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Comment by lifthrasiir | Hacker News Reader

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 is simply distinct, only connected by the fact that it represents some ratio in the logarithmic fashion. Treat any new dB unit you haven't seen as an alien.

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> The human-perceived loudness level is dB(A) and similar.

But db(A) doesn't really measure that for anything but sounds that could cause hearing damage, or test tones. You've essentially taken a newcomer's problem of underspecification and carried it into the given domain.

I feel like it'd be better to say dB(A) measures flaunkis, which is defined by the human frequency response. Then the newcomer's next question will be something like, "how do I use flaunkis to compute the loudness of a music recording?" And that's the right question to ask, because the answer is: it's complicated. :)

This is exactly it. The people who get confused by decibels are treating it a unit in it's own right when it's really just a ratio of some unit.

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.

I would agree.

Right now what we've got is basically "millis", and you just have to know whether the speaker is talking about length or mass. I like your proposal.

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.

This is right and all... But this usage still leads to confusion about what you are measuring your filter by.

There are filters we measure on power, there are filters we measure on signal amplitude, and "signal amplitude" can be ambiguous on some contexts too. There should be a way to specify this one better.

Well, dB is fully specified in that regard. It's always power. You can calculate the voltage gain from it under certain assumptions, and under normal assumptions you get that factor 2. But a -20dB attenuator will always reduce the power by a factor of 100.

There is also antenna gain in decibels.

dBi

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.

people are often confused by decibels because the necessary disambiguation is more often than not absent (see: spec sheets of some kind of appliance talking about noise)

That is of course not true. dB without reference is perfectly fine to use for gain and attenuation. dBm or any of the variants would be flat out wrong.

Even then, you'll get different results if it's 10 dB of voltage gain or 10 dB of power gain. You need to know what the actual units are.

No, dB are always power gain.

Exactly, and that's why a voltage gain of factor 10 is 20dB, because it's a power gain of factor 100.

...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.

It's correct to use dB without a suffix to indicate a pure ratio. (It's ok if some people don't see why that's correct. That's acceptable.)

Nobody (I hope) says that 100mW is "10 dBm" more than 10mW. That would be wrong. Readers would be confused. Most people would cross their fingers and correct it to "10 dB", but who knows what it might mean?

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