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

Re: The scientific “unit” we call the decibel

#401

Post by 00N8 » Thu, May 22, 2025, 5:09 PM UTC

klodolph wrote: post #265

viraptor wrote: post #243

Earlier quoted context omitted.

That only works if you're already familiar with the context and system and assume other people are too and don't care about anyone new to that area. (Good luck coming to the audio equipment datasheet with no experience and figuring out what the dB means in each case) "He's 3" works because of the previous question and because everyone had experience of talking about age. dB for anyone not already knowing the answer i…

Maybe I just live on the planet, but I don’t have this problem with dB and to me, it sounds like you’re the alien. Maybe you could elaborate, or give a motivating example? I just don’t remember encountering the problem you’re describing, and it’s unfamiliar to me. There’s something about your experience that I don’t understand, but I don’t know what it is.

I often see pop sci articles saying something like '400 dB would represent a sound strong enough to tear the world apart', or 'military sonar is X dB -- strong enough to liquefy your organs at Y distance'. It's rarely clear to me which of these usages of 'dB' are directly comparable. I think the dB measurement for sonar is a different scale/unit than the one for hearing damage thresholds in air, but I couldn't figure out how to convert between the two last time I spent a few minutes trying to look it up, so in my opinion it can be fairly confusing.

Re: The scientific “unit” we call the decibel

#402

Post by kazinator » Thu, May 22, 2025, 5:12 PM UTC

dj3l4l wrote: post #113

kazinator wrote: post #49

> This means that if you’re talking about watts, +1 bel is an increase of 10×; but if you’re talking about volts, it’s an increase of √10×. 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. Well no, because even if you are focusing on a signal measured in volts, the bel continues to be related to power and not voltage. As…

The Bel is a unitless quantity. Yes, by convention, in certain fields, it applies to the logarithm of the ratio of powers. But in other fields (for example, quantifying a change in the degree of evidence for a hypothesis, as in Bayesian probability theory) it is applied to a ratio of different quantities (in the Bayesian case, a ratio of probabilities). There is no reason why dB can't be used for any unit, and its me…

Mathematically, the Bel is unitless because it is a function applied to a ratio of measurements. Whatever units those measurements have disappear due to the cancelation.

If that measurement is linked to another one in a nonlinear way, that nonlinearity doesn't disappear: the fact that if the unitless fraction in which certain units canceled out is 2, then the corresponding unitless fraction obtained via different units is 4.

Just because the units disappeared in a fraction doesn't mean they are not relevant.

Re: The scientific “unit” we call the decibel

#403

Post by bigstrat2003 » Thu, May 22, 2025, 5:15 PM UTC

jayd16 wrote: post #351

klodolph wrote: post #234

Earlier quoted context omitted.

Sure, but the author is wrong. “How old is your son?” “He’s 3.” Clear in context. People write things like dB SPL (A-weighted) in spec sheets because spec sheets benefit from being unambiguous. Most of the time it’s really clear, like you’re talking about insertion loss or amplifier gain and there’s only one reasonable way to interpret it.

When you say 35 it would be strange for it to be anything but years. 3 days, weeks, months or years are ironically all common units when someone is "3".

Most of the time if someone says "he's 3", it is a good bet that they mean 3 years. People usually specify if they mean days/weeks/months with respect to someone's age. Not always, of course, but it's definitely uncommon to drop the unit when it's anything except years.

Re: The scientific “unit” we call the decibel

#404

Post by jameshart » Thu, May 22, 2025, 5:18 PM UTC

tadfisher wrote: post #399

jameshart wrote: post #378

Earlier quoted context omitted.

‘There is no such thing as zero volume’ - can you expand on that? Surely constant unchanging air pressure has zero volume? Volume is the range of variation in air pressure, right? That can surely go down to zero. It can also only meaningfully go up to about double the absolute mean air pressure, before what you are talking about becomes shockwaves of overpressure.

Air pressure is a statistical measure; given a room with zero net variation in pressure, I can always find a volume in that room with positive pressure, down to measuring Brownian motion of atoms. Now try to design a measurement apparatus that can only sample a small volume to measure variations in pressure, and you can understand why (sound) volume can never go to zero.

Over meaningful statistical sampling areas, like an eardrum, say, pressure can be effectively constant, surely?

Volume can be a statistical property, like temperature.

Re: The scientific “unit” we call the decibel

#405

Post by skrebbel » Thu, May 22, 2025, 5:21 PM UTC

_kb wrote: post #300

lxgr wrote: post #287

Earlier quoted context omitted.

> They're not the same unit, at all. Exactly, so why label two different things using the exact same letters in a potentially ambiguous context.

Because it’s not ambiguous. If I pay for something in Australia and the bill comes to $50 this has meaning within that context. I receive a bill in Zimbabwe for $50 this also has meaning within that context. These values are not equivalent. Ditto if I were to say it’s 30 degrees out. You may interpret that as either a good day for the beach, nice weather for ice skating, or we need to bear north-northeast depending o…

This is unnecessary complexity. My rant is against that exactly. You're defending confusing shit that doesn't get any better from being confusing. If all countries had a different currency, things would be clearer too. Ask any Australian shopping for digital products on international sites. Half of the sites write $ but forget to specify whether it's "we geolocated you and guessed AUD" or "haha it's USD but we just wrote $ because we forgot that there's a world outside the US". If Switzerland would rename the CHF to Euro but not change its value to match the existing Euro, everybody would agree that that's a terrible idea. There wouldn't be edgy HN commenters explaining that well, actually, there's precedent so it's fine! No, it'd just be bad. The dB ambiguity is a mess for the same reason. The situation has no benefit and in the world of units, where most other things (eg the most of the SI) are actually relatively usable and non-ambiguous, it's a fuckup. And the power vs voltage aspect of it makes it even worse than the $ situation.

Your argument that it isn't so bad in practice doesn't change the fact that it has no benefits whatsoever.

It's just ambiguity for the sake of it, because way back when people started measuring sound stuff, nobody bothered to go "but wait is this actually handy?" and then we got stuck with whatever the first guy came up with. It's just like the whole kilobyte/kibibyte crap and the whole Wh vs mAh vs kilojoule soup. It's all downside.

Re: The scientific “unit” we call the decibel

#406

Post by lambdaone » Thu, May 22, 2025, 5:21 PM UTC

weinzierl wrote: post #193

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…

"decibels are simply a dimensionless ratio, used as a multiplier for some known value of some known quantity." Except they are not. 1 dB can sometimes mean a ratio of ~ 1.26 and other times it can mean a ratio of ~ 1.12. "In every context where decibels are used, either the unit they qualify is explicitly specified, or the unit is implicity known from the context." Maybe in university, but certainly not in the real w…

In the "real world", techically competent people understand decibels just fine.

But this is not arcane knowledge only known by a priesthood. Since you are clearly confused about what a decibel is and how and why it is used, you can read

https://en.wikipedia.org/wiki/Decibel

and all will be revealed.

Re: The scientific “unit” we call the decibel

#407

Post by kazinator » Thu, May 22, 2025, 5:23 PM UTC

paipa wrote: post #216

dj3l4l wrote: post #113

Earlier quoted context omitted.

The Bel is a unitless quantity. Yes, by convention, in certain fields, it applies to the logarithm of the ratio of powers. But in other fields (for example, quantifying a change in the degree of evidence for a hypothesis, as in Bayesian probability theory) it is applied to a ratio of different quantities (in the Bayesian case, a ratio of probabilities). There is no reason why dB can't be used for any unit, and its me…

The denominator isn't the issue. The context-dependent base of the logarithm is, which makes 1 Bel = 10x for some things and 1 Bel = 3.16x for others. I've never heard of decibels used in probability theory. Did they adopt it with the same baked-in bastardizations? Please tell me +10dB(stdev) = +10dB(variance) isn't a thing.

(Funny you should mention that because while writing my grandparent comment above I vaguely ruminated about some kind of example involving standard deviation and variance, due to those being linked by squaring.)

Even if there isn't a +10dB(stddev), logarithmic graphs are a thing, in many disciplines. You just refer to the axes as "log ". Any time you are dealing with data which has a wide dynamic range, especially with scale-invariant patterns.

Back in the realm of electronics and signal processing, we commonly apply logarithm to the frequency domain, for Bode plots and whatnot. I've not heard of a word being assigned to the log f axis; it's just log f.

Re: The scientific “unit” we call the decibel

#408

Post by margalabargala » Thu, May 22, 2025, 5:35 PM UTC

camtarn wrote: post #394

margalabargala wrote: post #367

Earlier quoted context omitted.

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.

I have no idea what you mean here. Exotic containers? I'm talking about something like a measuring cylinder or a straight-sided glass or mug. This StackOverflow answer probably does the subject more justice than I can: https://earthscience.stackexchange.com/questions/14587/what-... but I'll try and explain it succinctly: What you're measuring is the volume of rain falling per unit area in a given time (usually 24 hou…

I couldn't have explained it better myself. Glad you understand!

Aside from the last bit. The rainfall per unit area is not relevant to the shape of a particular container. 5mm of rain falls regardless of your container shape. Whether that 5mm of rain falling, also means that the water in your container is 5mm deep, is a function of your container shape.

It's not a unit problem. If you're trying to measure rainfall in a conical vessel, you can do that, and the conversion from collected volume to fallen rain will still yield the same 5mm out of the sky.

Re: The scientific “unit” we call the decibel

#409

Post by radiowave » Thu, May 22, 2025, 5:39 PM UTC

formerly_proven wrote: post #355

radiowave wrote: post #317

Earlier quoted context omitted.

Though that's still incomplete, because (more of the context stuff that you just have to know already) the "A" here refers to the frequency-weighting scheme used in the measurement, and not to the reference level (which is SPL). It should probably be given as: dB(A) SPL, or dB SPL (A-weighted).

Though that's still incomplete, because (more of the context stuff that you just have to know already) the "SPL" here refers to the reference level in air (20 µPa) and not water or oil (1 µPa). It should probably be given as: dB(A) SPL (1=20 µPa). Though that's still incomplete, because (more of the context stuff that you just have to know already) the 20 µPa only applies at the standard temperature and pressure. It…

Granted, I should have been clearer about my intended point, which is just the hazard of assuming that the letter(s) after the dB tell you the reference, because sometimes they don't.

Re: The scientific “unit” we call the decibel

#410

Post by drob518 » Thu, May 22, 2025, 5:43 PM UTC

Nevermark wrote: post #390

drob518 wrote: post #349

Earlier quoted context omitted.

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…

You’re arguing both sides. If nobody does it then it’s not a problem.