ref name=Hamby>{{Cite web |url=http://www.makeitlouder.com/Decibel%20Level%20Chart.txt |title=Ultimate Sound Pressure Level Decibel Table |archive-url=https://web.archive.org/web/20051019001826/http://www.makeitlouder.com/Decibel%20Level%20Chart.txt |archive-date=2005-10-19 |url-status=live |author=William Hamby}}</ref>
| At ear
| align="right" | 2.00×10<sup>−5</sup>
| align="right" | 0
|-
| [[Anechoic chamber]], Orfield Labs, [[A-weighted]]<ref>{{Cite web |url=http://www.orfieldlabs.com/pdfs/chamber.pdf |title="THE QUIETEST PLACE ON EARTH" – GUINNESS WORLD RECORDS CERTIFICATE, 2005 |publisher=Orfield Labs }}</ref><ref>{{Cite web |url=http://www.audiojunkies.com/blog/902/the-quietest-place-on-earth-orfield-labs |title=The Quietest Place on Earth – Orfield Labs |last=Middlemiss |first=Neil |date=December 18, 2007 |website=Audio Junkies, Inc. |archive-url=https://web.archive.org/web/20101121101851/http://www.audiojunkies.com/blog/902/the-quietest-place-on-earth-orfield-labs |archive-date=2010-11-21 }}</ref>
| Ambient
| align="right" | 6.80×10<sup>−6</sup>
| align="right" | −9.4
|-
| [[Anechoic chamber]], [[University of Salford]], [[A-weighted]]<ref>{{Cite web |url=http://www.acoustics.salford.ac.uk/facilities/?content=anechoic |title=Anechoic Chamber |last=Eustace |first=Dave |publisher=University of Salford |access-date=2016-09-21 |archive-date=2019-03-04 |archive-url=https://web.archive.org/web/20190304052554/http://www.acoustics.salford.ac.uk/facilities/?content=anechoic |url-status=dead }}</ref>
| Ambient
| align="right" | 4.80×10<sup>−6</sup>
| align="right" | −12.4
|-
| [[Anechoic chamber]], Microsoft, [[A-weighted]]<ref>{{Cite web |url=http://www.guinnessworldrecords.com/news/2015/10/microsoft-lab-sets-new-record-for-the-worlds-quietest-place-399444 |title=Microsoft lab sets new record for the world's quietest place |date=2015-10-02 |access-date=2016-09-20 |quote=The computer company has built an anechoic chamber in which highly sensitive tests reported an average background noise reading of an unimaginably quiet −20.35&nbsp;dBA (decibels A-weighted).}}</ref><ref>{{Cite web |url=http://news.microsoft.com/stories/building87/audio-lab.php |title=Check out the world's quietest room |website=Microsoft: Inside B87 |access-date=2016-09-20}}</ref>
| Ambient
| align="right" | 1.90×10<sup>−6</sup>
| align="right" | −20.35
|}
{{noteslist}}

==See also==
*[[Acoustics]]
*[[Phon]] (unit)
*[[Loudness]]
*[[Sone]] (unit)
*[[Sound level meter]]
*[[Stevens's power law]]
*[[Weber–Fechner law]], especially [[Weber–Fechner law#Sound|The case of sound]]

==References==
{{Reflist}}

;General
*Beranek, Leo L., ''Acoustics'' (1993), Acoustical Society of America, {{ISBN|0-88318-494-X}}.
*Daniel R. Raichel, ''The Science and Applications of Acoustics'' (2006), Springer New York, {{ISBN|1441920803}}.

==External links==
*{{Commons category-inline}}
*[http://www.sengpielaudio.com/SoundPressureAndSoundPower.pdf Sound Pressure and Sound Power, Effect and Cause]
*[http://www.sengpielaudio.com/calculator-soundlevel.htm Conversion of Sound Pressure to Sound Pressure Level and Vice Versa]
*[http://www.sengpielaudio.com/TableOfSoundPressureLevels.htm Table of Sound Levels, Corresponding Sound Pressure and Sound Intensity]
*[http://www.sengpielaudio.com/calculator-ak-ohm.htm Ohm's Law as Acoustic Equivalent, Calculations]
*[http://www.sengpielaudio.com/RelationshipsOfAcousticQuantities.pdf Relationships of Acoustic Quantities Associated with a Plane Progressive Acoustic Sound Wave]
*[http://www.usmotors.com/products/ProFacts/sound_power_and_sound_pressure.htm Sound Pressure and Sound Power, Two Commonly Confused Characteristics of Sound] {{Webarchive|url=https://web.archive.org/web/20110929111953/http://www.usmotors.com/products/ProFacts/sound_power_and_sound_pressure.htm |date=2011-09-29 }}
*[http://www.sengpielaudio.com/calculator-levelchange.htm How Many Decibels Is Twice as Loud? Sound Level Change and the Respective Factor of Sound Pressure or Sound Intensity]
*[http://www.gcaudio.com/resources/howtos/loudness.html Decibel (Loudness) Comparison Chart]

{{Orders of magnitude}}
{{Authority control}}

[[Category:Acoustics]]
[[Category:Sound]]
[[Category:Sound measurements]]
[[Category:Physical quantities]]
[[Category:Acoustic equations]]