Hacker News Board Hacker News, browsed like a classic message board
Live data from Hacker News

The scientific “unit” we call the decibel

lcamtuf.substack.com

Post reply on HN
Print view View original

381–390 of 511 posts

Re: The scientific “unit” we call the decibel

#381

Post by bitdivision »

BenjiWiebe wrote: post #320
remram wrote: post #267

Earlier quoted context omitted.

I didn't get that reading the article. The author acknowledge that ratios are useful, but that there are specific problems in how we use this unit and how we picked and express the reference scales. > On the face of it, the idea makes sense. Your specific example is a pure ratio so there's no problem with it (there is no reference). Apart from the fact that I have to guess whether you are measuring volts or watts thr…

So what I learned today, via another comment and confirmed with a dictionary, is that decibels (unspecified reference) is NOT pure ratio - it's a _power_ ratio. So it will be Watts, not Volts.

But that's why it's so confusing. Did they purposefully leave off the reference to indicate Watts, or did they mean volts (as I'd guess audio engineers typically do when talking about amplifiers).

Re: The scientific “unit” we call the decibel

#382

Post by jameshart »

teknopaul wrote: post #369

"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 mixe…

Megapixels aren’t ‘squared’ though?

Ah, unless you’re trying to make ‘pixels’ the same unit as in ‘pixels per inch’…

The problem there isn’t how ‘mega’ is applied but how ‘pixel’ means both an area pixel as well as the linear size of a pixel.

Re: The scientific “unit” we call the decibel

#383

Post by marcosdumay »

davrosthedalek wrote: post #332
lifthrasiir wrote: post #159

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.

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.

Re: The scientific “unit” we call the decibel

#384

Post by strbean »

teknopaul wrote: post #369

"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 mixe…

I think the biggest issue by far is that many of the different contextual uses for dB are all in the same domain, or very close.

When you're talking about the loudness of sound, in the same exact context you might care about SPL, perceived loudness, AND gain.

If it was just a matter of "in electrical engineering / physics, dB implies this unit + baseline, when dealing with acoustics, it implies this other unit + baseline", it would be less problematic.

Re: The scientific “unit” we call the decibel

#385

Post by davrosthedalek »

viraptor wrote: post #336
drob518 wrote: post #325

Earlier quoted context omitted.

But it’s not. He’s raging against dB.

Yes. He's complaining against dB with no reference, not against dB(A) for example. (Apart from the naming of some of them being silly)

But dB without reference makes sense in many many occasions. Either because the reference is implicit (not ideal, but we have many implicit assumptions in communication), or because it's genuinely a ratio. Attenuation, gain.

If you every find an "official" written document that uses dB not as attenuation/gain and is not specifying the reference (at least in a footnote), it's written either by idiots or for idiots, or both.

Re: The scientific “unit” we call the decibel

#386

Post by Nevermark »

drob518 wrote: post #301
sanderjd wrote: post #200

Earlier quoted context omitted.

> 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!

Just because the author is ignorant of the context doesn’t mean that the engineers working in those fields are. They use it because it all makes sense in context.

The point of units is to indicate both what dimension, and relative magnitude in that dimension, is being talked about clearly.

In virtually any other situation, leaving off units and counting on context to fill them in would be considered to be at the extreme end of unacceptable.

The unit problems in question, are only accepted because they are an historically created anomaly. Not because they are a good idea, or anyone wanted that outcome.

Re: The scientific “unit” we call the decibel

#387

Post by elfrinjo »

Aachen wrote: post #276
elfrinjo wrote: post #227

I'll summarize: It's annoying that dB is often used as a physical unit without the necessary suffix. It's annoying that the suffix seems quite random for most units It's annoying that the factor for power differs from the factor for voltage, etc. It's slightly annoying that it's usually decibel instead of Bel. However, we all agree that dBs are really useful.

> However, we all agree that dBs are really useful. Which part of the article was that again? I was surprised to see that conclusion as summary because it's not what the takeaway seemed like to me, so went back to the article and I don't see this mentioned, perhaps I'm overlooking it because I didn't do a careful read a second time

oh sorry; thats the comments

Re: The scientific “unit” we call the decibel

#388

Post by amelius »

mik1998 wrote: post #358
amelius wrote: post #357

Speaking of which. Does anyone know why, in Physics, we only use the operators +, * and exponentiation? And why higher operators in the hyperoperation sequence are never used? https://en.wikipedia.org/wiki/Hyperoperation

You would use it if it was useful or convenient

It was more of a question about the nature of our universe, similar to: why are there only n spatial dimensions?

Re: The scientific “unit” we call the decibel

#389

Post by marcosdumay »

badlibrarian wrote: post #71

Horsepower is a unit of measurement. There's hp and bhp and hpE and... "Other names for the metric horsepower are the Italian cavallo vapore (cv), Dutch paardenkracht (pk), the French cheval-vapeur (ch), the Spanish caballo de vapor and Portuguese cavalo-vapor (cv), the Russian лошадиная сила (л. с.), the Swedish hästkraft (hk), the Finnish hevosvoima (hv), the Estonian hobujõud (hj), the Norwegian and Danish hestekr…

Yes, those are the reasons why nobody uses horsepower in an engineering context.

Re: The scientific “unit” we call the decibel

#390

Post by Nevermark »

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.

> But in many fields they aren’t.

I am lost. What fields you are talking about?

1. I am unaware of any field operating within its own echo/context chamber using unit-less numeric notation for anything but actual unit-less quantities. Except for informal slap-dash arithmetic, on trivial calculations.

2. Units indicate the dimension being measured, not just the relative magnitude within that dimension. Nobody is going to know from any shared context, except in person, what a bare number measures.

3. Virtually every measurable quantity has multiple possible units of different relative magnitude, depending on micro context, so even people within a field, who agree on the dimension measured, still need units. Meters, light years, AU, angstroms?

4. You cannot apply standard formulas of physics, or anything else, without specific units. Formulas operate on dimensions, but to interpret and calculate any numbers, you need to know the specific unit being used for each dimension.

(In any context, but a late night napkin argument between two well acquainted colleagues in a bar, units are universally used. And in that case, the opportunity for serious misunderstandings is more likely to be from missing units, than the quantity of scotch each has imbibed, or how much they have spilled on the napkin.)

Post reply on HN