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The scientific “unit” we call the decibel

lcamtuf.substack.com

421–430 of 511 posts

Re: The scientific “unit” we call the decibel

#421

It's a bit like hating numbers, and saying "What do you mean by 'three'; what is it - three volts, three amps, three metres? Clearly 'three' is meaningless, and we should stop using it and all the other numbers besides." decibels are simply a dimensionless ratio, used as a multiplier for some known value of some known quantity. In every context where decibels are used, either the unit they qualify is explicitly speci…

> People who assume that everyone is an idiot but themselves are rarely correct.

This is an off-by-one error.

Re: The scientific “unit” we call the decibel

#422
post #206

Earlier quoted context omitted.

Interviewer: "So what's the fastest you've gone on your bike?" Kid: "Thirty." Interviewer" "Thirty what?" Kid: "Umm..." Kid: "Speed." [0] https://www.youtube.com/watch?v=OYt1kqDNlMY

KSP measures funds in some unitless value, it drives me nuts. Everything else is almost (but not quite) perfect.

https://en.wikipedia.org/wiki/Currency_sign_(generic)

Re: The scientific “unit” we call the decibel

#423

Earlier quoted context omitted.

> I don't see a practical reason why dB's normal use can't have been covered by normal prefixes Because instead of numbers going like 1, 10, 100, 1k, 10k, 100k, 1M, 10M, 100M, 1G, and so on when using prefixes, we get a much more smoother numbers of 0 dB, 10 dB, 20 dB, 30 dB, 40 dB, 50 dB, 60 dB, 70 dB, 80 dB, 90 dB. You can see the the number for the dB get bigger, while when using prefixes the numbers get bigger tw…

> You can see the the number for the dB get bigger, while when using prefixes the numbers get bigger two times in a row and then go back to smaller Interesting, I don't have any issues with that, and I see the numbers getting bigger and bigger no problemo. Perhaps it's an issue of metric/imperial, as I grew up in a metric country: I have a mental visual model of decades, while dB feels linear. The opposite is likely…

Sure, log scales are done quite often with SI prefixes for absolute (non-ratio) data that doesn't have a standard reference (which is the case for component datasheets). But dB can be more convenient when you want to present logarithmically-varying ratios, particularly for gain or attenuation (which are relative to unity) or when you have a commonly-accepted reference of your absolute data that is a useful standard to compare against. This gives your 0-point typically at the top or bottom of the axis a standard meaning across all graphs in a particular domain, and then datapoints typically have about a couple significant digits above the "." of the dB datapoint that you generally are interested in.

I grew up in a metric country too, but still it is much easier to speak of 0 to 100 or so of decibels with decades of power being in increments of 10 rather than having to say 10, 100, 1k, 10k, 100k, 1M, 10M, 100M, 1G, 10G, etc of gain or 100m, 10m, 1m, 100u, 10u, 1u, 100n, 10n, 1n, 100p, etc of attenuation. Especially when gains or attenuations would be multiplied, then decibel makes it really easy to just add and or subtract in decibels. For an example, a signal with 10M of gain (or that is 10M times some reference) that gets passed through 100m attenuation would result in a 1M signal (which takes my brain some fiddling with those letters and numbers), but in decibel we are just dealing with simple addition & subtraction: 70 dB minus 10 dB equals 60 dB.

Re: The scientific “unit” we call the decibel

#424

It's a bit like hating numbers, and saying "What do you mean by 'three'; what is it - three volts, three amps, three metres? Clearly 'three' is meaningless, and we should stop using it and all the other numbers besides." decibels are simply a dimensionless ratio, used as a multiplier for some known value of some known quantity. In every context where decibels are used, either the unit they qualify is explicitly speci…

I think college undergrad physics and engineering is an important time to gain exposure to a variety of concepts and measures. Then you learn the people who discovered or invented these concepts were writing by candlelight and not washing their hands and didn’t have modern mathematics or computers and didn’t want to piss off the church. Oftentimes working at the pleasure of a rich sponsor family or king.

Like you have to describe relative intensity of waves in some way and these were experimental scientists, not commerce merchants looking for absolute interchangeability like weights and measures.

Computer blog people like standards and languageisms but science isn’t determined by big tech sponsored committee. It’s the best tool put forward so far and db is a physics concept and with a reference denominator you can calculate the absolute value. It’s fine. If you dabble with physics and expect the universe to make intuitive sense then you need an education.

Read books not blogs.

Re: The scientific “unit” we call the decibel

#425

The only thing you should know is that any use of bel and thus decibel should ideally have the reference level suffixed (usually in parentheses or subscript), not implied. The absolute sound pressure level is dB(SPL). The human-perceived loudness level is dB(A) and similar. The RMS voltage expressed in power is dB(u) (formerly dB(v), not same as capital dB(V)). And so on. And then each different instance of dB unit i…

...unless we're talking about a unitless ratio, like "this has 10dB less power than that". Which happens a lot.

Re: The scientific “unit” we call the decibel

#426
post #352

Earlier quoted context omitted.

> The units are implied by the context. This is the absurd part. There do exist other ratios that masquerade as units, e.g. specific gravity, and its meaning also changes depending on what you’re using it for - liquids are compared to pure water at 4 C, gases are compared to air at 20 C. As the parent comment points out, you can get used to things with experience. That doesn’t make them any less absurd. Look at Fahre…

> Look at Fahrenheit, for example. I’m American, and I still think it’s absurd, but because I’m extremely used to it, it feels natural. What do you find absurd about Fahrenheit? If it is that its 0 point is not absolute 0 then I think you can make a good case, at least for scientific work. It's a bit harder to make the case for absolute 0 being the 0 point a scale for ordinary day to day use since all temperatures mo…

It's likely as simple as Celsius befriending/visiting France, while Fahrenheit - England...

Re: The scientific “unit” we call the decibel

#427
post #291

When I worked on a radar project, my fellow radar engineers (I'm software) used dB a lot. A lot of them would actually agree with the article, but historical sometimes wins even when you're aware of its shortcomings. Aren't we the same in software anyway? The email protocol, terminal escape sequences, the UX of git command line, etc... Each of those could have an "X is ridiculous" blog post (and I would enjoy every s…

> A lot of them would actually agree with the article, but historical sometimes wins Indeed – as evidenced by some parts of the world still using non-metric units in daily life or even engineering :)

As a European if somebody tells me the diagonal size of a display in cm or meter, I'm simply not able to "grasp" it. I need to whip out the calculator and divide by 2.54, turning it into inches. It's just how monitors are measured in my head at this point.

Re: The scientific “unit” we call the decibel

#428
post #259

Earlier quoted context omitted.

> Below a certain point you can get legionnaires. I know you mean legionnaires' disease, but the idea of a bunch of soldiers getting to your house because you turned your boiler too low made me chuckle. Good thing the US have the third amendment to protect against this.

In this day and age I believe anything is possible.

"What have the Romans ever done for us ?

... hot water baths ..."

Re: The scientific “unit” we call the decibel

#429

Earlier quoted context omitted.

I actually want those suffixes mandatory, because there may be multiple plausible suffixes for each use. For example the loudness might be dB(A), dB(B), dB(C), dB(D) depending on the exact curve or even dB(SPL) if the sound pressure level is used as a proxy. So it is much more confusable than, say, "millis" when suffixes are implied.

There is a legitimate use of dB without a reference point. An attenuator attenuates by -20dB, not by -20dBm.

There is also antenna gain in decibels.

Re: The scientific “unit” we call the decibel

#430
post #425

The only thing you should know is that any use of bel and thus decibel should ideally have the reference level suffixed (usually in parentheses or subscript), not implied. The absolute sound pressure level is dB(SPL). The human-perceived loudness level is dB(A) and similar. The RMS voltage expressed in power is dB(u) (formerly dB(v), not same as capital dB(V)). And so on. And then each different instance of dB unit i…

...unless we're talking about a unitless ratio, like "this has 10dB less power than that". Which happens a lot.

But we used to use the same unit for the absolute measure and the relative measure, like degrees Celcius/Fahrenheit. (Okay, % vs. %p is different but is probably an exception.) I see no particular reason to avoid suffixes in such situations.
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