Talk:Decibel/Archive 2

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Noise floor of air

Earlier we discussed the maximum SPL possible in air (when it turns to vacuum during rarefaction). On the other end of the scale, what's the minimum noise always present in still air in a perfect anechoic chamber due to thermal motion? I'm sure it depends on temperature. Is it ever relevant? (Probably not.) — Omegatron 15:27, 26 February 2007 (UTC)

In water of density ρ, sound speed c, the noise floor of mean square pressure is given by A T f², where A = 4 π k ρ/c= 0.00012 nPa²/(K Hz³), T is absolute temperature, f is frequency and k is Boltzmann's constant k =1.381 10^-23 J/K. It comes out as about -15 dB re 1 μPa at 1 kHz, and is important at frequencies above 100 kHz. That doesn't answer your question but perhaps it helps. Thunderbird2 17:34, 31 July 2007 (UTC)

Hmmm... I found some references that give formulas, but I don't think I'm understanding the formulas correctly. To rewrite yours for water:

${\overline {P}}^{2}={4\pi \rho kTf^{2} \over c}\quad \left({\tfrac {\mathrm {Pa^{2}} }{\mathrm {\sqrt {Hz}} }}\right)$

So:

${P}^{2}=\int _{f_{1}}^{f_{2}}{4\pi \rho kTf^{2} \over c}\,df={4\pi \rho kT \over c}\int _{f_{1}}^{f_{2}}{f^{2}}\,df={4\pi \rho kT \over c}{{f_{2}^{3}-f_{1}^{3}} \over 3}\quad (\mathrm {Pa} ^{2})$

So RMS pressure would be:[1]

$P={\sqrt {{4\pi \rho kT \over c}{{f_{2}^{3}-f_{1}^{3}} \over 3}}}=303\ \mathrm {\mu Pa}$

which is +50 dB re 1 µPa. Is your "at 1 kHz" measurement a narrow bandwidth? — Omegatron (talk) 17:53, 4 May 2008 (UTC)

Goodness, I'd forgotten about this. It looks like I was being lazy. What I meant was a spectrum level of -15 dB re 1 μPa²/Hz at 1 kHz. What values are you using for f₁, f₂? Thunderbird2 (talk) 18:02, 4 May 2008 (UTC)

20 Hz to 20 kHz. Sound pressure lists +67 dB for the auditory threshold for divers, so it seems correct. — Omegatron (talk) 18:07, 4 May 2008 (UTC)

Does this mean that the noise is not white?

Refs for air:

The "softest," or quietest sound that humans can perceive is nearly the same as the softest sound that all animals (or insects) can perceive: the vibrational amplitude of the air at zero decibels (the softest sound) is only about the diameter of a hydrogen atom, and any further sensitivity of the listener would be overcome by the ambient "hiss" of the Brownian motion associated with the thermal agitation of molecules.[2]

How sensitive can one really hear? Vibrations of the ear-drum at the threshold of hearing can correspond to about the diameter of a hydrogen atom! As stated before, some people, especially those living in the countryside away from machinery and big city sounds can actually hear random motion of air molecules bouncing against their eardrums![3]

The amplitude range is substantially broader, beginning at a level so low that we can almost "hear" the fluctuations in air pressure due to random motion of air molecules near the ear drum[4]

At 4 kHz, which is about the frequency of the sensitivity peak, the pressure amplitude variations caused by the Brownian motion of air molecules, at room temperature and over a critical bandwidth, corresponds to a sound pressure level of about -23 dB. Thus the human hearing system is close to the theoretical physical limits of sensitivity. In other words there would be little point in being much more sensitive to sound, as all we would hear would be a 'hiss' due to the thermal agitation of the air! [5]

According to Watson, Franks, and Hood (1972), who were the first to control the signal duration (125 ms), the absolute threshold at 1000 Hz is about 9 dB SPL. The energy of this signal is approximately 10⁻¹⁶ W s/cm². We estimate the N₀ of Brownian motion over the same area to be about 1.7×10²⁰W s/cm2. Thus, the ratio of signal energy to noise power density at absolute threshold is about 5000. Our ability to hear weak auditory signals is impressive, but the fundamental limitation is not the thermal agitation of molecules of the air. Is human hearing limited by Brownian motion? David M. Green and Huanping Dai

Also [6] and doi:10.1121/1.1906831, which gives the equation:

${\overline {P}}^{2}={8\pi \rho \nu ^{2}h\nu \over c(e^{h\nu /kT}-1)}d\nu$

• ρ = density of gas
• c = velocity of wave
• ν = frequency
• dν = bandwidth
• h = Planck's constant
• k = Boltzmann's constant
• T = temperature in K

— Omegatron (talk) 18:12, 4 May 2008 (UTC)

That last equation is in the same form as the one I mentioned, provided that h.ν/k.T is small, but there is a factor of 2 discrepancy between them that I don't understand. Perhaps one is defined for negative frequencies and the other not. I'll dig up my source reference for thermal noise. (re whiteness: In the sea the audible noise spectrum is certainly not white, but it's not blue either because thermal noise is not important at audible frequencies. The actual spectrum depends on conditions but is usually reddish) Thunderbird2 (talk) 18:35, 4 May 2008 (UTC)

Is that environmental noise or thermal noise, though? — Omegatron (talk) 19:03, 4 May 2008 (UTC)

The red noise is environmental (often caused by wind-generated breaking waves) and dominates up to about 100 kHz. Above that the thermal noise takes over. The reference I was looking for is [R. H. Mellen, The thermal-noise limit in the detection of underwater acoustic signals, J. Acoust. Soc. Am. 24, 478-480 (1952).]. Thunderbird2 (talk) 19:24, 4 May 2008 (UTC)

That's relevant to the Colors of noise article, then, which is confused and lists "red noise" as a separate oceanographic spectrum rather than a synonym for Brownian noise. Can you fix it and add a reference? — Omegatron (talk) 20:10, 4 May 2008 (UTC)

I removed the statement about ocean noise, which was incorrect. In fact it's rare to hear the terms "red" and "blue" applied to ocean acoustics, so the relevance to the "colors" article is questionable. Instead, spectra are usually described in the form "N dB per octave". Is there a still a reference you'd like me to add? Thunderbird2 (talk) 20:57, 4 May 2008 (UTC)

Thanks for the Boyarsky 1951 reference. (It's not mentioned by Mellen so I was not previously aware of it). I don't understand his Eq. (4) because the total energy density is mean square pressure divided by (ρ c²), and not half that as he claims. If you correct for that factor of 2 and assume small h.ν, you end up with Mellen's formula, which I think is correct. Thunderbird2 (talk) 19:56, 4 May 2008 (UTC)

The lede

This sentence is poor: "The decibel is useful for a wide variety of measurements in acoustics, physics, electronics [...]". Acoustics and Electronics are a part of Physics, not separate entities as the sentence seems to implies. It's better to say "[...] of measurements in physics, (such as in acoustics and electronics) [...]. —Preceding unsigned comment added by 75.12.148.193 (talk) 05:19, 1 January 2008 (UTC)

No, it's just fine. Not everyone agrees that acoustics and electronics are part of physics. I mean, by that argument, everything is part of physics. It is clearer and more understandable in its current version than what you propose. jhawkinson (talk) 05:31, 1 January 2008 (UTC)

The decibel is overwhelmingly used by engineers rather than scientists. Therefore I don't agree that "The decibel is useful for a wide variety of measurements in science and engineering (e.g. acoustics and electronics)" is an improvement over "The decibel is useful for a wide variety of measurements in engineering (e.g. acoustics and electronics)". Thunderbird2 (talk) 14:16, 2 January 2008 (UTC)

Broken link no. 3

Link no. 3 "Decibel chart, small spectrum " is broken. Who can correct this? Georgegh (talk) 16:10, 28 January 2008 (UTC)

Real life examples that people can use

A lot of encyclopedias etc. have examples of what various decibels might sound like e.g a bee is .. a concorde is ... a train is ... . This would be more for kids, and might make the subject a bit more approachable?? —Preceding unsigned comment added by 41.241.5.105 (talk) 09:45, 9 May 2008 (UTC)

You mean like this? Thunderbird2 (talk) 10:26, 9 May 2008 (UTC)

Add dBsm?

One use of dB that I don't see here, and there's no article for it either, is dBsm. That's dB relative to 1 square meter. Used in radar when talking about the signal return from a target, compared to that from a 1 square meter sphere of a perfect conductor (usually considered smooth metal, and assuming that's big enough for several wavelengths of the radar's transmitted signal). So the (Greek letter lowercase sigma) or radar cross section (RCS) of a target is expressed in dBsm in the two-way radar power equation. Lets you calculate the maximum range you can detect a given target, especially when considering stealthy ones (low positive or even negative dBsm). Also useful when you design self-protection ECM, as you often have to provide some given ratio over the platform's RCS for the jamming technique to work.
Reference: radarproblems.com Chapter 6, top of chapter & part 11
Please consider adding dBsm to the article on dB. And if somebody has the time, write an article for dBsm and link to it. Also, I just noticed that somehow the article on RCS has no mention of dBsm, just numbers without any dB anywhere, and one implied calculation about reducing detection range by decreasing RCS. So a talk page entry is going there as well. Thanks! 138.163.0.42 (talk) 18:09, 8 July 2008 (UTC)

Gee, you've written most of the text right there - dig us up a reference from a popular radar systems design textbook (sounds like you have one within arms' reach) and paste it into the article. You're somebody, you had the time to notice it and the knowledge - you're eminently qualified to WP: Be Bold and add it. --Wtshymanski (talk) 17:50, 9 July 2008 (UTC)

estimating

There used to be a useful section on estimating dB from ratios and the other way around? Why was it removed? -Fundamentisto (talk) 16:53, 6 August 2008 (UTC)

It was removed with the edit summary "rm big fat OR" (OR = WP:original research). Whilst that's somewhat over the top, I agree with the sentiment. Most of that material seems to be a load of stuff that someone's figured out for themselves and added (e.g. I can't find anything on the web for "789 rule" that isn't just a mirror of the Wiki material). Secondly, this material reads a lot like how-to material, which isn't suitable for a Wikipedia article. Thirdly, most of the material is written quite badly, especially the "Round numbers" section.

I notice you've re-introduced the material; I'm planning on removing it again for the reasons above, but I'll hold off to give you chance to reply! Oli Filth^{(talk|contribs)} 19:21, 13 August 2008 (UTC)

Far be it from us to have useful material in Wikipedia! Good thing you removed it. - Fundamentisto (talk) 20:44, 2 October 2008 (UTC)

dB uV

Why has http://en.wikipedia.org/wiki/User:Wtshymanski removed my addition that 1uV = 0dBuV ?

I thought that 1uV did equal 0dBuV and it is not a straightforward concept to understand either. If it is correct, than I feel it should be included. If it is incorrect then can someone explain why it is incorrect ? --JustinSmith (talk) 15:36, 17 September 2008 (UTC)

It is correct that 0 db(FOO) = 1 FOO , where FOO is any unit. We don't need to repeat this again and again, since we can't teach the reader about logarithms that way. The replaced edit said Therefore 1 μV = 0 dBμV, since 1 μV is 0 dB relative to 1 μV and 0 x 1 = 0. 60 dBμV = 0 dBmV. which is redundant, and wrong, since 0 x 1 is not 60 or 0.60 or whatever this was meant to say. --Wtshymanski (talk) 16:26, 17 September 2008 (UTC)

I don`t really get your point. The reference to dBmV is the thing which is misleading as it`s a different unit, it`s dB ratio to 1mV so is just confusing things. On the other hand dBuV is dB ratio to 1uV. I don`t know about you but I initially found it hard to get my head round the concept of 1uV being 0dBuVs, maybe because I`ve always come across it in the real world where on one`s meter a dBuV is a reading, not a theoretical concept. I can`t be bothered to change it back, but I still think it`s relevant, and interesting.

Incidentally, can anyone confirm that 1 uV is indeed 0 dBuVs, as it dooesn`t actually tell you in this Wikipedia article  ! --JustinSmith (talk) 07:49, 18 September 2008 (UTC)

If the reader doesn't already know that log(1) =0, then the whole article is over his head in the first place. Excess repetition in articles makes them unreadable. Wikipedia editors seem to have a collective fascination with every power-of-ten prefix to a unit being worthy of a separate encyclopedia article. Thus we get to 2 million+ articles. --Wtshymanski (talk) 13:55, 18 September 2008 (UTC)

Optical dB vs Electrical dB

In the current article under the optics section is (quote)

"Optics In an optical link, if a known amount of optical power, in dBm (referenced to 1 mW), is launched into a fiber, and the losses, in dB (decibels), of each electronic 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." (unquote)

Should not "connectors, splices, and lengths of fiber" be referred to as optical components, not electronic components? Yeah, I know, this is nitpicky, not the main reason for this post.

One item that could be added to the article is that 3 dB optical loss equates to 6 dB electrical loss.

At the optical receiver, the optical detector current is directly proportional to the optical power. Cut the power by half (-3 dB), the current is also halved. But once converted from photons to electrons, half current in electronics translates to half voltage and becomes a 6 dB reduction. Circuit impedance is not an issue, this ratio is measured at the same point in the electronic circuit.

Thus a fiber rated at -3dB/Km power attenuation is actually -6dB/Km in terms of electrical attenuation. A -3dB change in optical level produces the same result (half voltage at the transimpedance amp output) as if an electrical attenuation of -6dB had been inserted into the system. That is why the bit error rate curve of an optical receiver is twice as steep as that of an electrical receiver when plotted against the same dB scale.

An interesting effect to consider adding to the article, after discussion of course. Zeeglen (talk) 23:19, 8 October 2008 (UTC)

did the bel prove inconveniently small?

Is there a mistake here? "the bel proved inconveniently large, so the decibel has become more common" wouldn't this make sense only if the bel proved inconveniently *small* so that a larger unit is required . . . the decibel, ten times larger than the bel . . . The article looks pretty good so I'm just checking here before I change anything —Preceding unsigned comment added by 128.197.61.103 (talk) 13:52, 20 October 2008 (UTC)

decibel = 1/10 of a bel
decabel = 10 bels (and very inconveniently large) Zeeglen (talk) 22:25, 22 October 2008 (UTC)

Format suggestion

The information under the subsection "Absolute measurements" should be presented in the form of a table. Fuzzform (talk) 04:59, 30 October 2008 (UTC)

Let's rename the article "decibel" instead of "Decibel".

Let's rename the article "decibel" instead of "Decibel" since the unit is lowercase by default when spelled out. -- Another Stickler (talk) 09:34, 20 December 2008 (UTC)

Let's not. All Wikipedia article titles start with an uppercase letter, except if the word would be written in lowercase even at the start of a sentence (iPod, pH meter, etc). Unit names (whether SI-prefixed or not) do not fall into that class. Hqb (talk) 09:50, 20 December 2008 (UTC)

Can you point me to the wikipolicy? -- Another Stickler (talk) 20:14, 20 December 2008 (UTC)

It's a technical restriction, not a policy per se. But the convention that unit names are treated like ordinary nouns (i.e., "decibel" should be capitalized the same as "dog") is a general BIPM convention to be followed in all writing, unless there's a specific wikipolicy to the contrary. Hqb (talk) 21:23, 20 December 2008 (UTC)

Ah, I see that forced first-letter capitalization is built in to the wikipedia software and requires a template to override. That's good enough for me to accept that wikipedia article names are "capitalized material" according to BIPM, unlike dictionary entries, which are lowercase by default. -- Another Stickler (talk) 01:37, 21 December 2008 (UTC)

The unit is named after a person, so I wonder why it is not spelled deciBel since the unit is abbreviated as dB. Also, dB is a unit of power, Watts, not intensity in Watts per square meter. Engineers never get this right. They think that it is simply the logarithm of a ratio. Gary Hefley —Preceding unsigned comment added by 202.20.20.202 (talk) 04:50, 2 March 2009 (UTC)

Units names are always lowercase in the SI system when written in full, whether or not they are named after a person. They are only capitalised (if a person) when abbreviated. Engineers do get this right, they are following the current international standards, see my reply in Talk:Decibel#Amplitude vs power above. SpinningSpark 14:21, 9 June 2009 (UTC)

Link consideration

please consider this link: http://www.fourier-series.com/rf-concepts/decibels.html

I think it will help people understand decibels. —Preceding unsigned comment added by 76.181.44.43 (talk) 19:54, 9 June 2009 (UTC)

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)

Decibels always express power ratios. dB = 10*log10(x), where x is a ratio of powers p1/p2. A dB can be "mapped" to a voltage if either the impedances are known or it is assumed that both components (p1 and p2) are into the same impedance. —Preceding unsigned comment added by 71.70.224.93 (talk) 21:29, 28 January 2008 (UTC)

Contra "This 20log10 crap is WRONG! dB are NEVER 20log10 of ANYTHING.":

Not according to the ISO, which now defines the decibel this way. The reason they did this is because they based the neper on field level ratios rather than power ratios. They decided for consistency's sake to redefine the bel based on field level ratios too. Since power goes as the square of field level, they redefined the number of bels as twice the base 10 log of the field level ratio. Equivalently, a bel is now defined not as a decade of power ratio, but as half a decade of field level ratio. In short, a bel is now half a decade, and a decibel is a twentieth of a decade, and the bel and the neper are both based on field level ratios (or voltages or whatever goes as the square root of power in your application...)

Alma Teao Wilson (talk) 23:20, 15 October 2008 (UTC)

OMG This is appalling ! Wikipedia loses all credibility when something as simple as this can not be explained properly. For goodness' sake will someone please edit the main article and get it right.

First simple fact - dBs are always a ratio of power levels and never anything else. dB are calculated as 10log(P1/P2)

Second simple fact - in an electrical circuit average power is proportional to V^2/R - proportional because the constant of proportionality depends on the waveform of an ac signal.

Therefore, by simple arithmetic, it is clear that a power ratio in dB can be calculated from a voltage ratio V1/V2 as 20log(V1/V2) + 10 log(R2/R1) where R1 and R2 are the resistances across which voltages V1 and V2 are measured.

Again, by simple arithmetic, if R1 and R2 are the same, the power ratio in dB can be calculated as 20log(V1/V2).

In practice this calculation is absolutely appropriate and correct when V1 and V2 are measured at the same point in an electric circuit, and the changes in power occur as a result of a change in signal level.

But the calculation is rarely valid when V1 and V2 are measured at different points (for example the input and the output of an amplifier), since the requirement that R1 = R2 is probably not then met. This is particularly true in the case of operational amplifiers.

dBu and dBV cannot possibly be defined as power ratios relative to 1v, etc, for the simple reason that 1V is not a unit of power. However, if a particular value of resistance is stated, then dBu and dBV will have a meaning, since power can then be calculated.

Dr Andrew Smith C.Eng. —Preceding unsigned comment added by 82.32.50.77 (talk) 17:29, 5 June 2009 (UTC)

Not according to NIST. On this page of their website citing IEC 60027-3 they state that decibels can be a ratio of power or a ratio of a field quantity. Furthermore, decibel originally referred to sound pressure level, so even from its inception it has never been exclusively about power ratios. From a purely pragmatic point of view, I can also confirm that it is common practice in my industry, where dBV and dBu are widely used, to deal with voltage ratios regardless of impedance. This is both because voltage is the quantity we can most readily measure and because power ratios are meaningless when dealing with high impedance inputs as is commonly the case in electronics. SpinningSpark 20:54, 5 June 2009 (UTC)

I see how you play the game. If I don't get everything right to your satisfaction, you just undo the whole thing. Why not be less lazy and only back out the stuff you dont like and then allow a discussion of what you backed out? I put a lot of effort into improving this page and just doing a complete back out has demonstrated to me that working with you in good faith is not going to happen. You don't even have the guts to tell me what you don't like, and based upon the rules you have set out for me, if I put in the stuff you don't like , I get blocked. Why don't YOU take the time to leave in what is uncontroversial, so we don't have to go into a 20 question game about what YOU don;t like?

By the way, every text book out there shows it like I presented it. (The Dr. above has said the whole way that amplitude is presented is appalling and requested a rewrite- i suspect he would be happier with my wording than what is currently in there.). —Preceding unsigned comment added by 76.181.44.43 (talk) 10:47, 11 June 2009 (UTC)

No one said anything about blocks, there's no need to keep this anything other than friendly. I normally try not to revert wholesale, but I have a problem with the majority or what you wrote, all that would have been left if I had tried to unpick it is a few tidy ups. Since you are writing in this section, I think you must already realise my objection. NIST allows, and it is widespread practice in some industries to use voltage ratios, follow the link above. In particular, the definition of dBu is not related to impedance or power. In the end we will go with what the references say, if you have references that are more authoritative than NIST, please present them. SpinningSpark 12:32, 11 June 2009 (UTC)

(note: I am same as 76.181.44.43) If you want to adhere by the NIST document, then why not strip out every referenece to dBm? After all, it would seem that dBm is not allowed by the rules that you cite as being what we must be consistent with (the point being, why pick and choose where we must be true to the NIST document, and where we get to ignore it?) I do see from this document that dBs need to be separately considered for both power comparisons and field comparisons. It is valid to not allow the EE's here to minimize the role that decibels play in field strength. However, in taking this posistion steps for converting voltages to decibles via the route of showing that they must first be considered as two powers , is completely lost on this page. Every text book for EE's show this relationship, and somehow, this wiki page cannot show this becuase it is not faithful to an NIST document (which we have already said does not have to be adhered to completely anyhow). Your target audience for this webpage is going to consist of a lot of people who are more interested in how voltages need to be converted to powers, than in the academics arguments of amplitude and squares and field strength , with no tie whatsoever on how a voltage actually converts to a power.

Perhaps a dual route showing decibels as ratios of powers , and decibels as ratios of field strengths would be appropriate, in which case, the v^2/r could be shown. Not ever showing the v^2/r and how the R drops out , at the highest level, is not correct. To not show that basic idea really minimizes this page. It should be worked in somehow into this page. —Preceding unsigned comment added by 70.62.154.132 (talk) 14:47, 11 June 2009 (UTC)

You make two good points here. First of all that the 20log rule arises because V²/R and I²R are power and will equal 10logPower if R is the same on numerator and denominator. The article does attempt to explain this with 10 is because it is in decibels (10ths of bels), and 2 is because it is a ratio of powers (squares of amplitudes): the product is 20 but that is about as clear as mud and could be improved. What we must be careful not to claim, as many seem to think, is that the dB is defined as a power ratio. This is not what the National Institute of Standards and Technology say, it is not what the International Electrotechnical Commission say, and it is not even what the International Bureau of Weights and Measures say (although in the latter case they do not define a 20log case, they would call it a "mean square ratio"). The "power ratio" definition may well be how the gentlemen at Bell labs originally defined it when they thought it up and it can be mentioned as a historical fact (a reference for that would be good) but that is in the past and current practice is different, a ratio of voltages is allowed, regardless of impedance.

Your other good point is that dBm is not compliant with SI nomenclature. In fact very nearly all the dB mentioned in the article fall into this category, not just dBm. However, it is not the job of Wikipedia to promulgate the SI system of units for them, or to rap the knuckles of anyone who won't comply. An encyclopedia article should neutrally cover the actual facts: and the facts are that in some industries, especially telecommunications and audio engineering, this kind of notation is widespread. But I fully support the article covering the "official" notation if you want to put it in - that is certainly notable even if no-one in my industry uses it.

SpinningSpark 16:14, 11 June 2009 (UTC)

When I wrote 'dBs are always a ratio of power levels and never anything else', I was, of course speaking from within my own discipline, which is Electrical Engineering. If dBs are used differently in other disciplines it is appropriate for the Wikki entry to describe all usages. The other qualification I would like to make is that I am well aware that variations occur even within electrical engineering.

It is common practice, for example, to quote the voltage gain of an operational amplifier as 20log(Vout/Vin) even though the output and input impedances of opamps are widely different by many orders of magnitude. This points to an unsatisfactory situation where the same symbol is used with different meanings within the same discipline. This can only be seen as a form of colloquialism or undisciplined usage; the 'correct' way to describe voltage gain is as a simple ratio (Vout/Vin).

So it is necessary to state clearly what the 'proper' meaning of dB is, and this is clearly and without doubt 10log(p1/p2); no professional engineer would understand the term differently unless he had reason to suspect a colloquialism of the kind mentioned above. If anyone wants to dispute this they must explain why so-called 'voltage dB' are calculated as 20log(V1/V2), and say why we should not see this as an unacknowledged admission that dB do in fact express a power ratio, and the calculation as a misunderstood way of attempting to calculate power ratio (see my previous posting on this). Nevertheless, in a context such as Wikipedia it would be appropriate to explain all usages and comment on how they have arisen and how they relate to each other. I would strongly recommend a caveat concerning the inappropriateness of introducing an alternative to 10log(p1/p2) within engineering.

Andrew Smith —Preceding unsigned comment added by 82.32.50.77 (talk) 19:01, 12 June 2009 (UTC)

That's why there is a profusion of qualifying postcript letters. There is no doubt that the 20log form started out that way because of its relation to power but current use is different, and is most definitely not considered a colloquialism, it is taught that way. SpinningSpark 19:22, 12 June 2009 (UTC)

Arbitrary editing break

As this whole discussion section demonstrates, there is definitely confusion regarding the usage of decibels following the 10- and 20-log rules. While the present version of the article is clear about these rules, I offered an example [7] further illustrating it. The modification was reverted, on the basis that it's "a pointless belabored example". While I might agree that the example was verbose, it does serve a purpose, namely, to clarify a well documented recurrent confusion. So could you please either improve on the example or argue why you think the confusion is irrelevant. 128.138.43.51 (talk) 03:40, 23 August 2009 (UTC)

I didn't say the confusion is irrelevant, I said it was "a pointless belabored example". OK, maybe "pointless" was stretching the point, but I think the situation is already well explained in the article. If the confusion is important, find a reliable source that says so and incorporate some of its explanation with a citation. Dicklyon (talk) 03:48, 23 August 2009 (UTC)

Actually, I don't think that many people are confused by this (so it doesn't need extensive examples to explain). What is a problem is that a certain faction (mostly from the physics community) disagree with using dB as a voltage ratio under any circumstances. Both sides understand the issues, and the only confusion to the reader occurs when the argument spills over into the article. SpinningSpark 09:20, 23 August 2009 (UTC)

Spinningspark, please let other people contribute! I took great care to make my modifications in three separate pieces, but you reverted all of them! One [8] was a correction to something plain wrong, a second one [9] was an interpretation completely compatible with the mathematical definitions, and a third one [10] was a sourced statement from an authoritative source (search for "(Note that if P/P0 = (F/F0)2, then LP = LF.)" in that reference). Let me remind you that "Reversion throws away proposed changes by the other editor (even those made in good faith and for well intentioned reasons), rather than improving upon them or working with the editor to resolve any differences of opinion. Therefore reverting is not to be undertaken without good reason." -- "Try to avoid deleting things as a matter of principle." So could you please put some time to expose why you disagree with each of my latest three modifications. Thanks for your time. 128.138.43.51 (talk) 03:33, 25 August 2009 (UTC)

I support the revert; at first I thought your first edit looked OK, but on careful reading to see what that sentence was about, I realized it was not; the original "Note also that no constant factor is needed for the power (one can take power to be the square of amplitude, whatever the units), since any constant cancels in the ratio: the ratio of two quantities or their squares are scale-invariant." -- here the fact about ratios that you changed to in the parenthetical part is already stated in the non-parenthetical part, and doesn't need to be said again. The point it's trying to make in the parens is the same point it's making about power, which is that you don't need to worry about constant factors and units; like in I^2/R, you don't need the constant R. The reason follows immediately "since any constant cancels in the ratio." So it's OK as is, and your other edits were just belaboring a point that needs no belaboring. In the "sourced" bit, I was unable to find anything in the source to support your wording. Dicklyon (talk) 03:54, 25 August 2009 (UTC)

First: can't you see the danger of people quoting "one can take power to be the square of amplitude, whatever the units" (sic) and referencing Wikipedia? Furthermore, my first edit does no harm to the article; I think it is a necessary caveat, and if you don't agree with it, you have no right to revert it -- let other people contribute! Even SpinningSpark eventually said "I restored your first edit which I agree should not have been reverted".

About my second edit, you might think this is "belaboring a point that needs no belaboring", but please put things in perspective and recognize that you are probably an expert in the subject, and that aspects that are obvious to you can be even surprising to beginners! So, again, I found it necessary, and if the edit does not introduce any mistake in the article, you have no right to revert it! Could you please explain why you think the interpretation given in my second edit is wrong.

About my third edit, I went to great length to quote literally from it (again: "Note that if P/P0 = (F/F0)2, then LP = LF"). Spinningspark found it, I trust you can easily search and find it in the source, too. Then if you still have reservations, please say why exactly you don't agree with NIST.

I have restored your first edit which I agree should not have been reverted. As for the rest, "if $\scriptstyle P/P_{0}=(F/F_{0})^{2}$, then $\scriptstyle L_{P}=L_{F}$" is a very long way from claiming that as a definition, there is a qualification "if" in there. In fact the NIST article makes no such statement. In fact it implies the opposite, if they were defined to be equal there would not be a separate definition for L_P and L_F. Also, such a definition would be in plain contradiction to many of the specific examples in the articles, and in contradiction to widespread industry practice. SpinningSpark 07:42, 25 August 2009 (UTC)

Thanks for reconsidering your revert, Spinningspark. I hope you understand how frustrating unnecessary reverts can be for a beginner to Wikipedia, and realize that these repel potential contributors to Wikipedia. As for my other edits, let me treat them separately.

Regarding my third edit [11], you say that "'if P/P_0 = (F/F_0)^2, then \scriptstyle L_P = L_F' is a very long way from claiming that as a definition": I didn't intend to claim that as a definition, only as a consequence for the original definition, which I make no objections to; I said: "The two definitions below are such that the result ... is the same". You go on to say that "there is a qualification 'if' in there": sure there is! That's what I meant when I said "as long as...". You contine "In fact the NIST article makes no such statement": but I'm quoting literally! Why exactly do you think that I'm stretching NIST's statement? You go on: "In fact it implies the opposite, if they were defined to be equal there would not be a separate definition for LP and LF.": separate definitions exist for the case when the caveat ("as long as...") does not hold true. Finally: "such a definition would be in plain contradiction to many of the specific examples in the articles...": I did read all the article and couldn't find any such examples; care to quote, please? In fact, I just added another modification illustrating my point. "...and in contradiction to widespread industry practice.": I need a source, please. :o) 128.138.43.51 (talk) 14:29, 25 August 2009 (UTC)

Regarding my second edit [12], why exactly do you think it is wrong?

Your edits read to me as if you were claiming that $\scriptstyle P/P_{0}=(F/F_{0})^{2}$ by definition. It now appears that you do not actually believe this, but your edit was very unclear and was not helping the article. Your latest edit looks fine to me (after a few tweaks). By the way, you said to Dicklyon "...you have no right to revert it...". Not so, anyone can remove any challenged unsourced claim on Wikipidia without discussion. Next time you meet opposition, you might want to consider making a proposed change to the text on the talkpage, that way it can be thrashed out and agreed before it goes in the article. SpinningSpark 16:42, 25 August 2009 (UTC)

Thanks for the corrections in that fourth edit of mine; it was a little sloppy. I'm glad you corrected it; I was afraid it'd be simply reverted.

Still about my third edit [13], since you accepted the succeeding illustrating example [14], it seems that you concede that the idea is acceptable, only the particular wording needs improvement. So let's discuss that. What about: "Provided that power ratios equal amplitude ratios squared, the two definitions below --- 10-log and 20-log rules --- yield the same result in decibels ($L_{\mathrm {dB} }$)." Again, I'm only trying to convey NIST's statement that "if P/P0 = (F/F0)^2, then LP = LF". Please make me ensure that I'm not stretching NIST's statement.

Do you still see any problem with my second edit [15]? If not, could you please revert your reversion. You said "anyone can remove any challenged unsourced claim". To me, my second edit seems a perfectly valid, logical interpretation of the definitions laid down in the article. I will assume that you are not challenging the content of that claim -- otherwise, please detail your reasons -- only the need for it. Again, that claim might well be obvious to you, but the whole thread above documents the confusion and warrants a clarification, and my edit was a very concise one. The bottom line is: I think it is necessary, you don't, so which opinion prevails? I suggest that the decision should follow the "do no harm rule": if you think an edit is unnecessary, but another editor does see it as necessary, don't revert it unless the edit harms the article. Otherwise people will start scraping Wikipedia from claims that they find unnecessary, however true and informative they might be. (I'm the same as 128.138.43.51) 128.138.64.151 (talk) 17:32, 25 August 2009 (UTC)

"Provided that the power ratio...." sounds fine to me, but I don't like your proposed order. The definitions should come first, then the condition under which they are equivalent. Discussing the thing before defining it does not flow very well. Following that we should go on to emphasise that in some usages of dB they are not (necessarily) equivalent, eg dBu and dBV. I've read your second edit again and that phrasing really could be taken with the opposite meaning you intended, and, I think, is probably going to be unecessary after you put in your proposed edit above. SpinningSpark 18:06, 25 August 2009 (UTC)

Again, thanks for taking the time to improve this article, Spinningspark. I gave it another try, in two new parts: [16] and [17]. I've abandoned my previous third edit, since you insisted it was confusing. Feel free to modify them, of course. Thanks. 128.138.64.151 (talk) 19:26, 25 August 2009 (UTC)

...except when it's not...

Could we get an authoritative reference for the definition of dB(FS)? I find it a common and annoying weakness in Wikipedia articles to define " A "foo" is <this>, except when it's not." I don't think an article on decibels needs to wander off into the tall grass of explaining the limitations of digital recording systems, average responding vs. peak responding instruments, the definitions of RMS values, and other peripheria. The whole "explanation" that some systems have "headroom" irresistably reminds me that some amplifiers go to 11. Very murky. Leave it at "full scale is the maximum value the system can process" and the digital audio heads can go some other place to explore the wonders of "headroom". -Wtshymanski

I looked at a lot of books online trying to find a clear or authoritative definition, a week or two ago, and this was just a summary from memory. I'll see if I can find a suitable ref; but you could also help with that, instead of just removing info that is essentially true just not well expressed. -Dicklyon

This is a very authoritative source for all matters broadcast and seems to define dBFS nicely.

• Edwin Paul J. Tozer (ed), Broadcast engineer's reference book, p133, Focal Press, 2004 ISBN 0240519086

Full disclosure, I know some of the authors. SpinningSpark 19:33, 19 June 2009 (UTC)

Like most sources in that space, it doesn't really define it at all well. I know what "clip level" means, but I don't what what they mean when they talk about referencing to a signal at clip level. Is the rms level of a signal compared to that of a sinusoid at clip level? One source talked a 10 ms metering time constant, but it still wasn't clear is they were smoothing rms or peak or what, nor was it clear whether 0 dBFS was referenced to a sinusoid; that's what the comments in there were about; I don't know what the answer is, but it's clear why someone said it was unclear. Dicklyon (talk) 05:49, 20 June 2009 (UTC)

I also find it very patronizing of an encyclopedia to say things like "probably the most common usage in everyday speech...". Our readers (presumably) already know this, and don't need to be talked down to. This conveys no useful information unless some naive AI is reading this and learning about what people consider common - AIs are not our target audience. --Wtshymanski (talk) 14:47, 19 June 2009 (UTC)

Why would a reader know this already? It seems to me to be very useful info, if verifiable. Dicklyon (talk) 15:07, 19 June 2009 (UTC)