Editing Decibel (section)

== Uses ==

=== Perception ===
The human perception of the intensity of sound and light more nearly approximates the logarithm of intensity rather than a linear relationship (see [[Weber–Fechner law]]), making the dB scale a useful measure.<ref>{{Google books |id=1SMXAAAAQBAJ |page=268 |title=Sensation and Perception}}</ref><ref>{{Google books |id=BggrpTek5kAC |page=SA19-PA9 |title=Introduction to Understandable Physics, Volume 2}}</ref><ref>{{Google books |id=ukvei0wge_8C |page=356 |title=Visual Perception: Physiology, Psychology, and Ecology}}</ref><ref>{{Google books |id=-QIfF9q6Q_EC |page=407 |title=Exercise Psychology}}</ref><ref>{{Google books |id=oUNfSjS11ggC |page=83 |title=Foundations of Perception}}</ref><ref>{{Google books |id=w888Mw1dh_EC |page=304 |title=Fitting The Task To The Human}}</ref>

=== 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>

=== Telephony ===
The decibel is used in [[telephony]] and [[Audio signal|audio]].  Similarly to the use in acoustics, a frequency weighted power is often used.  For audio noise measurements in electrical circuits, the weightings are called [[psophometric weighting]]s.<ref name="Reeve">{{Cite book |last=Reeve |first= William D. |year= 1992 |title= Subscriber Loop Signaling and Transmission Handbook – Analog |edition= 1st |publisher=IEEE Press |isbn= 0-87942-274-2}}</ref>

=== Electronics ===
In electronics, the decibel is often used to express power or amplitude ratios (as for [[Gain (electronics)|gains]]) in preference to [[arithmetic]] ratios or [[percent]]ages. One advantage is that the total decibel gain of a series of components (such as amplifiers and [[Attenuator (electronics)|attenuators]]) can be calculated simply by summing the decibel gains of the individual components. Similarly, in telecommunications, decibels denote signal gain or loss from a transmitter to a receiver through some medium ([[free space optical communication|free space]], [[waveguide]], [[coaxial cable]], [[fiber optics]], etc.) using a [[link budget]].

The decibel unit can also be combined with a reference level, often indicated via a suffix, to create an absolute unit of electric power. For example, it can be combined with ''m'' for ''milliwatt'' to produce the ''dBm''.  A power level of 0&nbsp;dBm corresponds to one milliwatt, and 1&nbsp;dBm is one decibel greater (about 1.259&nbsp;mW).

In professional audio specifications, a popular unit is the [[dBu]].  This is relative to the root mean square voltage which delivers 1&nbsp;mW (0&nbsp;dBm) into a 600-ohm resistor, or {{sqrt|1&nbsp;mW &times; 600&nbsp;Ω }}≈ 0.775&nbsp;V<sub>RMS</sub>.  When used in a 600-ohm circuit (historically, the standard reference impedance in telephone circuits), dBu and dBm are [[#dBu or dBv|identical]].

=== Optics ===
In an [[optical link]], if a known amount of [[Optics|optical]] power, in [[dBm]] (referenced to 1&nbsp;mW), is launched into a fiber, and the losses, in dB (decibels), of each component (e.g., connectors, splices, and lengths of fiber) are known, the overall link loss may be quickly calculated by addition and subtraction of decibel quantities.<ref>
{{cite book
 | title = Fiber optic installer's field manual
 | author-first = Bob |author-last=Chomycz
 | publisher = McGraw-Hill Professional
 | year = 2000
 | isbn = 978-0-07-135604-6
 | pages = 123–126
 | url = {{Google books |plainurl=yes |id=B810SYIAa4IC |page=123 }}
}}</ref>

In spectrometry and optics, the [[absorbance|blocking unit]] used to measure [[optical density]] is equivalent to −1&nbsp;B.

=== Video and digital imaging ===
In connection with video and digital [[image sensor]]s, decibels generally represent ratios of video voltages or digitized light intensities, using 20&nbsp;log of the ratio, even when the represented intensity (optical power) is directly proportional to the voltage generated by the sensor, not to its square, as in a [[CCD imager]] where response voltage is linear in intensity.<ref>
{{cite book
 | title = The Colour Image Processing Handbook
 | author = Stephen J. Sangwine and Robin E. N. Horne
 | publisher = Springer
 | year = 1998
 | isbn = 978-0-412-80620-9
 | pages = 127–130
 | url = {{Google books |plainurl=yes |id=oEsZiCt5VOAC |page=127 }}
}}</ref>
Thus, a camera [[signal-to-noise ratio]] or dynamic range quoted as 40&nbsp;dB represents a ratio of 100:1 between optical signal intensity and optical-equivalent dark-noise intensity, not a 10,000:1 intensity (power) ratio as 40&nbsp;dB might suggest.<ref>
{{cite book
 | title = Introduction to optical engineering
 | author = Francis T. S. Yu and Xiangyang Yang
 | publisher = Cambridge University Press
 | year = 1997
 | isbn = 978-0-521-57493-8
 | pages = 102–103
 | url = {{Google books |plainurl=yes |id=RYm7WwjsyzkC |page=120 }}
}}</ref>
Sometimes the 20&nbsp;log ratio definition is applied to electron counts or photon counts directly, which are proportional to sensor signal amplitude without the need to consider whether the voltage response to intensity is linear.<ref>
{{cite book
 | title = Image sensors and signal processing for digital still cameras
 | chapter = Basics of Image Sensors
 | author = Junichi Nakamura
 | editor = Junichi Nakamura
 | publisher = CRC Press
 | year = 2006
 | isbn = 978-0-8493-3545-7
 | pages = 79–83
 | chapter-url = {{Google books |plainurl=yes |id=UY6QzgzgieYC |page=79 }}
}}</ref>

However, as mentioned above, the 10&nbsp;log intensity convention prevails more generally in physical optics, including fiber optics, so the terminology can become murky between the conventions of digital photographic technology and physics. Most commonly, quantities called ''dynamic range'' or ''signal-to-noise'' (of the camera) would be specified in {{nowrap|20 log dB}}, but in related contexts (e.g. attenuation, gain, intensifier SNR, or rejection ratio) the term should be interpreted cautiously, as confusion of the two units can result in very large misunderstandings of the value.

Photographers typically use an alternative base-2 log unit, the [[F-number#Stops.2C f-stop conventions.2C and exposure|stop]], to describe light intensity ratios or dynamic range.