The scientific “unit” we call the decibel • Hacker News Board

Re: The scientific “unit” we call the decibel

#361

Post by duped » Thu, May 22, 2025, 3:32 PM UTC

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 about 2m away before the inverse square law kicks in, unless they've been designed to operate as line sources which decay linearly for a very long distance. For loud sound sources near barriers like the ground they behave like half point sources, which will eventually act like point sources but there's a good bit of distance before it is really measurable.

Re: The scientific “unit” we call the decibel

#362

Post by BobaFloutist » Thu, May 22, 2025, 3:35 PM UTC

sanderjd wrote: post #200

lambdaone wrote: post #187

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…

> In every context where decibels are used, either the unit they qualify is explicitly specified, or the unit is implicity known from the context. The author's whole point is that this is not true. To adapt your analogy, it's not like being mad at the number three, it's like being mad about people not attaching any units to the number three, arguing that it's clear in context. It isn't!

I mean it's always a little bizarre when the default unit used to measure something is dimensionless. Why not set a default (like STP) that's assumed to be the baseline unless otherwise specified? It would be like if Celsius had no reference point and every time you said water boils at 100 degrees Celsius people came out of the word work to smugly correct you "Only if you first say 0 degrees C is when water freezes, which you can't assume. What if 0C is actually absolute 0?"

Re: The scientific “unit” we call the decibel

#363

Post by ashoeafoot » Thu, May 22, 2025, 3:40 PM UTC

drob518 wrote: post #349

strbean wrote: post #333

Earlier quoted context omitted.

Except the various disasters caused by assuming the wrong units (Mars Climate Orbiter, for example).

Which proves what? That misunderstandings happen? Yes they do. Get over it. But most amplifiers and recordings don’t crash and burn, so there’s your counter proof. Use units when they might be ambiguous. But in many fields they aren’t.

No, this are the sounds of underspecification.

Re: The scientific “unit” we call the decibel

#364

Post by WhitneyLand » Thu, May 22, 2025, 3:46 PM UTC

lambdaone wrote: post #187

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 things are not complex because they understand them are rarely correct.

The complexity absolutely is not necessary. Maybe you mean to say it’s understandable or it’s coherent if you know the rules.

Physics doesn’t require us to create ambiguity by assigning DB to mean multiple possible ratios. If it needs to be disambiguated, then they both didn’t need to be DB in the first place.

Re: The scientific “unit” we call the decibel

#365

Post by gregschlom » Thu, May 22, 2025, 3:48 PM UTC

"The bel is named in the honor of Alexander Bell; this is in the same tradition that prompted us to name the “wat” in honor of James Watt."

This line killed me. I literally laughed out loud.

Re: The scientific “unit” we call the decibel

#366

Post by marcosdumay » Thu, May 22, 2025, 3:57 PM UTC

hulitu wrote: post #360

LastTrain wrote: post #356

Earlier quoted context omitted.

Yeah hence his comment “If you know you know.” The author is far from ignorant on the subject, he’s pointing out the unit is often used without context.

[flagged]

You may have missed the short article explaining how it could mean 3 different things in an audio context, or 3 other different things in a radio context. Or how it doesn't actually mean anything by itself and yet people insist on using it that way.

Re: The scientific “unit” we call the decibel

#367

Post by margalabargala » Thu, May 22, 2025, 3:57 PM UTC

camtarn wrote: post #156

margalabargala wrote: post #125

Earlier quoted context omitted.

Could you elaborate on this? If 10mm rain falls, and you have an open collection container, then the depth of the water in that container would be more, less, or the same as the thickness of the branch?

...as long as the container has straight sides. The area of the opening needs to be the same as the area of the container all the way down, otherwise the two areas don't cancel out.

Sure you can do that, and now you're measuring something different. The volume of an exotic container is measured in different units than the depth of rain falling from the sky evenly.

Re: The scientific “unit” we call the decibel

#368

Post by grues-dinner » Thu, May 22, 2025, 3:59 PM UTC

wl wrote: post #330

grues-dinner wrote: post #171

Earlier quoted context omitted.

It's not that arcane, it's referenced to a sound pressure level of 1 pascal. Which, yes, is still arbitrary in that it all ends up back at the arbitrary values of things like metres, seconds, kilograms, etc. Volts per pascal might make sense in some contexts (like your input buffer power supply), but log volts per pascal makes sense in others, particularly for audio applications where you stack gains and attenuations…

dB SPL is referenced to 20 µPa, not 1 Pa. You might be confused by the fact that the 94 dB SPL (1 Pa) is the default for microphone calibrators and specifying microphone sensitivity.

I was referring to the latter, which is why volts are involved at all. But I suppose if they'd chosen to follow the SPL zero point for the voltage-pressure scale as well as the absolute pressure scale that's would be equally arbitrary in its own way, even if more consistent in another.

Re: The scientific “unit” we call the decibel

#369

Post by teknopaul » Thu, May 22, 2025, 4:01 PM UTC

"This is nuts: it’s akin to saying that the milli- prefix should have different meanings depending on whether we’re talking about meters or liters."

Were were here recently with "mega": Sometimes mega is squared as in megapixels. Sometime not as in megabytes.

No biggie.

Db in audio is a relative scale and that makes perfect sense. If you mixer goes + or - 6db that makes sense but can't be measured as power, your mixer might not be plugged in to any speakers so relation to real power is moot in the digital realm.

3 eq bands with -+6db makes sense too. Doesn't need to be precisly specified to be of immediate value, +-12db is clearly something else and users know what.

Re: The scientific “unit” we call the decibel

#370

Post by seanhunter » Thu, May 22, 2025, 4:01 PM UTC

monster_truck wrote: post #341

strbean wrote: post #333

Earlier quoted context omitted.

Except the various disasters caused by assuming the wrong units (Mars Climate Orbiter, for example).

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.