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>{{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|upright=1.1|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|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|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|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>v</math> is the particle velocity,
* <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>

==Sound pressure level==
<!--"Sound pressure level" redirects here.-->
{{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>{{cite book |last1=Bies |first1=David A. |last2=Hansen |first2=Colin |date=2003 |title=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 {{nowrap|''L''<sub>''p''/(20 μPa)</sub>}} or {{nowrap|''L''<sub>''p''</sub> (re 20 μPa)}}, but the suffix notations {{nowrap|dB SPL}}, {{nowrap|dB(SPL)}}, dBSPL, and 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 {{nowrap|1 Pa}} will equal an SPL of <math>20 \log_{10}\left(\frac{1}{2\times10^{-5}}\right)~\text{dB}\approx 94~\text{dB}</math>. In other media, such as [[underwater acoustics|underwater]], a reference level of {{nowrap|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 {{nowrap|0 dB}}, but the upper limit is not as clearly defined. While {{nowrap|1 [[atmosphere (unit)|atm]]}} ({{nowrap|194 dB peak}} or {{nowrap|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 |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. |last2=King |first2=P.F. |author3=North Atlantic Treaty Organization Advisory Group for Aerospace Research and Development Aerospace Medical Panel |date=1972 |url=https://books.google.com/books?id=LvRJAQAAIAAJ&q=191+dB+SPL |title=Aeromedical Aspects of Vibration and Noise |publisher=North Atlantic Treaty Organization, Advisory Group for Aerospace Research and Development |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|upright=1.1|[[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 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 |date=2003 |isbn=978-0-670-91430-2 |page=218 |author-link=Simon Winchester }}</ref>
|165&nbsp;km
|
| align="right" | 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=Gregory A. |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 |last1=Brueck |first1=Scott E. |last2=Kardous |first2=Chuck A. |last3=Oza |first3=Aalok |last4=Murphy |first4=William 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 journal |title=Did You Know How Loud Balloons Can Be? |journal=Canadian Audiologist |date=9 January 2014 |volume=3 |issue=6 |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 |doi-broken-date=12 July 2025 |pmid=20459912 |doi-access=free |hdl=2263/13136 }}</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" | 20–100
| align="right" | 120–134
|-
| 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
|-
| [[Hearing damage]] (over long-term exposure, need not be continuous)<ref name="Hamby" />
| At ear
| align="right" | 0.36
| align="right" | 85
|-
| [[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://www.epa.gov/archive/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
|-
| [[Automobile|Passenger car]] at 30&nbsp;[[Kilometres per hour|kph]] ([[Electric vehicle|electric]] and [[combustion engines]])<ref name=":2">{{Cite journal |last1=Misdariis |first1=Nicolas |last2=Pardo |first2=Louis-Ferdinand |date=August 2017 |title=The sound of silence of electric vehicles – Issues and answers |url=https://hal.science/hal-01708883 |journal=Inter.noise (International Congress & Exposition on Noise Control Engineering) |location=Hong-Kong, China |quote=Figure 1 shows the noise level generated when three vehicles go by, according to their speed. At low speed, the difference between a vehicle with an engine and an electric vehicle can be significant (over 10 dB(A)). Above 20 to 30 km/h, the noise made by the tyres on the road surface becomes dominant and the differences become less pronounced.}}</ref>
| 10 m
| align="right" | 0.045–0.063
| align="right" | 67–70
|-
| 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
|-
| Passenger car at 10&nbsp;[[Kilometres per hour|kph]] (combustion)<ref name=":2" />
| 10 m
| align="right" | 12.6×10<sup>−3</sup>
| align="right" | 56
|-
|Passenger car at 10&nbsp;[[Kilometres per hour|kph]] (electric)<ref name=":2" />
|10 m
| align="right" |6.32×10<sup>−3</sup>
| align="right" |50
|-
| 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 |first=William |last=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 |last=Middlemiss |first=Neil |date=December 18, 2007 |url=http://www.audiojunkies.com/blog/902/the-quietest-place-on-earth-orfield-labs |title=The Quietest Place on Earth – Orfield Labs |website=Audio Junkies |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 |last=Eustace |first=Dave |url=http://www.acoustics.salford.ac.uk/facilities/?content=anechoic |title=Anechoic Chamber |publisher=University of Salford |archive-url=https://web.archive.org/web/20190304052554/http://www.acoustics.salford.ac.uk/facilities/?content=anechoic |archive-date=2019-03-04 }}</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
|}
{{Notelist}}

==See also==
* {{Annotated link|Acoustics}}
* {{Annotated link|Phon}}
* {{Annotated link|Loudness}}
* {{Annotated link|Sone}}
* {{Annotated link|Sound level meter}}
* {{Annotated link|Stevens's power law}}
* {{Annotated link|Weber–Fechner law}}

==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.usmotors.com/products/ProFacts/sound_power_and_sound_pressure.htm Sound Pressure and Sound Power, Two Commonly Confused Characteristics of Sound]
* [http://www.gcaudio.com/resources/howtos/loudness.html Decibel (Loudness) Comparison Chart]

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

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