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The scientific “unit” we call the decibel

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Re: The scientific “unit” we call the decibel

#491

Post by formerly_proven »

nothacking_ wrote: post #466
formerly_proven wrote: post #18

Decibels make sense and it’s usually only laymen who use only „dB“. I’d be surprised if you’d find „-45 dB“ as the specification for a microphone. Here’s two random examples: Sensitivity at 1 kHz into 1 kohm: 23 mV/Pa ≙ –32.5 dBV ± 1 dB Sensitivity: -56 dBV/Pa (1.85 mV)

Except finding '-45 dB' is frustratingly common. Have a look at this random microphone datasheet from DigiKey: https://www.sameskydevices.com/product/resource/cmm-3125at-4... They do state the test conditions "at 94 dB SPL, 1 kHz", but don't specify the units attacked to the actually measurement. It's given as a ratio to an unspecified quantity.

Thanks, it's a good example for this bad practice. Especially because this could reasonably be both dBu and dBV, so it's actually ambiguous. (On the other hand with that tolerance the difference between the two is not that significant)

Re: The scientific “unit” we call the decibel

#492

Post by filterfish »

duped wrote: post #361
etskinner wrote: post #293

Here's another related one that always bothers me: When you say something's loudness in decibels, you also need to specify a measurement distance. The author of this article even accidentally makes this omission: > It’s 94 dB, roughly the loudness of a gas-powered lawnmower And that distance is very important; the actual sound pressure measured is proportional to distance^2. So for a lawnmower measuring 94dB, let's s…

> And that distance is very important; the actual sound pressure measured is proportional to distance^2. While we're sniping nerds, the inverse square law only applies in the far field (which is tautologically "far enough away for the source to behave as a point source and follow the inverse square law"). That's probably a good bit further than 1m for a lawnmower in the physical world. For loudpseakers you have to be…

Whether something is near field or far field is frequency dependent

Re: The scientific “unit” we call the decibel

#493

Post by mwbajor »

holowoodman wrote: post #56
kristjank wrote: post #33

This seems exceedingly ignorant of the work decibels do in telecommunications, RF and fibre engineering. The voltage vs power relationship is something that exists and is a core memory of beginner blunders in the field, but it boils down to a simple 10 vs 20 division operation. Besides that, the decibel simplifies a lot of multiplying very small and very big numbers to summing of two-digit numbers that you can do in…

Decibels are completely ridiculous. They are only useful if you are a cable monkey just adding and subtracting amplification/dampening factors. As soon as you need to do any kind of non-trivial conversion or computation, dB* is more of a hindrance to understanding and simplicity, because you will always need to look up in some strange table of dB weirdness. Integrate over a spectral power density in dB/Hz? You better…

They're just ratios....

Re: The scientific “unit” we call the decibel

#494

Post by oddthink »

kragen wrote: post #223

This is an incredibly valuable article for anyone who's trying to make sense of decibels in some context. Michał clearly explains almost all of the gotchas you have to understand. I realized recently, after years of doing it for signal powers, that dB are a pretty convenient way to do mental logarithmic estimates for things that have nothing to do with power or signals, with only a small amount of memorization. Logar…

I've been using decibels as a scale for log-odds for a while when reporting results. It works pretty well, since the relevant part of the scale is wide, unlike using nepers, which I had never heard of until I started noodling around with this.

Re: The scientific “unit” we call the decibel

#495

Post by mikewarot »

rcxdude wrote: post #181
mikewarot wrote: post #75

Earlier quoted context omitted.

20 dB is always the same, actually. If you multiply voltage (and thus current as well) by 10:1 (20 db) the power is multiplied by 100:1 (also 20 dB)

That's the point that's confusing. Especially in contexts where it's not obvious whether it's power-like or amplitude-like. It should be 40dB of power gain or 20dB of amplitude gain, with the context made explicit.

I don't see how it's confusing. Thanks to the scaling of voltage dB, you never have to worry, 10 dB of voltage gain is the same as 10 dB of power gain.

Re: The scientific “unit” we call the decibel

#496

Post by monster_truck »

seanhunter wrote: post #370
monster_truck wrote: post #341

Earlier quoted context omitted.

The team that wrote their code in English units instead of Metric defied specifications, that has nothing to do with this. > The Software Interface Specification (SIS), used to define the format of the AMD file, specifies the units associated with the impulse bit to be Newton-seconds (N-s). Newton seconds are the proper units for impulse (Force x Time) for metric units. The AMD software installed on the spacecraft us…

Hey don't blame the English for that. I would be prepared to wager you couldn't find a single English engineer who uses lbfs or anything similar. Everyone in physics or engineering uses metric for everything to do with forces even those who might use mph for a speed informally.

Nobody is blaming the english for anything, those are simply the units they used.

Re: The scientific “unit” we call the decibel

#497

Post by kragen »

oddthink wrote: post #494
kragen wrote: post #223

This is an incredibly valuable article for anyone who's trying to make sense of decibels in some context. Michał clearly explains almost all of the gotchas you have to understand. I realized recently, after years of doing it for signal powers, that dB are a pretty convenient way to do mental logarithmic estimates for things that have nothing to do with power or signals, with only a small amount of memorization. Logar…

I've been using decibels as a scale for log-odds for a while when reporting results. It works pretty well, since the relevant part of the scale is wide, unlike using nepers, which I had never heard of until I started noodling around with this.

Like, how many decibels of evidence does this observation provide for this hypothesis? That seems potentially pretty workable, as long as you can ensure conditional independence.

Re: The scientific “unit” we call the decibel

#498

Post by WorldMaker »

lifthrasiir wrote: post #130
margalabargala wrote: post #128

Earlier quoted context omitted.

Disagree. The people who get confused by decibels, are exposed to other people treating it like it's a unit in its own right. I agree that what the parent described, should be done. If it was what was done, this article wouldn't exist.

As I've said in the other comment, I believe this should be ultimately addressed by the SI.

It's SI that caused some of this weirdness in the first place by discouraging "same unit" ratio units like m/m or kg/kg and then intentionally disallowing those derived units to be individually named. On the one hand, there is a sort of "clean sense" that km/km cancels out and disappears, but on the other hand there are too many "unitless quantities" in SI that are very formula-specific that a ratio unit would better explain and help save the wrong thing from being plugged into the formula. It's enough of a need that scientists found themselves using worse tools like deciBels for some of these "same unit" ratio formulas just to have "some unit at all" to avoid accidental unit-less mistakes.

Some Scientists and Mathematicians would rather use random log_10(x) functions than allow units like km/km in their formulas. It's wild, and SI has been a part of those decisions all along.

Re: The scientific “unit” we call the decibel

#499

Post by hunter2_ »

dagw wrote: post #190
gwd wrote: post #148

Earlier quoted context omitted.

> Aspect ratio is a ratio. Right, but in this case they only give you one of the two numbers. Imagine being told that your TV had an aspect ration of ":16", and you just have to magically know what the other number means in the context. And sometimes ":16" actually means ":4", because quadratic mumble mumble, and sometimes the number is scaled according to some other "how big it seems to humans" factor; all of which…

Imagine being told that your TV had an aspect ration of ":16" We kind of have that with people talking about a screen or image being "2k" and then expect you to infer what the actual resolution and aspect ratio is from context.

The real absurdity of 2k/4k/etc is that those refer to approximate horizontal pixel counts, when "lines" (vertical pixel count) such as 480/525/1080/etc has been the typical dimension for such a long time. Why the switch? In the 16:9 world, ~2k horizontal is close enough to ~1k (1080) vertical, and ~4k horizontal is close enough to ~2k (2160) vertical, etc. so it can't be that the horizontal numbers round to the nearest k better than the vertical numbers.

Re: The scientific “unit” we call the decibel

#500

Post by hatthew »

kragen wrote: post #223

This is an incredibly valuable article for anyone who's trying to make sense of decibels in some context. Michał clearly explains almost all of the gotchas you have to understand. I realized recently, after years of doing it for signal powers, that dB are a pretty convenient way to do mental logarithmic estimates for things that have nothing to do with power or signals, with only a small amount of memorization. Logar…

Does this have anything to do with decibels specifically or is it just general log-space arithmetic?
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