Talk:Decibel

Amplitude and power ...

@Dicklyon: ... are related in a simple way for harmonic signals, but not in general. For this reason the wording "(usually equivalently)" is incorrect. Dondervogel 2 (talk) 07:31, 12 November 2020 (UTC)

@Dicklyon: I shall wait until a week has passed and then implement this change. Dondervogel 2 (talk) 08:16, 17 November 2020 (UTC)

Sorry, I missed your first ping. Please explain; I don't see how "harmonic" is relevant here (what it means, even). Dicklyon (talk) 17:08, 17 November 2020 (UTC)

I don't think it needs to be harmonic, but it does need to be a non-reactive (resistive) load. If current is proportional to voltage, then either squared is proportional power. Gah4 (talk) 20:52, 17 November 2020 (UTC)

It has to be a root-power quantity, but not necessarily a non-reactive load. E.g the voltage across a parallel RC circuit is OK, even though the proportionality of current to voltage is frequency dependent. Dicklyon (talk) 22:09, 17 November 2020 (UTC)

Hello. I would like to hear what exactly is meant by harmonic. In one textbook I have, it just means the the time variation is sinusoidal. Constant314 (talk) 22:12, 17 November 2020 (UTC)

What I object to is the use of “usually equivalently” in “Two signals whose levels differ by one decibel have a power ratio of 10^1/10 … or (usually equivalently) an amplitude … ratio of 10^1/20”. It would be more accurate to say “usually inequivalently” but I am not suggesting that because I find it unhelpful. The wording I suggested was “sometimes equivalently”, the accuracy of which surely cannot be disputed, but my edit was reverted. I was using the word “harmonic” in the sense of a harmonic oscillator (Constant314 is correct), but let me spell out my concern more precisely.

• First consider two harmonic signals, both of the form y = A sin(wt – p) , where A is the amplitude, w is the angular frequency, t is time and p is the phase, and let postfixes 1 and 2 denote each of the two signals. Thus y1 = A1 sin(wt – p) and y2 = A2 sin(wt – p), where for simplicity I am assuming the two signals have the same frequency and phase, as any change in these do not affect my main point. Now imagine that y represents current, such that the power is 0.5 A²/r, where r is resistance. If r is the same in both cases we can write P1/P2 = A1²/A2² and the statement I am objecting to is therefore correct for a harmonic signal if the resistance is unchanged.
• Now consider a situation with some other form of current fluctuation. The two currents could be random noise but they could be anything (in real life, most fluctuations are not sinusoidal). One can measure the current fluctuations and from these compute the ratio of their powers P1/P2, but in general there is no amplitude here. One can address the absence of a clearly identifiable current amplitude by replacing it instead with a root-power quantity (R) proportional to the square-root of the power, such that R = constant * sqrt(P). In this situation the statement I am objecting to is incorrect (because there is no amplitude) but becomes correct if one replaces either "usually equivalent" with "sometimes equivalent" or “amplitude ratio” with “root-power ratio”.

Dondervogel 2 (talk) 09:28, 18 November 2020 (UTC)

dBSPL

As much as I love all the technical stuff in this article, probably 90% of the people who find this are just looking for information about "how loud is x vs y" sound pressure levels, which are often reported for point sources in "dB" without any distance or reference level specified.

It might be good to have a simple explainer in the introduction that the "dB" people have heard of is only one of many types, that it's short for "dBSPL", and that the measurements they've heard of are largely meaningless because they don't include distance. — Omegatron (talk) 22:37, 3 December 2020 (UTC)

I think you need a space in "dB SPL". Dicklyon (talk) 04:24, 4 December 2020 (UTC)

Maybe improve the hatnote to include a link to dB SPL or sound pressure level? ~Kvng (talk) 15:13, 7 December 2020 (UTC)

I have improved the hatnote. ~Kvng (talk) 19:56, 14 December 2020 (UTC)

Acoustics: intensity, pressure, or what?

The part about uses in acoustics is confusing. First it talks about pressures, for which a factor 20 must be used. Then it apparently mixes between pressures and intensities, and it's not clear what happens with the formulae (see italic):

"The human ear has a large dynamic range in sound reception. The ratio of the sound intensity that causes permanent damage during short exposure to that of the quietest sound that the ear can hear is equal to or greater than 1 trillion (1012).[39] Such large measurement ranges are conveniently expressed in logarithmic scale: the base-10 logarithm of 10^12 is 12, which is expressed as a sound pressure level of 120 dB re 20 μPa. "

However, 20 x log (10^12) = 20 x 12 = 240 ... I've corrected using only intensities for now, but I'm not sure how correct this is. Am I missing something? Can it be explained better and correctly? Kruiser (talk) 15:24, 14 December 2020 (UTC)

The statement is correct as quoted above, but I agree it's confusing (the 20*log10 is a red herring; better to think of SPL as being 10*log10(p^2/p_0^2) dB). One option might be to replace the sound pressure level of 120 dB re 1 uPa with a sound intensity level of 120 dB re 1 pW/m^2, but I'm not sure that really helps. The whole paragraph could do with a spring clean. Dondervogel 2 (talk) 15:42, 14 December 2020 (UTC)

You are probably seeing the word intensity being used differently between sound and electromagnetics. In E&M, intensity is a field quantity, such the magnetic field intensity, H or the electric field intensity, E. The field quantities go as 120 x log10. Power in E&M is represented by E X H; it goes as 10 x log10. In sound, intensity is power. It goes as 10 x log10. Constant314 (talk) 16:49, 14 December 2020 (UTC)

I'm not too confident with sound intensity, but I'm not really relating to electromagnetism or other fields. Is just that exchanging pressures and intensities is confusing for a reader, I believe. To answer Dondervogel 2, 10*log10(p^2/p_0^2) is exactly 20*log10(p/p_0), so I don't see how that helps. To me, the only explanation is that SPL is 10*log10(p/p_0) or 10*log10(I/I_0), which is the same for a constant velocity (I=pv). Kruiser (talk) 11:48, 16 December 2020 (UTC)

The equations for sound pressure level (SPL) and sound intensity level (SIL) are

• SPL = 10*log10(p^2/p_0^2) dB
• SIL = 10*log10(I/I_0) dB

Dondervogel 2 (talk) 13:03, 16 December 2020 (UTC)

@Dondervogel 2

I'll try to explain, why this formula is correct: dB SPL = 20*log10(p/p_0)

In Math, 20*log10(p/p_0) = 10*log10(p^2/p_0^2), but in the real world, the real sound pressure in Pascals will change for 1 000 000 times for 120 dB (for example), but Intensity will change 1 000 000 000 000 times with the same 120dB because Pressure is root-power quantity and Intensity is power quantity

These quantities are always confusing an I think it's better not use them in one sentence or even in one paragraph

English is not my native language so it's very difficult for me to explain math and physics. Hope you'll understand

Yes it's me who corrected trillion to million here

37.214.77.175 (talk) 23:14, 4 December 2021 (UTC)

Improper use of attachments to dB

@Kvng:@Dondervogel 2: I am afraid this is a misinterpretation. As a unit, there is one decibel only, equal to the ratio 10^1/10:1 for a power quantity and 10^1/20:1 for a root-power quantity. All the attachments belong to the quantity name, not to the quantity unit name, see ISO standards: "Any attachment to a unit symbol as a means of giving information about the special nature of the quantity or context of measurement under consideration is not permitted." . Particularly for dB, see .JOb (talk) 10:52, 24 December 2020 (UTC)

Can you suggest a way to improve the article? Dondervogel 2 (talk) 12:06, 24 December 2020 (UTC)

OK, but it will take quite a time. I shall prepare something. JOb (talk) 12:58, 24 December 2020 (UTC)

Just I modified 3rd paragraph of "Suffixes and reference values". Do you agree so? JOb (talk) 15:26, 24 December 2020 (UTC)

Apart from a minor edit I just made it looks fine to me. Dondervogel 2 (talk) 15:41, 24 December 2020 (UTC)

Fine. @Kvng:@Dondervogel 2: Btw, in shortest time, a new "ISO/IEC 80000-15:2021 Quantities and units. Part 15 - Logarithmic quantities" will occur. When it appears officially, I shall reflect it here. JOb (talk) 16:02, 27 December 2020 (UTC)

References

Power and amplitude ratios in lead

I think the lead may be too technical or confusing for some people. I have a very rough idea of these topics but let's look at it from a layperson's perspective, to which I'm closer than to that of a knowledgeable person. It says:

The decibel (symbol: dB) is a relative unit of measurement corresponding to one tenth of a bel (B). It is used to express the ratio of one value of a power or root-power quantity to another, on a logarithmic scale. A logarithmic quantity in decibels is called a level. Two signals whose levels differ by one decibel have a power ratio of 10^1/10 (approximately 1.25893) or (sometimes equivalently) an amplitude (field quantity) ratio of 10^1⁄20 (approximately 1.12202).

I feel very strongly that many people will be totally confused by this little paragraph. If they wanna know things such as "what's the loudness difference from one decibel to another?" (I know perceived loudness is measured differently, but bear with me, since again, we're looking at it from a layperson's perspective). Unlike centimeters or inches, which many people can visualize, I don't think the bel is a familiar unit of measurement. I know experts will cringe at the suggestion, but maybe we could add some plainer explanation, with some visual analogies, somewhere in the lead? I would attempt to do it myself, but I don't know much about the topic beyond what one picks up producing and compressing music at home for fun. By the way, if you look up "power ratio" on Google the first result is from Investopedia (finance-related website) and the next seem to be references for engineers or physicists. --Paper wobbling sound (talk) 06:15, 13 March 2021 (UTC)

You are right (I agree to your position). Bel or db is not a "class" of it´s own. First "comes" a Ratio (r)of Power P, for example r = P2/P1. In many cases it may be of advantage, to say: x = log P2/P1 where x is said to be x Bel or xB. Where is the trouble for nontechnicians? Here it is: Bel oder decibel (dB) is not a unit! You should not recognize xB as a mathematical product like x multiplied by B. B is not a factor like all (!) other Units. It`s an How To Do, nothing else. Edgar Wollenweber (Germany) --79.204.169.180 (talk) 17:42, 16 April 2021 (UTC)

I have made an incremental simplification reducing the amount of different technical terminology used in the opening paragraph. Readers don't really need to know what a bel is in the first paragraph. The key point to get across in the first paragraph is that it's a relative unit of measurement using a logarithmic scale. ~Kvng (talk) 15:05, 19 April 2021 (UTC)

I suppose so. It might be worth saying that "root" means "square root" in case people don't know that. It might be nice to say somewhere why everyone uses decibel instead of bel, when all the other log units use the log10 version. (pH, optical density, for two). Gah4 (talk) 16:02, 19 April 2021 (UTC)

Gah4, I have imporved Power, root-power, and field quantities to explain where root comes from.

Why is it that other log units don't use 10log10? ~Kvng (talk) 14:08, 22 April 2021 (UTC)

Why would they? Dondervogel 2 (talk) 15:44, 22 April 2021 (UTC)

Why do it here? It is a little easier to work with whole numbers. Easier to pronounce, easier to think about. Optical absorption is commonly log10, but the filters used in color printing are numbered with two digits after the decimal point, and then forgetting the decimal point. That would be centibels if one wanted a unit for them. But they are commonly unitless. For pH, often enough whole steps are fine, but in real experiments often 0.1 steps. Gah4 (talk) 06:45, 23 April 2021 (UTC)

New page for logarithmic units

I believe that Wikipedia should feature a conversion table of units that express ratio. This is not only decibel, but also neper, decade, music intervals from semitone to octave and cents as well... Shall I make a new page and cross-link? --FDominec (talk) 08:40, 8 June 2021 (UTC)

We already have Logarithmic_scale#Logarithmic_units. I suggest expanding that before creating a new page. Dondervogel 2 (talk) 08:56, 8 June 2021 (UTC)