Decibel: Difference between revisions

{{short description|Logarithmic unit expressing the ratio of physical quantities}}

{{about|the logarithmic unit|use of this unit in sound measurements|Sound pressure level|other uses}}

{{use dmy dates|date=February 2014}}

{{infobox unit

| inunits1 = 0.1 bel

The '''decibel''' (symbol: '''dB''') is a relative [[unit of measurement]] equal to one tenth of a '''bel''' ('''B'''). It expresses the [[ratio]] of two values of a [[Power, root-power, and field quantities|power or root-power quantity]] on a [[logarithmic scale]]. Two [[signals]] whose [[level (logarithmic quantity)|levels]] differ by one decibel have a power ratio of 10^1/10 (approximately {{val|1.26}}) or root-power ratio of 10^1/20 (approximately {{val|1.12}}).<ref name="auto">{{cite book |author-last=Mark |author-first=James E. |title=Physical Properties of Polymers Handbook |publisher=Springer |date=2007 |page=1025 |bibcode=2007ppph.book.....M |quote=[...] the decibel represents a reduction in power of 1.258 times [...]}}</ref><ref name="auto1">{{cite book |author-last=Yost |author-first=William |title=Fundamentals of Hearing: An Introduction |url=https://archive.org/details/fundamentalsofhe00yost |url-access=registration |publisher=Holt, Rinehart and Winston |edition=Second |date=1985 |page=[https://archive.org/details/fundamentalsofhe00yost/page/206 206] |isbn=978-0-12-772690-8 |quote=[...] a pressure ratio of 1.122 equals +1.0&nbsp;dB [...]}}</ref>

As it originated from a need to express power ratios, two principal types of scaling of the decibel are used to provide consistency depending on whether the scaling refers to ratios of power quantities or root-power quantities. When expressing a power ratio, the corresponding change in decibels is defined as ten times the [[Common logarithm|logarithm with base 10]] of that ratio.<ref>{{cite book |title=IEEE Standard 100: a dictionary of IEEE standards and terms |edition=7th |publisher=The Institute of Electrical and Electronics Engineering |location=New York |year=2000 |isbn=978-0-7381-2601-2 |page=288}}</ref> That is, a change in ''power'' by a factor of 10 corresponds to a 10&nbsp;dB change in level. When expressing root-power ratios, a change in [[amplitude]] by a factor of 10 corresponds to a 20&nbsp;dB change in level. The decibel scales differ by a factor of two, so that the related power and root-power levels change by the same value in linear systems, where power is proportional to the square of amplitude.

In 1924, [[Bell Labs|Bell Telephone Laboratories]] received a favorable response to a new unit definition among members of the International Advisory Committee on Long Distance Telephony in Europe and replaced the MSC with the ''Transmission Unit'' (TU). 1&nbsp;TU was defined such that the number of TUs was ten times the base-10 logarithm of the ratio of measured power to a reference power.<ref>{{cite book |title=Sound system engineering |edition=2nd |author-first1=Don |author-last1=Davis |author-first2=Carolyn |author-last2=Davis |publisher=[[Focal Press]] |date=1997 |isbn=978-0-240-80305-0 |page=35 |url={{Google books|plainurl=yes|id=9mAUp5IC5AMC|page=35}}}}</ref> The definition was conveniently chosen such that 1&nbsp;TU approximated 1&nbsp;MSC; specifically, 1&nbsp;MSC was 1.056&nbsp;TU. In 1928, the Bell system renamed the TU as the ''decibel'',<ref>{{cite journal |journal=Bell Laboratories Record |title='TU' becomes 'Decibel' |author-first=R. V. L. |author-last=Hartley |author-link=R. V. L. Hartley |volume=7 |issue=4 |publisher=AT&T |pages=137–139 |date=December 1928 |url={{Google books|plainurl=yes|id=h1ciAQAAIAAJ}}}}</ref> being one tenth of a newly defined unit for the base-10 logarithm of the power ratio. It was named the ''bel'', in honor of the telecommunications pioneer [[Alexander Graham Bell]].<ref>{{cite journal |author-last=Martin |author-first=W. H. |date=January 1929 |title=DeciBel—The New Name for the Transmission Unit |journal=[[Bell System Technical Journal]] |volume=8 |issue=1}}</ref> The bel is seldom used, as the decibel was the proposed working unit.<ref>{{Google books |id=EaVSbjsaBfMC |page=276 |title=100 Years of Telephone Switching}}, Robert J. Chapuis, Amos E. Joel, 2003</ref>

The naming and early definition of the decibel is described in the [[National Institute of Standards and Technology|NBS]] Standard's Yearbook of 1931:<ref>{{cite journal |title=Standards for Transmission of Speech |journal=Standards Yearbook |volume=119 |author-first=William H. |author-last=Harrison |date=1931 |publisher=National Bureau of Standards, U. S. Govt. Printing Office}}</ref>

{{anchor|Logit|Decilog}}The word decibel was soon misused to refer to absolute quantities and to ratios other than power. Some proposals attempted to address the resulting confusion. In 1954, J. W. Horton considered that 10{{sup|0.1}} be treated as an elementary ratio and proposed the word ''logit'' as "a standard ratio which has the numerical value 10{{sup|0.1}} and which combines by multiplication with similar ratios of the same value", so one would describe a 10{{sup|0.1}} ratio of units of mass as "a mass logit". This contrasts with the word ''unit'' which would be reserved for magnitudes which combine by addition and reserves the word ''decibel'' specifically for unit transmission loss.<ref>{{cite journal |first=J. W. |last=Horton |title=The bewildering decibel |journal=Electrical Engineering |volume=73 |issue=6 |pages=550–555 |year=1954|doi=10.1109/EE.1954.6438830 |bibcode=1954ElEng..73..550H |s2cid=51654766 }}</ref> The ''decilog'' was another proposal (by N. B. Saunders in 1943, A. G. Fox in 1951, and E. I. Green in 1954) to express a division of the logarithmic scale corresponding to a ratio of 10{{sup|0.1}}.<ref name=":0">{{cite journal |last=Green |first=E. I. |date=July 1954 |title=The decilog: A unit for logarithmic measurement |journal=Electrical Engineering |volume=73 |issue=7 |pages=597–599 |doi=10.1109/EE.1954.6438862 |bibcode=1954ElEng..73..597G |issn=2376-7804}}</ref>

{| class="wikitable floatright" style="font-size:85%; margin-left:1em"

| style="text-align:right; border:none; padding-right:0" |                      0 || style="border:none; padding-left:0;" | .708 ≈ {{sfrac|{{sqrt|2}}}}

The IEC Standard [[IEC 60027|60027-3:2002]] defines the following quantities. The decibel (dB) is one-tenth of a bel: 1&nbsp;dB&nbsp;= 0.1&nbsp;B. The bel (B) is {{sfrac|1|2}}&nbsp;ln(10) [[neper]]s: 1&nbsp;B&nbsp;= {{sfrac|1|2}}&nbsp;ln(10)&nbsp;Np. The neper is the change in the [[level (logarithmic quantity)|level]] of a [[root-power quantity]] when the root-power quantity changes by a factor of [[e (mathematical constant)|''e'']], that is {{nowrap|1=1 Np = ln(e) = 1}}, thereby relating all of the units as nondimensional [[Natural logarithm|natural ''log'']] of root-power-quantity ratios, {{val|1|u=dB}}&nbsp;= {{val|0.11513|end=...|u=Np}}&nbsp;= {{val|0.11513|end=...}}.  Finally, the level of a quantity is the logarithm of the ratio of the value of that quantity to a reference value of the same kind of quantity.

When referring to measurements of ''[[Power (physics)|power]]'' quantities, a ratio can be expressed as a level in decibels by evaluating ten times the [[base-10 logarithm]] of the ratio of the measured quantity to reference value. Thus, the ratio of ''P'' (measured power) to ''P''₀ (reference power) is represented by ''L''_''P'', that ratio expressed in decibels,<ref>{{cite book |title=Microwave Engineering |author-first=David M. |author-last=Pozar |edition=3rd |publisher=Wiley |date=2005 |author-link=David M. Pozar |isbn=978-0-471-44878-5 |page=63}}</ref> which is calculated using the formula:<ref>IEC 60027-3:2002</ref>

Although power and root-power quantities are different quantities, their respective levels are historically expressed in the same units, typically decibels.  A factor of 2 is introduced to make ''changes'' in the respective levels match under restricted conditions such as when the medium is linear and the ''same'' waveform is under consideration with changes in amplitude, or the medium impedance is linear and independent of both frequency and time.  This relies on the relationship

: <math> \frac{P(t)}{P_0} = \left(\frac{F(t)}{F_0}\right)^2 </math>

Since logarithm differences expressed in these units often represent power ratios and root-power ratios, values for both are shown below. The bel is traditionally used as a unit of logarithmic power ratio, while the neper is used for logarithmic root-power (amplitude) ratio.

A change in power ratio by a factor of 10 corresponds to a change in level of {{nowrap|10 dB}}. A change in power ratio by a factor of 2 or {{sfrac|2}} is approximately a [[Cutoff frequency|change of 3&nbsp;dB]]. More precisely, the change is ±{{val|3.0103}}&nbsp;dB, but this is almost universally rounded to 3&nbsp;dB in technical writing.{{citation needed|date=January 2025}}  This implies an increase in voltage by a factor of {{nowrap|{{sqrt|2}} ≈}} {{val|1.4142}}.  Likewise, a doubling or halving of the voltage, corresponding to a quadrupling or quartering of the power, is commonly described as 6&nbsp;dB rather than ±{{val|6.0206}}&nbsp;dB.

The [[logarithmic scale]] nature of the decibel means that a very large range of ratios can be represented by a convenient number. For example, 50&nbsp;dB is easier to say than "the two powers bear a 100,000 to 1 ratio" or that "one power is 10{{sup|5}} the other".<ref name=":0" /> Decibels express huge changes of a quantity with few digits of dB.

However, according to its critics, the decibel creates confusion, obscures reasoning, is more related to the era of [[slide rule]]s than to modern digital processing, and is cumbersome and difficult to interpret.<ref name="Hickling">R. Hickling (1999), Noise Control and SI Units, J Acoust Soc Am 106, 3048</ref><ref>Hickling, R. (2006). Decibels and octaves, who needs them?. Journal of sound and vibration, 291(3-5), 1202-1207.</ref> Quantities in decibels are not necessarily [[Dimensional homogeneity|additive]],<ref>Nicholas P. Cheremisinoff (1996) Noise Control in Industry: A Practical Guide, Elsevier, 203 pp, p. [{{Google books |plainurl=yes |id=rrpEuUOkT3UC |page=7}} 7]</ref><ref>Andrew Clennel Palmer (2008), Dimensional Analysis and Intelligent Experimentation, World Scientific, 154 pp, p. 13</ref> thus being "of unacceptable form for use in [[dimensional analysis]]".<ref>J. C. Gibbings, ''Dimensional Analysis'', [{{Google books |plainurl=yes |id=Q6iflrgVaWcC |page=37}} p.37], Springer, 2011 {{ISBN|1849963177}}.</ref> Thus, units require special care in decibel operations. Take, for example, [[carrier-to-noise-density ratio]] ''C''/''N''₀ (in [[hertz]]), involving carrier power ''C'' (in W) and noise power spectral density ''N''₀ (in W/Hz). Expressed in decibels, this ratio would be a subtraction {{nowrap|1=(''C''/''N''₀)_dB = ''C''_dB − ''N''_0&nbsp;dB}}. However, the linear-scale units still simplify in the implied fraction, so that the results would be expressed in dB-Hz.

{{further|Logarithmic addition}}

Addition on a logarithmic scale is called [[logarithmic addition]], and can be defined by taking exponentials to convert to a linear scale, adding there, and then taking logarithms to return. For example, where operations on decibels are logarithmic addition or subtraction and logarithmic multiplication or division, while operations on the linear scale are the usual operations:

: <math>87\,\text{dBA} \ominus 83\,\text{dBA} = 10 \cdot \log_{10}\bigl(10^{87/10} - 10^{83/10}\bigr)\,\text{dBA} \approx 84.8\,\text{dBA}</math>

: <math>

[[Attenuation]] constants, in topics such as [[optical fiber]] communication and [[radio propagation]] [[path loss]], are often expressed as a [[Fraction (mathematics)|fraction]] or ratio to distance of transmission. In this case, {{nowrap|dB/m}} represents decibel per meter, {{nowrap|dB/mi}} represents decibel per mile, for example. These quantities are to be manipulated obeying the rules of dimensional analysis, e.g., a 100-meter run with a {{nowrap|3.5 dB/km}} fiber yields a loss of {{nowrap|1=0.35 dB =}} {{nowrap|3.5 dB/km ×}} 0.1&nbsp;km.

The original choice of the decibel over the bel as a log unit of change of intensity is because a single change in a property of sound which is below the [[just-noticeable difference]] (JND) does not affect perception of the sound. For amplitude, the JND for humans is around 1&nbsp;dB.<ref name=Middlebrooks>{{citation |author=Middlebrooks, John C. and David M. Green |title=Sound Localization by Human Listeners|journal=Annual Review of Psychology |volume=42|issue=1|year=1991|pages=135–159|issn=0066-4308|doi=10.1146/annurev.ps.42.020191.001031|pmid=2018391}}</ref><ref name=Mills>{{citation |author=Mills, A.W. |title=Lateralization of High-Frequency Tones|journal=The Journal of the Acoustical Society of America|volume=32|issue=1|year=1960|pages=132–134|issn=0001-4966|doi=10.1121/1.1907864|bibcode=1960ASAJ...32..132M }}</ref>

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>

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]].

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 × 600&nbsp;Ω }}≈ 0.775&nbsp;V_RMS.  When used in a 600-ohm circuit (historically, the standard reference impedance in telephone circuits), dBu and dBm are [[#dBu or dBv|identical]].

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

; dBu or dBv : [[File:Relationship between dBu and dBm.png|thumb|upright=1.25|Schematic of a 0&nbsp;dBu [[voltage source]] dissipating 0&nbsp;dBm of power as [[heat]] in a 600&nbsp;Ω [[resistor]]]] 0 dBu is defined as the RMS voltage that would dissipate 0&nbsp;dBm (1&nbsp;mW) in a 600&nbsp;Ω [[Electrical load|load]]. Per [[Ohm's law]], this voltage equals:<math display="block">\sqrt{\text{resistance} \cdot \text{power}} = \sqrt{600\ \mathsf{\Omega} \ \cdot\ 0.001\ \mathsf{W}\;} = \sqrt{0.6} \ \mathsf{V_{RMS}} \approx 0.7746\ \mathsf{V_{RMS}}\, . </math>Therefore, 1&nbsp;V{{Sub|RMS}} corresponds to:<ref name=clqgmk/> <math display="block">20 \cdot \log_{10} \left( \frac{ 1\ \mathsf{V_{RMS}}}{\sqrt{0.6}\ \mathsf{V_{RMS}}} \right) \approx 2.218\ \mathsf{dB_u} ~.</math>Originally called dBv, it was changed to dBu to avoid confusion with dBV.<ref>{{cite web |first=Stas |last=Bekman  |title=3.3 – What is the difference between dBv, dBu, dBV, dBm, dB&nbsp;SPL, and plain old dB? Why not just use regular voltage and power measurements? |website=stason.org |department=Entertainment audio |series=TULARC |url=http://stason.org/TULARC/entertainment/audio/pro/3-3-What-is-the-difference-between-dBv-dBu-dBV-dBm-dB.html }}</ref> According to [[Rupert Neve]], the ''u'' originated from the [[volume unit|volume ''unit'']] displayed on a [[VU meter]].<ref>{{cite AV media |first=Rupert |last=Neve |author-link=Rupert Neve |date=9 October 2015 |title=Creation of the dBu standard level reference |medium=video |url=https://www.youtube.com/watch?v=b02P4f3CBuM | archive-url=https://ghostarchive.org/varchive/youtube/20211030/b02P4f3CBuM |archive-date=2021-10-30 }}{{cbignore}}</ref> The ''u'' has also been interpreted as ''unloaded''.<ref>{{cite web |last=White |first=Paul |date=February 1994 |title=Decibels Explained |url=https://www.soundonsound.com/sound-advice/decibels-explained |url-status=live |archive-url=https://web.archive.org/web/20250121014734/https://www.soundonsound.com/sound-advice/decibels-explained |archive-date=2025-01-21 |access-date=2025-05-31 |website=[[Sound on Sound]]}}</ref>{{paragraphbreak}}

: In [[professional audio]], equipment may be calibrated to indicate a "0" on the VU meters some finite time after a signal has been applied at an amplitude of {{nowrap|+4 dBu}}. Consumer equipment typically uses a lower "nominal" signal level of {{nowrap|−10 dBV}}.<ref>{{cite web |title=dB or not dB&nbsp;? |website=deltamedia.com |url=http://www.deltamedia.com/resource/db_or_not_db.html |url-status=dead |access-date=2013-09-16 |archive-url=https://web.archive.org/web/20130620064637/http://www.deltamedia.com/resource/db_or_not_db.html |archive-date=20 June 2013 }}</ref> Therefore, many devices offer dual voltage operation (with different gain or "trim" settings) for compatibility. A switch or adjustment that covers at least the range between {{nowrap|+4 dBu}} and {{nowrap|−10 dBV}} is common in professional equipment.

; dBmV : dBmV: dB(mV_RMS) – root mean square voltage relative to 1&nbsp;millivolt across 75&nbsp;Ω.<ref>

</ref> Widely used in [[cable television]] networks, where the nominal strength of a single TV signal at the receiver terminals is about 0&nbsp;dB{{sub| mV}}. Cable&nbsp;TV uses 75&nbsp;Ω coaxial cable, so 0&nbsp;dB{{sub| mV}} corresponds to −78.75&nbsp;dBW, −48.75&nbsp;dBm or approximately 13&nbsp;nW.

 

; dBmV0s : Defined by ''Recommendation ITU-R V.574''.

Probably the most common usage of "decibels" in reference to sound level is dB{{sub| SPL}}, sound pressure level referenced to the nominal threshold of human hearing:<ref>

; dB(A), dB(B), and dB(C) : These symbols are often used to denote the use of different [[weighting filter]]s, used to approximate the human ear's [[stimulus (psychology)|response]] to sound, although the measurement is still in dB&nbsp;(SPL). These measurements usually refer to noise and its effects on humans and other animals, and they are widely used in industry while discussing noise control issues, regulations and environmental standards. Other variations that may be seen are dBA or dB(A). According to standards from the International Electro-technical Committee ([[IEC 61672|IEC 61672-2013]])<ref>{{cite book |title=IEC 61672-1:2013 Electroacoustics - Sound Level meters - Part 1: Specifications |date=2013 |publisher=International Electrotechnical Committee |location=Geneva}}</ref> and the American National Standards Institute, [[ANSI S1.4]],<ref>[[ANSI]] [https://law.resource.org/pub/us/cfr/ibr/002/ansi.s1.4.1983.pdf S1.4-19823 Specification for Sound Level Meters], 2.3 Sound Level, p. 2–3.</ref> the preferred usage is to write {{nowrap|{{mvar|L}}{{sub| A}} {{=}} {{mvar|x}}&nbsp;dB}}. Nevertheless, the units dB(A) are still commonly used as a shorthand for A{{nbhyph}}weighted measurements. Compare [[dBc]], used in telecommunications.

; dBHL : dB [[hearing level]] is used in [[audiogram]]s as a measure of hearing loss. The reference level varies with frequency according to a [[minimum audibility curve]] as defined in ANSI and other standards, such that the resulting audiogram shows deviation from what is regarded as 'normal' hearing.{{citation needed|date=March 2008}}

; dBQ : sometimes used to denote weighted noise level, commonly using the [[ITU-R 468 noise weighting]]{{citation needed|date=March 2008}}

; [[dBm0]] : Power in dBm (described above) measured at a [[zero transmission level point]].

; [[dBFS]] : dB([[full scale]]) – the amplitude of a signal compared with the maximum which a device can handle before [[Clipping (signal processing)|clipping]] occurs. Full-scale may be defined as the power level of a full-scale [[Sine wave|sinusoid]] or alternatively a full-scale [[Square wave (waveform)|square wave]]. A signal measured with reference to a full-scale sine-wave appears 3&nbsp;dB weaker when referenced to a full-scale square wave, thus: 0&nbsp;dBFS(fullscale sine wave) = −3&nbsp;dBFS (fullscale square wave).

; [[dBZ (meteorology)|dBZ]] : dBZ – decibel relative to Z&nbsp;= 1&nbsp;mm{{sup|6}}⋅m{{sup|−3}} :<ref>{{cite web |title=Terms starting with&nbsp;'''D''' |department=Glossary |publisher=U.S. [[National Weather Service]] |website=weather.gov |url=https://www.weather.gov/jetstream/glossary_d<!-- Former URL: http://www.srh.noaa.gov/jetstream/append/glossary_d.htm --> |access-date=2013-04-25 |archive-url=https://web.archive.org/web/20190808140856/https://www.weather.gov/jetstream/glossary_d |archive-date=2019-08-08 |url-status=live}}</ref> energy of reflectivity (weather radar), related to the amount of transmitted power returned to the radar receiver. Values above 20&nbsp;dBZ usually indicate falling precipitation.<ref>{{cite web |title=Frequently Asked Questions |department=RIDGE Radar |publisher=U.S. [[National Weather Service]] |website=weather.gov |url=https://www.weather.gov/jetstream/radarfaq#reflcolor |access-date=2019-08-08 |archive-url=https://web.archive.org/web/20190331123302/https://www.weather.gov/jetstream/radarfaq#reflcolor |archive-date=2019-03-31 |url-status=live }}</ref>

; dBsm : dB(m{{sup|2}}) – decibel relative to one square meter: measure of the [[radar cross section]] (RCS) of a target. The power reflected by the target is proportional to its RCS. "Stealth" aircraft and insects have negative RCS measured in dBsm, large flat plates or non-stealthy aircraft have positive values.<ref>{{cite web |title=dBsm |department=Definition |website=Everything&nbsp;2 |url=http://everything2.com/title/dBsm |access-date=2019-08-08 |archive-url=https://web.archive.org/web/20190610170944/https://everything2.com/title/dBsm?%2F |archive-date=10 June 2019 |url-status=live }}</ref>

; [[dBc]] : relative to carrier – in [[telecommunications]], this indicates the relative levels of noise or sideband power, compared with the carrier power. Compare dB(C), used in acoustics.

; dBmJ : energy relative to 1&nbsp;[[joule|millijoule]], or 1 milliwatt per hertz.

; [[dBm]] : dB(mW) – power relative to 1&nbsp;[[milliwatt]]. Usually referenced to a 50&nbsp;Ω load, so 0&nbsp;dBm corresponds to 0.224&nbsp;volts.<ref>{{cite book |last=Carr |first=Joseph |author-link=Joseph Carr |year=2002 |title=RF Components and Circuits |publisher=Newnes |isbn=978-0750648448 |pages=45–46 }}</ref> 0&nbsp;dBm&nbsp;= -30&nbsp;dBW.

; dBμV/m, dBuV/m, or dBμ :<ref name="dBμ">{{cite web |title=The dBμ vs. dBu mystery: Signal strength vs. field strength? |date=24 February 2015 |website=Radio Time Traveller (radio-timetraveller.blogspot.com) |type=blog |via=blogspot.com |url=http://radio-timetraveller.blogspot.com/2015/02/the-db-versus-dbu-mystery-signal.html |access-date=13 October 2016 }}</ref> dB(μV/m) – [[electric field strength]] relative to 1&nbsp;[[microvolt]] per [[meter]]. Related to power flux density through the impedance of free space (&eta;{{sub|0}}&nbsp;= 376.73&nbsp;Ω), so 0&nbsp;dB&nbsp;μV/m corresponds to (1&nbsp;μV/m){{sup|2}}/&eta;{{sub|0}}&nbsp;= 2.65x10{{sup|-15}} W/m{{sup|2}}&nbsp;= -145.76&nbsp;dBW/m{{sup|2}}&nbsp;= -115.76 dBm/m{{sup|2}}.

; dBW : dB(W) – power relative to 1&nbsp;[[watt]]. 1&nbsp;dBW&nbsp;= +30&nbsp;dBm.

; dBW/m{{sup|2}} : dB(W/m{{sup|2}}) - power flux density (of electromagnetic radiation) relative to 1&nbsp;W per square meter.

; dBk : dB(kW) – power relative to 1&nbsp;[[kilowatt]], 0&nbsp;dBk = +30&nbsp;dBW = +60&nbsp;dBm.  Not to be confused with dBK, temperature relative to 1 Kelvin.

; dBo : dB optical. A change of 1&nbsp;dBo in optical power can result in a change of up to 2&nbsp;dBe in electrical signal power in a system that is thermal noise limited.<ref>{{cite journal |last1=Chand |first1=N. |last2=Magill |first2=P.D. |last3=Swaminathan |first3=S.V. |last4=Daugherty |first4=T.H. |year=1999 |title=Delivery of digital video and other multimedia services ({{nowrap|> 1 Gb/s}} bandwidth) in passband above the {{nowrap|155 Mb/s}} baseband services on a FTTx full service access network |journal=Journal of Lightwave Technology |volume=17 |issue=12 |pages=2449–2460 |doi=10.1109/50.809663 }}</ref>

; dBd : dB(dipole) – the [[antenna gain|gain]] of an [[antenna (electronics)|antenna]] compared with the gain of a half-wave [[dipole antenna]]. 0&nbsp;dBd&nbsp;= 2.15&nbsp;dBi

; dBq : dB(quarterwave) – the [[antenna gain|gain]] of an antenna compared to the gain of a quarter wavelength whip. Rarely used, except in some marketing material; {{nowrap|0 dBq}}&nbsp;= {{nowrap|−0.85 dBi}}

; dBsm : dB(m{{sup|2}}) – decibels relative to one square meter: A measure of the [[antenna effective area|effective area]] for capturing signals of the antenna.<ref>{{cite book |first=David |last=Adamy |year=2004 |title=EW&nbsp;102: A second course in electronic warfare |series=Artech House Radar Library |place=Boston, MA |publisher=Artech House |isbn=9781-58053687-5 |page=[{{Google books |plainurl=yes |id=-AkfVZskc64C |page=118 }} 118] |url={{Google books |plainurl=yes |id=-AkfVZskc64C }} |via=Google |access-date=2013-09-16}}</ref>

; dBm{{sup|−1}} : dB(m{{sup|−1}}) – decibels relative to reciprocal of meter: measure of the [[antenna factor]].

; dBHz : dB(Hz) – bandwidth relative to one hertz; e.g., 20&nbsp;dBHz corresponds to a bandwidth of 100&nbsp;Hz. Commonly used in [[link budget]] calculations. Also used in [[carrier-to-receiver noise density|carrier-to-noise-density ratio]] (not to be confused with [[carrier-to-noise ratio]], in dB).

; [[dBFS|dBov or dBO]]: dB(overload) – the amplitude of a signal (usually audio) compared with the maximum which a device can handle before [[Clipping (signal processing)|clipping]] occurs. Similar to dB FS, but also applicable to analog systems. According to ITU-T Rec. G.100.1 the level in dB ov of a digital system is defined as: <math display = "block">L_\mathsf{ov} = 10 \log_{10} \left( \frac{ P }{\ P_\mathsf{max}\ } \right)\ [\mathsf{dB_{ov}}],</math> with the maximum signal power <math>P_\mathsf{max} = 1.0</math>, for a rectangular signal with the maximum amplitude <math>x_\mathsf{over}</math>. The level of a tone with a digital amplitude (peak value) of <math>x_\mathsf{over}</math> is therefore <math>L_\mathsf{ov} = -3.01\ \mathsf{dB_{ov}}</math>.<ref>{{cite report |title=The use of the decibel and of relative levels in speech band telecommunications |date=June 2015 |id=ITU-T Rec. G.100.1 |publisher=[[International Telecommunication Union]] (ITU) |place=Geneva, CH |type=tech spec |url=https://www.itu.int/rec/dologin_pub.asp?lang=e&id=T-REC-G.100.1-201506-I!!PDF-E&type=items }}</ref>

; dBK{{sup|−1}} or dB/K : dB(K{{sup|−1}}) – decibels relative to 1&nbsp;K{{sup|−1}}.<ref>{{cite book |first=Ali Akbar |last=Arabi |year= |title=Comprehensive Glossary of Telecom Abbreviations and Acronyms |page=[{{Google books |plainurl=yes |id=DVoqmlX6048C |page=79 }} 79] |url={{Google books |plainurl=yes |id=DVoqmlX6048C }} |via=Google |access-date=2013-09-16 |df=dmy-all }}</ref> — ''not'' decibels per kelvin: Used for the {{mvar|{{sfrac| G | T }} }} [[G/T|(G/T) factor]], a [[figure of merit]] used in [[satellite communications]], relating the [[antenna gain]] {{mvar|G}} to the [[receiver (radio)|receiver]] system noise equivalent temperature {{mvar|T}}.<ref>{{cite book |first=Mark E. |last=Long |year=1999 |title=The Digital Satellite TV Handbook |place=Woburn, MA |publisher=Newnes Press |page=[{{Google books |plainurl=yes |id=L4yQ0iztvQEC |page=93 }} 93] |url={{Google books |plainurl=yes |id=L4yQ0iztvQEC }} |access-date=2013-09-16 |df=dmy-all }}</ref><ref>{{cite book |first=Mac E. |last=van&nbsp;Valkenburg |date=2001-10-19 |df=dmy-all |title=Reference Data for Engineers: Radio, electronics, computers, and communications |series=Technology & Engineering |editor-first=Wendy M. |editor-last=Middleton |place=Woburn, MA |publisher=Newness Press |isbn=9780-08051596-0 |page=[{{Google books |plainurl=yes |id=U9RzPGwlic4C |page=SA27-PA14 }} 27·14] |url={{Google books |plainurl=yes |id=U9RzPGwlic4C }} |via=Google |access-date=2013-09-16}}</ref>

; dBa : see [[dBrn adjusted]].

; [[dBFS]] : dB([[full scale]]) – the amplitude of a signal compared with the maximum which a device can handle before [[clipping (signal processing)|clipping]] occurs. Full-scale may be defined as the power level of a full-scale [[sine wave|sinusoid]] or alternatively a full-scale [[Square wave (waveform)|square wave]]. A signal measured with reference to a full-scale sine-wave appears 3&nbsp;dB weaker when referenced to a full-scale square wave, thus: 0&nbsp;dBFS (fullscale sine wave)&nbsp;= −3&nbsp;dBFS (full-scale square wave).

; dBiC : dB(isotropic circular) – the forward gain of an antenna compared to a [[circular polarization|circularly polarized]] isotropic antenna. There is no fixed conversion rule between dBiC and dBi, as it depends on the receiving antenna and the field polarization.

; dBm{{sup|2}} or dBsm : dB(m{{sup|2}}) – decibel relative to one square meter

; dBq : dB(quarterwave) – the forward gain of an antenna compared to a quarter wavelength whip. Rarely used, except in some marketing material. 0&nbsp;dBq&nbsp;= −0.85&nbsp;dBi

; dBsm : see dBm{{sup|2}}

; dBu or dBv : RMS voltage relative to <math>\ \sqrt{0.6\ }\ \mathsf{V}\ \approx 0.7746\ \mathsf{V}\ \approx -2.218\ \mathsf{dB_V} ~.</math>

; dBuV : see dBμV

; dB W·m{{sup|−2}}·Hz{{sup|−1}} : [[Jansky#dBW·m−2·Hz−1|spectral density]] relative to 1 W·m{{sup|−2}}·Hz{{sup|−1}}<ref>{{cite web |title=Units and calculations |website=iucaf.org |url=http://www.iucaf.org/sschool/mike/Units_and_Calculations.ppt |last1=Davis |first1=Mike |format=PPT |date=June 2002 |access-date=2025-03-12 |url-status=live |archive-url=https://web.archive.org/web/20160303223821/http://www.iucaf.org/sschool/mike/Units_and_Calculations.ppt |archive-date=2016-03-03 }}</ref>

; dBHz or dB-Hz : dB(Hz) – bandwidth relative to one [[hertz]]

; dBHz{{sup|2}} or dB/s{{sup|2}} : dB(Hz{{sup|2}}) – the squared magnitude of an impulse response (or impulse response envelope) relative to the squared magnitude of an impulse response with unity amplitude.

; dBK{{sup|−1}} or dB/K : dB(K{{sup|−1}}) – decibels relative to [[multiplicative inverse|reciprocal]] of [[kelvin]]

; dBm{{sup|−1}} : dB(m{{sup|−1}}) – decibel relative to reciprocal of meter: measure of the [[antenna factor]]

; mBm : {{anchor|Millibel}} mB(mW) – power relative to 1&nbsp;[[milliwatt]], in millibels (one hundredth of a decibel). 100&nbsp;mBm = 1&nbsp;dBm. This unit is in the Wi-Fi drivers of the [[Linux]] kernel<ref>{{cite web |title=Setting TX power |series=en:users:documentation:iw |website=wireless.kernel.org |url=http://wireless.kernel.org/en/users/Documentation/iw#Setting_TX_power }}</ref> and the regulatory domain sections.<ref>{{cite web |title=Is your Wi Fi ap missing channels&nbsp;12 and 13&nbsp;? |date=16 May 2013 |website=Pentura Labs |via=wordpress.com  |url=http://penturalabs.wordpress.com/2013/05/16/is-your-wifi-ap-missing-channels-12-13/ }}</ref>

* [[Day–evening–night noise level]]

* [[Day-night average sound level]]

* {{section link|One-third octave|Base 10}}

{{decibel}}

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