Talk:Decibel

120 dB breaks eardrums?

This sentence doesn't make sense: "In air, sound pressure levels above 85 dB are considered harmful, while 95 dB is considered unsafe for prolonged periods and 120 dB causes an immediate perforation of the ear drum (tympanic membrane)." But the table that follows lists that as the threshold of pain. -- jun

Introductory reasons

Okay, my phrase 'This is a direct result that many of our perceptory organs (like hearing and sight) are logarithmic in nature, therefore the dB is a more natural unit.' has been deleted due to '(rv badly phrased interjection)'. Care to elaborate, Light current?

Bel vs. Decibel, which came first?

The part until the next line has been moved over from Talk:Decibel: Earlier I thought Alexander Graham Bell coined the term "bel" as a measurement of sound and that it was later determined to be so coarse that 1/10th of it proved more useful (the decibel). On seeing someone on this page claim that Bell coined the phrase "decibel" I looked up its history in the Oxford English Dictionary. The OED has their earliest recorded uses in 1928 and 1929; Bell died in 1922.

• the earliest few quotations the OED has on file for "decibel": "1928 Electrical Communication VII. I. 33/2 If common logarithms are used, the reproduction is obtained in Decibels. 1929 W. H. MARTIN in Bell System Techn. Jrnl. VIII. 2 The Bell System has adopted the name ?decibel? for the ?transmission unit?, based on a power ratio of 10·1... For convenience, the symbol ?db? will be employed to indicate the name ?decibel?. 1930 Discovery Dec. 398/2 The band-pass filter, which follows the low frequency modulator, allows the lower side-band to pass with an attenuation of six decibels."
• the earliest few quotations the OED has on file for "bel": "1929 W. H. MARTIN in Bell System Techn. Jrnl. VIII. 2 It was further suggested that the naperian unit be called the ?neper? and that the fundamental decimal unit be called the ?bel?, these names being derived from..Napier..and Alexander Graham Bell. 1930 Gloss. Terms Electr. Engin. (B.S.I.) 13 The bel is a unit used in the comparison of the magnitudes of power, voltages or currents at two different points in a network of lines or apparatus."

Clearly the term "bel" was in use before the term for 1/10th of it came about. Yet it seems odd to me that the terms did not see print for almost a decade. Anyone? Specifically, I'd like to know who made the suggestion for the terms "napier" and "bel." I suspect it wasn't Alexander Graham Bell. --Koyaanis Qatsi

I've generalised the article a bit, since decibels are not just used for acoustics (e.g. they're used to measure the gain of amplifiers and loss of transmission lines.) -- DrBob

Think we ought to change the title to Bels? Scienceman123 20:40, 20 April 2006 (UTC)

No. We moved it here from bel because effectively no one uses bels. — Omegatron 20:52, 20 April 2006 (UTC)

Acoustic decibel reference

How likely is it, if I find a claim that a sound is at, say, 120dB, that the reference level is indeed 20 micropascals? Similarly, if author A claims that one sound is so many dB, and author B claims that one sound is so many dB, how safe is it to compare the measures given, if neither indicates the reference level? --Ryguasu 04:38 Feb 26, 2003 (UTC)

---

Assuming they are both talking about sound in air and that they are not trying to obscure, it's fairly safe. The standard reference for sound in air has been 20 micropascals since the early 1920s. Technically, of course, decibels without a reference have no meaning.

Watts vs watts per square meter

This 0 (zero), decibel level also corresponds to one billionth of a watt, 0.000 000 000 001 watt, roughly a mosquito flying 10 feet away.

Watt would be total power, not related to distance; also http://ccms.ntu.edu.tw/~karchung/decibels/decibels1.ppt says 40 dB. - Patrick 09:52 Apr 16, 2003 (UTC)

List of acoustic decibel levels

It'd be nice to have a list of example decibel levels on this article - Khendon

Decibel vs Bel, separate article for acoustics or not

1. Do acoustic decibels and general decibels really need to be separate articles?
2. Since decibel is much more commonly used than bel, shouldn't it be the title of the article? kind of like kilogram being the standard SI unit, even though it is a basic unit with a prefix.

- Omegatron 00:47, Apr 17, 2004 (UTC)

Agreed Omegatron, I think this article should move to decibel - anyone disagree? -- Rissa 00:45, 6 Jun 2004 (UTC)

I am moving it back to decibel. It is much more common. it sounds like the richter scale uses bels without calling it as such, but EVERYTHING else uses decibels. - Omegatron 14:12, Jun 27, 2004 (UTC)

20 micropascals or 2 pascals???

This seems to say that dB SPL is referenced to both 20 micropascals and 2 pascals. I'm sure the standard (used for dBA, etc.) is only one of those. Which is it? - Omegatron 17:43, Nov 23, 2004 (UTC)

20 micro. someone put in 2 N/m^2 without any exponent for some reason. fixed now. - Omegatron 17:51, Nov 23, 2004 (UTC)

which dB?

Please make sure the reference for all values listed as only dB are clear from context. (dB SPL, dBu, etc.) - Omegatron 17:43, Nov 23, 2004 (UTC)

The treatment of sound pressure level appears to be inconsistent with standard reference works across Wikipedia. Both Kinsler and Frey's "Fundamentals of Acoustics" (2nd edition) and Robert Urick's "Principles of Underwater Sound" (3rd edition) indicate that a measured intensity is a level (Urick p.15) or sound pressure level (K&F) relative to a reference effective pressure (K&F pp.125-126). Both of these sources recommend reporting decibels with an explicit listing of the reference effective pressure, like so: "74 dB re 20 micropascals", where the number and units following re is the reference effective pressure. I was working on the decibel article's misuse of SPL as if it specified a reference effective pressure when I realized that several other articles were similarly affected. I don't have time at the moment to correct all of them, but I will drop this comment into the discussion part of each article that I have come across so far with this problem. Level or sound pressure level in both these standard texts simply refer to a measurement in the sound field and are not indications of a specific reference pressure upon which the decibel is based. In other words, "dBSPL" is an incorrect means of attempting to refer to the in-air reference effective pressure. In no article thus far have I seen the "dBSPL" usage tied to an authoritative source. By contrast, the "dB re" formalism is common to both standard reference works that I have cited. I have worked over the "Acoustics In Air" section up to the table of very high SPL values; there is more work to be done to fix the whole article, though. Wesley R. Elsberry 11:23, 9 April 2006 (UTC)

Other sites using the "dB re" formalism: Oceans of Noise (explicit in defining SPL and SIL in terms of "dB re"), SURTASS LFA, NIST listing SPL in terms of "dB re", and Acoustic Impacts on Marine Mammals. But the best thing I've found has to be ASACOS Rules for Preparation of American National Standards in ACOUSTICS, MECHANICAL VIBRATION AND SHOCK, BIOACOUSTICS, and NOISE, which states:

3.16 Unit symbols

3.16.1 When to use unit symbols

In the text of the standard, the unit symbol for a quantity shall be used only when the unit is preceded by a numeral. When the unit is not preceded by a numeral, spell out the name of the unit. In text, even when a numerical value is given, it is desirable to spell out the name of the unit. Moreover, the name shall be spelled out when it first appears in the text, and more often if the text is lengthy.

Thus, in text write "...a sound pressure level of 73 dB; or "...a sound pressure level of 73 decibels." Do not write "sound pressure level in dB"; the correct form is "sound pressure level in decibels." Do not write "dB levels", "dB readings", or "dB SPL."

Levels or readings are not of decibels; they are of sound pressure levels or some other acoustical quantity. Write out the word "decibel" for such applications, and be sure that the word 'decibel' follows, not precedes the description of the relevant acoustical quantity.

The guidelines given for the National Standards clearly excludes the use of "dB SPL". Wesley R. Elsberry 17:03, 9 April 2006 (UTC)

The supposed "better reference" for use of "dB SPL" added to the decibel article ends up being a document that merely includes "dB SPL" in a list of terms. The glossary within the same document does not even list this supposed term, even though weighted decibel terms are defined. The glossary in the file does have an entry for "sound pressure level", which is

Sound pressure level: (1) Ten times the logarithm to the base ten of the ratio of the time-mean-square pressure of a sound, in a stated frequency band, to the square of the reference sound pressure in gases of 20 micropascals (µPa). Unit, dB; symbol, Lp. (2) For sound in media other than gases, unless otherwise specified, reference sound pressure in 1 µPa (ANSI S1.1-1994: sound pressure level).

Notice that the unit is "dB", NOT "dB SPL". The inclusion of "dB SPL" in their list of terms does NOT establish that their usage is correct, and even their own reference of the ANSI standard indicates that their usage is incorrect. SPL refers to a measurement, and is NOT an indication of the reference effective pressure. The ANSI standard referenced makes this clear, as SPL is defined as being used for other reference effective pressures, too. Wesley R. Elsberry 16:22, 10 April 2006 (UTC)

I have changed the article following your comments. Han-Kwang 17:22, 10 April 2006 (UTC)

Theatre?

One of the values in the table is "Theatre". That's too vague to be at all useful (to me, at least). An empty theatre? A filled theatre of whispering people? Sitting 5m away from Hamlet giving his soliloquy? A theatre showing a Jerry Bruckheimer movie? (From context, probably not the last one, but that's still a decent range.)

It's not a very explanatory entry, and would be better replaced with something more obvious. 30 dB is fairly quiet, the sort of environment where most people could fall asleep. The table entry probably means a theatre full of people who aren't intentionally making any noise at all (just breathing and rustling about in those uncomfy theatre seats as they wait for the show to start - of course all polite people cease their conversation at that point ;). Unfortunately I can't think of an equivalent value that would be more descriptive at the moment. - toh 19:04, 2005 August 22 (UTC)

Kerbside vs Curbside

The original spelling in this article was by Heron on 18 Nov as Kerbside. This is fine. Unless you wish to remove the entire entry, please leave the spelling in its original sense. Ian Cairns 00:29, 8 Dec 2004 (UTC)

Not that it's terribly important, but the policy seems to be one of consistency for the whole page:

• Wikipedia:How_to_copyedit#Correcting_spelling
• Wikipedia:Tutorial_(Keep_in_mind)#U.S._English_vs._British_English

- Omegatron 15:19, Dec 8, 2004 (UTC)

dBA incorrect?

according to whom? - Omegatron 02:57, Dec 12, 2004 (UTC)

Exactly my question. The link is to dB(A), and that's what I've seen the manufacturers use ([1]).--Jerryseinfeld 18:38, 23 Jan 2005 (UTC)

Well yeah, manufacturers use it all the time (not just "historical sources"). Someone is claiming that it's not official, though, because it implies a reference to an "A" unit, like dBV implies a reference to a volt. Either this is a small minority opinion, or it's a new opinion that just hasn't gained a foothold yet, like binary prefixes like "mebibyte". My personal opinion is that it's fine to use it the way it is. It's just a shorthand way of saying it's A-weighted. Like saying miles (survey). In addition, the standard acoustic version is just "dB" with no qualifier (it should be dB SPL or dBSPL or dB(SPL)) which leads laymen to believe that a dB is a unit by itself. I would much rather see people using dB SPL than see them stop using dB (A). - Omegatron 20:08, Jan 23, 2005 (UTC)

Unit symbol dB rather than dBA, dB(A) etc

When making acoustical measurements and determining, for example, an A-weighted sound pressure level, the measured sound pressure is still compared with the reference sound pressure (20 μPa), and the unit symbol should still be dB. This is standard usage in definitions given in modern ISO and IEC standards, and is now mandated for ANSI standards developed by the Acoustical Society of America in their ASACOS rules. The preferred form of expression is "the A-weighted sound level was xx dB" or L_A = xx dB.

There are many historical sources which use "dBA", or "dB(A)" or talk about "dBA levels", and these should be understood as indicating A-weighted sound levels, although the unit symbol is incorrect.

Unit symbols such as dBu or dBm indicate the reference value used to determine the level, and thus are correct. But the A denotes a frequency weighting, not a reference quantity.

Potential inclusion in SI

Any citation to support the claim that "(BIPM) has recommended its inclusion in the SI system."?

• This doesn't even seem plausible to me—it's outside BIPM's bailiwick. More likely the Comité International des Poids et Mesures (CIPM), or the Consultative Committee on Units which is, I think, under the CIPM and advises the CGPM

Also, does that recommendation include the neper as well as the decibel? Gene Nygaard 01:51, 19 Jan 2005 (UTC)

RMS

"dB(0.775 V)—(usually RMS) voltage amplitude"

Is it ever not RMS? Every site I see says it is referenced to RMS. - Omegatron 02:50, Jan 19, 2005 (UTC)

---

Yes, it may be the instantaneous sound pressure - eg, in measurements of sounds such as gunshots or sonic booms. Richardng 17:34, 2 June 2006 (UTC)

Is this website violating the GNU License??

http://encyclopedia.laborlawtalk.com/Decibel

if so, i find it funny that a "law" site is guilty of copyright violation.

No, it does not appear so. 203.26.206.129 07:09, 13 Apr 2005 (UTC)

Read the bottom of the page: "This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Decibel"." RoceKiller 10:16, 14 Apr 2005 (UTC)

dB Doubling Versus "Perceived" Doubling

So I always thought that 10dB represented a doubling of the sound intensity but now I find out that this may be technically incorrect. That 3dB represents a true doubling of sound pressure and that 10dB is where the human ear perceives the pressure as doubling. Can anyone confirm this?

It would be nice for the article to clarify this.

I'm not sure of the actual answer, but these are where you should look. I'm going to try to figure it out, too: 3.01 dB corresponds to a doubling of sound power, not sound pressure. How we perceive it depends on amplitude and frequency, and would be derived from the equal-loudness contour. Also see the units of perceived loudness, phon and sone. - Omegatron 16:36, Apr 29, 2005 (UTC)

Wow. You helped me realize there are a lot of confusing, overlapping sound measurement articles. I've created a template to keep them tied together: Template:Sound measurements - Omegatron 16:49, Apr 29, 2005 (UTC)

Here is the story on this: 10 dB in acoustic terms, at 1 kHz corresponds to "twice as loud" Actually it should be 9 dB, but people get sloppy and use 10 dB instead. This goes back to Harvey Fletcher, and the article by Fletcher and Munson (1933) in Jol of the Acoust. soc of Am.

 Jont Allen jun 11, 2006

Amplitude vs power

I'm still sketchy on the relationship between power and amplitude and dB. You can't convert from a power to a dB to a voltage, for instance, unless you know a load resistance. I'm especially confused because you can't convert amplitude --> dBFS --> power in digital land which doesn't even have load resistance. What does "power" even mean for digital?? - Omegatron 20:13, Apr 29, 2005 (UTC)

YEEARG - and I had explicitly edited this article to correct this, and it has been reverted.

decibels are DEFINED AS 10log10(x) - PERIOD. This 20log10 crap is WRONG! dB are NEVER 20log10 of ANYTHING.

Now, when you are talking about dBWatts (or any derived units like dBmW) you can compute the CHANGE in level by computing 20log10(V1/V2), but that is a simplification of the full formula 10log10( (V1*V1/R)/(V2*V2/R) ).

But it is also perfectly valid to speak of dBV - and a 10 dB increase of a 0dBv signal yeilds 10 volts, NOT 100!

dB absent any unit is a RELATIVE measurement - I can speak of a 6dB increase of my bank account (a thing much to be desired), but I cannot speak of having 6dB in my bank account - I can say I have 36dB$ in the bank, but I *have* to supply a unit for an absolute measure to be meaningful.

I design test equipment for a living, and this confusion causes us NO END of problems. People will increase the deivation of an FM signal by 2, and expect to see the audio spectrum analyzer increase by 6 dB. The spec-an is measuring deviation, so a x2 increase is 10log10(2) = 3dB. To get a 6dB increase we would have to be reporting (kHz deviation)^2 - now what physical property does that describe?

Please - dB is 10log10(x) ALWAYS - not 20log10!

Revert the reversion of my changes, please!

N0YKG 10 May 2005

Here's your chance to be bold and put the above explanation why 20 log 10(X/Xref) is wrong into the article. Though I admit I don't see why it's wrong to say 20 log ((V1/V2)) as opposed to 10 log((V1/V2)^2), assuming the circuit impedance is the same. dB should always be used to refer to ratios of power quantities, not to ratios of amplitude quantities...but we often cheat and say 20 log (amplitude/reference amplitude) --Wtshymanski 19:02, 10 May 2005 (UTC)

On the one hand, you say dB should only be used for power quantities, but then you say that dB could be used for your bank account. Also:

• 0 V_RMS = 0 dBV
• 10 V_RMS = 20 dBV
• 100 V_RMS = 40 dBV

According to this, which claims to be based on the ANSI T1.523-2001 definitions, which I would download, except it costs $175 - Omegatron 20:21, May 10, 2005 (UTC)

Mark Phillips 28 June 2005

Ref the statement above that "a 10 dB increase of a 0dBv signal yeilds 10 volts, NOT 100!", a 10 dB increase means an increase in power of 10^(10/10), i.e. an increase in power of 10 times. Assuming that the circuit impedance doesn't change whilst this increase is happening (nearly always true when the 'before' and 'after' measurements are made at the same place in a circuit or system), then to achieve this 10 times increase in power the voltage must increase by the square root of 10 (because power is proportional to the square of the voltage, for a constant impedance), i.e. by a factor of 3.162 (approx). Since, in your example, you started off with a voltage of 0 dBV, i.e. 1 volt, the 10 dB increase will actually yield 3.162 volts (approx). (Note that I have written 0 dBV rather than 0 dBv, because from the context I think that is what the author meant. The lower case v is now falling out of usage in favour of a lower case u - these indicate a reference level of 0.775 volts rather than the 1 volt reference indicated by the upper case V. On a finer point it is always preferable to set a space between the value and the units, in line with SI standards, although the decibel is not yet accepted as a 'unit' by SI.)

Excellent. Thank you. - Omegatron June 29, 2005 23:07 (UTC)

Be aware that there is a slight usage inconsistency between different fields. Originally, dB was always strictly a measure of relative power. This usage is preserved in physics. One can use 20·log₁₀ of an amplitude ratio, but only when the "impedences" are the same, so that the dB result is still a measure of power. This usage is not strictly preserved in engineering, where for example electrical engineers may express a ratio of voltages as $20\,\log _{10}(V_{1}/V_{2})$, even when V₁ and V₂ are measured at different points in the circuit, where the impedence differs. Anyone who calls $10\,\log _{10}(V_{1}/V_{2})$ a result in "dB", however, is simply mistaken. This unfortunate usage does, however, occur from time to time in engineering literature.--Srleffler 05:00, 28 November 2005 (UTC)

Confusion over this crept back into the "definition" section of the article. I have tried to resolve it. Again, for those unfamiliar with this: in physics, decibels are strictly a measure of power or intensity ratio. They are never a measure of voltage or amplitude. (This is not the case in engineering.) As long as the impedence is constant, $20\,\log _{10}(V_{1}/V_{2})$ is equal to the power ratio. If the impedence is not constant, it would be incorrect in physics to call the result a value in "dB".--Srleffler 22:20, 4 January 2006 (UTC)

--- "This 20log10 crap is WRONG! dB are NEVER 20log10 of ANYTHING." Decibels are used for all sorts of physical quantities - some where the power concept is obvious and others, such as sound pressure and vibratory acceleration, where it is not so obvious. I agree completely that decibels always express a power-like ratio - ie, 10log(power-like ratio). But decibels are widely used for many non-power quantities, such as sound pressure when it becomes 10log(ratio of pressure squared) and hence 20log(pressure ratio). So it's not wrong to say 20log(pressure ratio) but writing it that way tends to facilitate confusion by encouraging people to think that "sometimes it's 10log and sometimes it's 20log. For that reason I edited the sound pressure level page which originally said something like SPL = 20log(pressure ratio) = 10log(pressure squared ratio) so that it became SPL = 10log(pressure squared ratio) = 20log(pressure ratio). I think we should always use 10log() as the starting point in Wikipedia - not because 20log is wrong, but because 10log keeps the decibel concept clear and consistent. Richardng 17:35, 2 June 2006 (UTC)

modern usage of bel

"According to the World's standardization organizations should sound power level regarding computers and other kinds of IT equipment be expressed in bels (B) instead of in decibels (dB). Sound power level is here expressed in bels for to avoid confusion between decibels for sound power level and decibels for sound pressure level [1,2,3,4,5]. The computer industry is the only product group that uses sound power in bels, even if other product declaration standards tell that one can use bel for stating sound power level for to avoid confusion with sound pressure level measures." - bels for sound power level

This is the only modern usage I can find of bels being used instead of decibels. - Omegatron 14:37, July 25, 2005 (UTC)

Krakatoa dB level

According to article linked site [2]:

310 (Normalized) KRAKATOA VOLCANO ERUPTION-1883 A.D.

The table in this article:

1,000 Krakatoa (1883)

The linked article at least has references listed; I wonder where the reference for the 1000 dB Krakatoa is. It sounds (no pun intended...) like an awful lot anyway, when similar extreme volcano eruptions are also listed at 300 dB there. -- Jugalator 21:55, 8 November 2005 (UTC)

I'm going to change it to 310. 1000 is a ridiculous amount, given the exponential nature of the decibel system. Crovax 04:23, 10 November 2005 (UTC)

Measurements like this need a distance associated with them. Meaningless otherwise. — Omegatron 16:25, 11 November 2005 (UTC)

Here's some trustable measurements: [3], though they are of infrasound. — Omegatron 16:57, 11 November 2005 (UTC)

Talk:Krakatoa#Loudness

Definition

Somehow I didn't notice that the definition was changed from a power ratio to an acoustics measurement. [4] I think it should be a general power ratio. — Omegatron 22:36, 8 November 2005 (UTC)

Intensity and pressure

I deleted the statement "Neither ear drums nor microphones can convert sound intensity. We hear the pressure variations.", on the grounds that it is meaningless. There is only one physical phenomenon--molecules of air move and displace the eardrum, leading to detection of "sound". One can choose to characterize the motion of the molecules by their pressure or by the intensity of the wave. Which you choose is irrelevant. One can't even say that the response of the ear is proportional to pressure rather than intensity, since the response is logarithmic and log(pressure) is proportional to log(intensity).--Srleffler 06:42, 28 November 2005 (UTC)

You can and should say that the ear responds to sound pressure rather than intensity. Although sound pressure is proportional to intensity in a plane progressive wave, it is not always true. Sound pressure is a scalar quantity and intensity a vector. So, in a perfectly diffuse sound field (equal amounts of sound travelling is all directions, approximated by a highy reverberent space) you may have a high sound pressure, but very low sound intensity in any direction. RNG 2006-06-02—The preceding unsigned comment was added by Richardng (talk • contribs) .

Interesting. You may be right. If this were going to appear in the article, the scalar-vs-vector distinction would need to be explained.--Srleffler 15:32, 2 June 2006 (UTC)

dBZ?

Where would dBZ fit in or is it an alias for something else here? [5] defines it as (under "Base reflectivity"):

decibels of Z, where Z represents the energy reflected back to the radar

Cburnett 23:08, 1 December 2005 (UTC)

"dBZ" an alias for "dBr", perhaps? Cburnett 23:09, 1 December 2005 (UTC)

Correct explanation of dBm and dBu

dBm (or dBmW) and dBW are independent of impedance. 1mW is 1mW regardless of the impedance it driven into. RF engineers typically use dBm to measure the power into a 50Ω load.

dBu (or dBv) is dependant on 600Ω when you are trying to relate it to dBm, because dBm = dBu when the load used is 600Ω. 600Ω is not important to audio, rather it originated from the telephone companies because it is the characteristic impedance of telephone wire when stretched miles (or kilometers) over land. Somewhere along the way (I have no clue why) audio started using the same measurements even though the characteristic impedance of a microphone cable is rarely 600Ω let alone consistent or even long enough for this to matter. I suspect this came from the fact that some of the first audio engineers came from the telecom industry or were educated in universities teaching the practices used in the telecom world. The use of dBu in audio is diminishing greatly because it is being substituted for the more appropriate dBV as output and input stages are being properly designed for voltage transfer and not power transfer.

File:P-v2r.jpg

File:V-pr.jpg

File:0755VRMS.jpg

This is also the first time I've heard that the "u" in dBu means "unloaded." I've been told that the "u" was used to remove the "v" to avoid confusion with dB(1mV). Can anyone confirm this? --Babar77 11:17, 11 December 2005 (UTC)

Why would dBV be more appropriate than dBu?

I heard the same about the u being a replacement for lowercase v, though maybe "unloaded" is true, too. — Omegatron 00:50, 5 January 2006 (UTC)

I'm not an electrical engineer, but dBV seems like a more straightforward, intuitive unit, being based on the ratio of the voltage to 1 V. dBu, on the other hand, is based on the ratio of voltage to the odd value of 0.775 V. If you want something that agrees with dBm, just use dBm.--Srleffler 03:17, 5 January 2006 (UTC)

I am an electrical engineer who designs professional audio equipment, and I can verify what Srleffler said is true. dBV is more appropriate because you can do the Volts to dBV conversion in your head, easily. It reduces stupid arithmetic errors when designing under the gun and makes more sense. It's also getting harder to find test equipment that displays Voltage in dBu, and half the time it's wrong. The old HP8903B audio analyzer marks it's dBu measurement as dBm, which is completely wrong because it does not guarantee the load is 600Ω. This has caused A LOT of confusion in the office over the years and forced me to teach this on countless occasions.

Srleffler, I'm not sure if you are saying to make a voltage measurement using a power unit, but in case you are or any other reader is thinking the same thing I must say this is incorrect. If you are talking power use dBm, and if you are talking voltage, use dBu. This is because they have different scales and it is technically accurate. If it were up to me, I would remove dBu all together or make a small note of it for historical reasons only. It may have been very useful back in the day, but it needs to die - trust me. --Babar77 17:37, 5 January 2006 (UTC)

Hmm.. I am a (relatively inexperienced?) electrical engineer who designs professional audio equipment, and see dBu much more often than dBV. Just curious why someone would say that one is inherently better than the other.

Your explanation makes sense. Head calculations and conversions mean nothing to me, though; that's what computers are for.  :-) — Omegatron 18:58, 5 January 2006 (UTC)

Sorry for the confusion, Babar77. When I wrote "just use dBm", I meant that if one wants something that agrees with dBm, one should give a power ratio, not a voltage ratio (allowing that power ratio can be determined from voltage measurements). In general, I despise all use of dB to represent anything other than a power or intensity. If it were up to me, I would abolish not only dBu, but also dBV. Alas, convention and habit sometimes leave us stuck with inappropriate or confusing units. --Srleffler 02:13, 6 January 2006 (UTC)

I didn't say that dBu is gone, it's just going away and fairly quickly where I work. Largely because the only test equipment with dBu is audio analyzers, but not all. Try finding a fluke meter or any DMM these days that has dBu. Plus, even if you are using a calculator, it's still a lot easier to use dBV - less key strokes. Most of the newer mixing consoles I've seen don't even label what dB they are using.

Srleffler, if it were up to me, I would expand the use of dB, or some sort of logarithmic scale. I find using dB to be as generic as using percent, and easier. In fact, because most - if not all - of the human senses are logarithmic in nature, it makes sense to use it whenever you are dealing with human perception - like audio or light. Personally, I would go nuts if I had to design all my audio amps (voltage or power) using linear scales. I think it is safe to say the rest of the audio industry would agree, because any audio guy I've met speaks dB quite fluently. In fact if the consumer world used dB instead of Watts when spec’ing audio amps, people probably would not be getting ripped off when they spend the extra 50 bucks on a 250 Watt (24.0dBW) amp instead of buying the 225 Watt (23.5dBW) amp. Labeling the gain on a microphone preamp (which is voltage) would be a nightmare. Imagine having to label the knob from 3.16x to 1778x instead of 10dB to 65dB. Or trying to say your noise floor is 316,227 times lower than your max signal as opposed to saying your dynamic range is 110dB. I wish LED manufacturers would spec the intensity of an LED in some sort of dB instead of the linear "lumens" or "candela," because your eye is logarithmic as well. Anyway, enough of my rant - dB the planet! :-) --Babar77 06:54, 6 January 2006 (UTC)

Logarithmic units are great. What I object to is the unfortunate custom that has arisen in engineering, of calling any logarithmic ratio "dB". It leads to unnecessary confusion and mistakes, like mixing up when to use 10·log and when to use 20·log. Decibels were originally quite specific, a measure of relative power. The 20·log form only really makes sense in this context. Once you get away from dB as relative power, it's not always clear whether to use 10x or 20x. Physicists use logarithmic expressions of values all the time, we just don't call them all "dB". It's more common to simply plot log(x), or to plot x on a logarithmic scale. Anyway, this is just a minor rant, and I'm not seriously proposing to change the units everyone uses now, after the fact. I at least applaud efforts to be clear by specifying dBV, for example, when the quantity expressed is a voltage ratio that doesn't correspond to an actual power ratio (due to differing impedences). --Srleffler 07:24, 6 January 2006 (UTC)

By the way, does that microphone preamp go from 10dB to 65dB, or from 10dBV to 65dBV?--Srleffler 07:27, 6 January 2006 (UTC)

dB is being used as the generic logarithmic scale in the absence of anything else. I think the mistake was made when people agreed on the 20x for Voltage dB. They should have just left it at 10x for everything, I think it actually would have made more sense. BTW- I knew you were a physicist, you puritan! :-) That mic gain is dB not dBV because it is unitless and relative to the input signal, not a constant voltage. It's like saying, take whatever is coming in and add 20dB, or multiply it by 10. Not take what ever signal is coming in and make it 20dBV, or 10Vrms. If that was the case you would never hear a change in the volume of anything. In other words, a whisper and a shout would sound the same volume. BTW - That's called compression, which would take a whole other paper to explain completely.--Babar77 07:41, 6 January 2006 (UTC)

Just had a thought. Srleffler, you want to get together and create a generic logarithmic unit that can be used for anything? We could say that it would always be relative to 1 of the whatever the linear unit is. We can call it SrBabs or Wiki's. So the calculation would be 10*log(whatever) = 0 SrBab or 0 Wiki. So if we biked 100km, we could say that we biked 20 SrBabs(km) or 20 Wiki(km). BTW- I'm not poking fun at you, I'm just having fun in general. --Babar77 08:00, 6 January 2006 (UTC)

I wish I could access this:

Logarithmic units: a need in acoustics

A H Davis 1934

Abstract. In view of the confusion which prevails in the use of logarithmic units, the paper suggests the use of a new unit, to be named the "brig", for the ratio of two quantities, together with certain subsidiary changes, particularly in the nomenclature of acoustics. — Omegatron 17:08, 6 January 2006 (UTC)

Back to the u in dBu

• [This reference originally was labeled dBv (lower-case) but was too often confused with dBV (upper-case), so it was changed to dBu (for unterminated).] [6] copied? copied? copied?
• dBu
  The reference level is 0.775 volt rms across any impedance. See dBm above for the origin of the value. The "u" stands for unterminated. [7]
• This reference originally was the dBv and was often confused with the dBV, so now it is called the dBu (for unterminated), [8]
• The origin of the index of dBu is from "u = unloaded" and of dBV is from "V = 1 volt". Some say:
  The "u" in dBu implies that the load impedance is unspecified, unterminated, and is likely to be high. [9]
• dBu represents the level compared to 0.775 Volts RMS with an unloaded, open circuit, source (u = unloaded). [10]
• The corresponding unit for use in circuits where the exact impedance is unknown or irrelevent is the dBu. The "u" stands for "unloaded". [11]
• the new term dBu (u meaning unloaded) [12]

Summary: I have no idea. — Omegatron 15:28, 25 January 2006 (UTC)

Origin of bel

• "The bel as a unit of level was originally devised by engineers working in telephony as a way measure the loss of signal amplitude in telephone wires as a function of the length of wires. Developed in 1928, the bel (named after Alexander Graham Bell) was based upon a psychophysical law first stated by Gustav Fechner in 1860 and thought to be true at the time the bel was devised. The purpose of Fechner’s law was to summarize the relation between the growth of stimulation (a physical event) and the growth of sensation (a psychological event)." [13]
• "The decibel is derived from the less frequently used unit the bel, named in honor of Alexander Graham Bell (1847–1922). Two flux densities differ by 1 bel (10 dB) when the larger is 10 times greater than the smaller. It is to be noted that the logarithmic nature of the response of sensory organs, described in the Weber–Fechner law, underlies the definition of the bel." [14]
• "It was noted many years ago (Fechner, 1860) that the sensitivity of the ear to changes in intensity was not related linearly to either intensity or pressure. It was believed then that the ear's sensitivity to sound intensity or sound pressure was an approximately logarithmic relationship. Initially, it was proposed that a new measure of intensity be utilised which was derived from the log (base 10) of the ratio of two intensities." [15]

Interesting. I didn't know Fechner's law was first. Should be in the history section. — Omegatron 03:37, 6 January 2006 (UTC)

For a review with historical discussion, look at: Allen, J. B., (1996) "Harvey Fletcher's role in the creation of communication acoustics" J. Acoust. Soc. Am., 99(4), pp. 1825--1839

For a modern review of Fechner and Weber, and some "new" insights on this topic, look at: Allen, J. B. and Neely, S. T. (1997), "Modeling the relation between the intensity JND and loudness for pure tones and wide--band noise," 102(6), pp. 3628--3646

 June 11, 2006

Expressed in negative?

Out of curiosity why do some stereo amplifiers express units of decibels in negatives. Example, -90bB is louder the -10dB, topping out at 0dB. --Trode 19:58, 10 January 2006 (UTC)

-10dB should be louder than -90dB, not the other way around. It just depends what the reference is, i.e. what power 0dB corresponds to. If the 0dB reference is chosen to be the maximum power the amplifier can put out, then every volume setting will be a negative number in dB.--Srleffler 22:33, 10 January 2006 (UTC)

I was wondering that too, and I found the answer here: http://tomclegg.net/decibel 143.252.80.110 15:29, 18 January 2006 (UTC)

The first consumer stereo instance I remember seeing the 0 dB reference level was on a THX pre-amplifier. The idea is to calibrate your stereo with a pink noise reference source and an SPL meter at the 0 dB level. That way when I say I watched a certain movie at the -11 dB level you will have an idea how loud it was. Supposedly the audio playback volume at THX dubbing and THX movie theaters is set to the 0 dB level. Well in theory that's the idea, in practice though I've never been to a public THX theater that is anywhere close to 0 dB. About -7 dB is more like it which is still really loud. (Spectrogram 00:10, 27 January 2006 (UTC))

No way beyond 196!

There is mention of 220 and 300 decibels in the article. This is nonsense, since the absolute maximum is 196 in earth athmosphere, sea level. This is where air becomes vacuum in the dilutated phase of the soundwave and thus cannot be increased further. 195.70.32.136 13:34, 26 January 2006 (UTC)

I agree that the numbers are dubious, however, I don't think there's an inherent limit to the sound pressure level. Of course vacuum is a negative pressure limit, but the only limit in the positive pressure direction would be the point where air becomes liquid? So you could have asymmetrical waves with much higher pressures, right?

• Yet another amusing calculation reveals that with a sinusoid of 196dB SPL, the rarefying part of the fluctuation reaches vacuum. This is the theoretical limit on sinusoidal pressure fluctuations in normal atmospheric pressure, then. (Compressive impulses can, of course, reach much higher SPLs; cf. the hydrogen bomb.) [16]
• Sound is technically at its upper limit at 194.09 dB. Above this level it should be called a shock wave. Sound — Omegatron 15:20, 26 January 2006 (UTC)

I have always thought the upper limit in one atmosphere was 196 dB SPL and anything higher would be considered an overpressure. A sealed chamber can have greater than 1 ATM so in that special case a higher max SPL would be possible. Anybody know what the max SPL in sea water is? Base SPL levels are very different in water than in air. Liquid dB SPL's probably would make a nice wiki section topic if it isn't someplace else already. (Spectrogram 23:40, 26 January 2006 (UTC))

The math:

1. vacuum = 0 Pa = 0 atm, so pressure difference is 1 atm = 101,325 pascal [17]
2. $L_{\mathrm {p} }=20\,\log _{10}\left({\frac {p_{1}}{p_{0}}}\right)=10\,\log _{10}\left({\frac {{p_{1}}^{2}}{{p_{0}}^{2}}}\right){\mbox{ dB SPL}}$
   where $p_{0}$ is the reference sound pressure and $p_{1}$ is the sound pressure being measured. Sound pressure level
3. Reference for SPL is 2×10⁻⁵ Pa
4. $L_{\mathrm {p} }=20\,\log _{10}\left({\frac {101,325}{2\times 10^{-5}}}\right)=194.09{\mbox{ dB SPL}}$

---

I changed the article regarding values above 194 dB SPL. The list at makeitlouder.com from which the values like 200--240 dB for explosions, tornadoes, earthquakes apparently are taken uses various nontrivial approaches to get these numbers as a means to compare violent processes, for example the total released energy per second (including heat), the wind speed compared to the velocity of the air inside an acoustic wave, and so on. It makes no sense to put these numbers into a dB SPL comparison table as an absolute truth. Han-Kwang 12:24, 1 April 2006 (UTC)

I put in "Although pressures higher than 2 atm are possible in air", but is that correct? If a sound wave is at 194 dB SPL, the rarefactions will be at vacuum, but where will the compressions be? I assumed it would be 2 atm in a linear manner, but I could be wrong. — Omegatron 16:00, 1 April 2006 (UTC)

In order to get wave propagation in a medium, you need a linear compressibility, i.e., dp/dV = K (K is a constant). In a gas, the actual relation is IIRC dp/dV = K/V^(gamma+1), where gamma is typically 1.6 for a diatomic gas. If you imagine a two equal slabs of gas stacked on top of each other in a confined space, and expand one of the slab with a factor 1.9, then its pressure will be about 36% of the original. The other slab will be compressed with a factor 10 and its pressure will increase with a factor 40. So a true vacuum inside the rarefactions is impossible. Put in a different way: if you assume a sinusoidal pressure change over position, then you need to change the number of gas molecules in the system, since the decrease in density at the lower parts of the sine wave is larger than the gain in the higher parts. Since this is impossible, the wave must have a shape that differs considerably from a pure sine. Because of this asymmetry, this type of disturbance (shock wave) changes shape during propagation, unlike normal sound waves. Anyway, this is more about the definition of sound. As you said yourself above, it's not called sound anymore. I feel that sound is something with a well-defined frequency spectrum that is reasonably conserved during propagation, and that is not the case with a shockwave. Hence I object against the use of the term sound pressure level for a shockwave. The question is where you put the boundary between normal sound and shock waves, which is a matter of taste, but I'd guess that it will end up +/-6 dB from the 194 dB that you get from the naive calculation discussed before. Han-Kwang 19:42, 2 April 2006 (UTC)

-3 db

-3dB = 20 log (1/sqrt(2)) not 20 log (.5)

This is a very bad approximation in the table since -3dB is such an important value

This is an egregious error

Scott Curran

I'm not sure where you're looking. 20 log(0.5) would indeed be wrong. -3 dB is 10 log(0.5). Perhaps the tables of ratios at the very end of the article confused you? The ratios are power ratios not voltage ratios, since dB (as opposed to dBV) is a measure of relative power not voltage. --Srleffler 22:24, 4 February 2006 (UTC)

Clarifying factor of 10 vs 20

I tried to revise a bit of the definition to make the difference between intensity and pressure equations a bit more understandable, but when I saved the equation did not render and the entire page seemed to become bold font, so I reverted to the previous edit. If someone wants to give this another try, please do.

In electrical circuits, the dissipated power is typically proportional to the square of the voltage V, and for sound waves, the transmitted power is proportional to the square of the pressure amplitude p. Effective pressure is related to intensity by the following equation:

$I=p_{e}^{2}/\rho _{0}c$

Substituting a measured voltage or pressure and a reference voltage or pressure and rearranging terms leads to the following equations and accounts for the difference between the multiplier of 10 for intensity or power and 20 for voltage or pressure:

$V_{\mathrm {dB} }=20\log _{10}\left({\frac {V_{1}}{V_{0}}}\right)\quad \mathrm {or} \quad p_{\mathrm {dB} }=20\log _{10}\left({\frac {p_{1}}{p_{0}}}\right)\ ,$

etc. The added equation is on p.125 of Kinsler and Frye's "Fundamentals of Acoustics" 2nd edition, 1962. Wesley R. Elsberry 18:46, 12 April 2006 (UTC)

I think it's better to put all the mathematical derivations somewhere in a separate section. The equation pe^2/rho0 c isn't very helpful in the current form, i.e. without explaining what all the symbols mean (and you appear to change notation from p to p_e). OTOH, it would distract from the main point if you do explain all the symbols at the current location. Han-Kwang 21:26, 12 April 2006 (UTC)

This issue came up because someone altered the article page to ask about why 10 was used in one place, and 20 in another. While putting a question in the article is the wrong way to go about things, that person did have a point. If we defer the derivation, then we have people wondering why there is suddenly a different multiplier in the equation to obtain a decibel value. On the other hand, I can see that it would be nice to be able to offload the gory details to its own section. I'm not sure how we handle this to make it right for everyone. I'll note that I really liked how Omegatron handled putting in the info as it is now, with the explanation of the terms in the new equation added. Wesley R. Elsberry 00:14, 13 April 2006 (UTC)

Thank you for using correct dimensions!

After skimming through the article, I'm delighted to see that the expressions are dimensionally sound. That is, the decibel values aren't confused with the powers themselves, and the arguments of the logarithms are all dimensionless and nice. I was fearing a dimensional mess, since that is how it usually is, in my experience. (The index "dB" is even written upright. Great!) Bromskloss 16:58, 26 April 2006 (UTC)

We do our best.  :-) It certainly wasn't always this good...

I'm sure there are still a few problems somewhere. If you can find them, we would be happy to know. — Omegatron 18:27, 26 April 2006 (UTC)

real world examples

Since I'm not really familiar with this technical topic, I'll suggest this here first. I think a list of real world examples would be useful for lay people, similar to the one at the bottom of this page: [18]. I'll probably add something in a few days if no one else does. Thanks. --W.marsh 20:55, 17 June 2006 (UTC)

Yes this would be good! Could you add it please? But please dont copy it verbatim!--Light current 00:03, 18 June 2006 (UTC)

Origin of 600Ω in dBu

The origin of the 600Ω in dBu is not from the professional audio industry, it's from the telecom industry. The characteristic impedance of telephone cable when stretched over miles is 600Ω. This means that your input and output stages connected to these lines would have to be matched to 600Ω to prevent reflections. Granted, telephone companies do not use 600Ω transmission lines anymore, but that's its origin. --Babar77 15:54, 29 June 2006 (UTC)