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

201–210 of 511 posts

Re: The scientific “unit” we call the decibel

#201

Post by hgomersall »

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…

It's a power ratio. 1dB always means a power ratio of 1.26. That might mean a voltage or current ratio, say, of 1.12, but that is because the relationship between voltage ratios and power ratios is a simple square.

Re: The scientific “unit” we call the decibel

#202

Post by tpoacher »

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…

This sounds more like a problem with the 'real' world then.

Re: The scientific “unit” we call the decibel

#203

Post by sanderjd »

kristjank wrote: post #33

This seems exceedingly ignorant of the work decibels do in telecommunications, RF and fibre engineering. The voltage vs power relationship is something that exists and is a core memory of beginner blunders in the field, but it boils down to a simple 10 vs 20 division operation. Besides that, the decibel simplifies a lot of multiplying very small and very big numbers to summing of two-digit numbers that you can do in…

"It's weird and blunder prone but useful" is not actually a counterargument to the author, who is not arguing that it is not a useful thing.

Re: The scientific “unit” we call the decibel

#204

Post by hgomersall »

svara wrote: post #169
TheOtherHobbes wrote: post #119

Earlier quoted context omitted.

This like arguing that aspect ratios are stupid and wrong because sometimes they apply to a screen which is really big and sometimes it's really small and sometimes they apply to a physical print or a photo or a billboard or a vintage TV and sometimes it's a jpg or a PNG. Aspect ratio is a ratio. It can be a ratio between pixel counts, or between print dimensions, or physical display dimensions. All of which are usef…

Respectfully, I'm not sure you fully understand how dB is used. The analogy to aspect ratios only works for one of multiple uses of decibel. dB SPL and dB(A) are not ratios, they're absolute. You can derive them from a ratio and a reference level, but the former can be expressed in Pascal and the latter relates to Pascals after applying a perceptual correction function. Similarly, dBm can be expressed as an absolute…

Stop saying "one of two possibilities". It isn't. A dB is a power ratio. The fact that you can describe that in terms of voltage ratio is a simple reflection of the fact that power ratios can be described as a voltage ratio squared. When talking about voltage ratios directly from dB you're just short circuiting the necessary square root.

Re: The scientific “unit” we call the decibel

#205

Post by sanderjd »

rusk wrote: post #199
losvedir wrote: post #170

Earlier quoted context omitted.

Nothing in the article and nobody in the comments takes issue with the fact that it's logarithmic. It's everything else that's the problem. (It's a ratio where the base value is situation dependent, and the base of the logarithm varies.)

But the base value is well defined, qualitatively (“a quiet room”), which is fine for what we are talking about which is “experienced loudness”. Once you go past log(3) it really doesn’t matter what the noise was at log(0).

I'm sorry, but did you read the article? This is not the complaint.

If decibels were used only to measure sound relative to "experienced loudness" there would be no complaint.

The complaint is that it is used in many other ways, often without distinguishing what the base unit is.

Re: The scientific “unit” we call the decibel

#206

Post by xeonmc »

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…

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

Re: The scientific “unit” we call the decibel

#207

Post by skrebbel »

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…

To be frank, your comment just reads to me as a "stockholm syndrome" type reaction to a needlessly complex unit that you're intimately familiar with.

You see the same in HN threads where people complain that eg Git or Rust are needlessly complex, there's a swath of people who are so emotionally invested in how well they understand the ins and outs of Git resp Rust that any suggestion that maybe things could be better makes them angry.

It's possible for decibels to be usable and generally fine and also for them to needlessly complex, ie for there to exist better alternatives in each place they're used.

As an example, it makes no sense to me that eg in audio software, volume sliders start at 0 dB and then go down to negative $MUCHO, until complete silence at -Infinity. And then this same unit is also used to measure how loud my coffee machine is, somehow, but then it's suddenly positive and not a relative number at all? That's just weird shit, it's like expressing the luminosity of a pixel (in HSL terms) in lumen instead of a unitless percentage.

In the audio software context, it would be much more intuitive for "no sound" to be 0, and "full volume" to be 100, a bit like percentages. The "but volume needs to be logarithmic because that's how we hear it!" argument doesn't disallow that at all. Just because a slider goes from 0 to 100 doesn't mean that a 10 must mean 10% of the power output. Decibels are ridiculous.

Re: The scientific “unit” we call the decibel

#208

Post by moefh »

dagw wrote: post #190
gwd wrote: post #148

Earlier quoted context omitted.

> Aspect ratio is a ratio. Right, but in this case they only give you one of the two numbers. Imagine being told that your TV had an aspect ration of ":16", and you just have to magically know what the other number means in the context. And sometimes ":16" actually means ":4", because quadratic mumble mumble, and sometimes the number is scaled according to some other "how big it seems to humans" factor; all of which…

Imagine being told that your TV had an aspect ration of ":16" We kind of have that with people talking about a screen or image being "2k" and then expect you to infer what the actual resolution and aspect ratio is from context.

I think the difference is that if you write a blog post complaining about the silliness of these labels, you don't get people telling you that no, you don't get it, it's totally fine, these are just aspect ratios.

Re: The scientific “unit” we call the decibel

#209

Post by FRidh »

The problem with using decibels is that since it is a relative unit you need to know what it is relative to, and that is often not just the unit but also the quantity. Unfortunately, this part is (as expressed) at times omitted or put in the wrong place. And its often also the engineers in the respective fields that keep using this incorrect notation spreading the confusion for those outside the domain.

E.g., acoustic engineers often write db(A) for A-weighted sound pressure levels. Yes, it is often noted this way, but it is incorrect. The correct way is to specify the quantity and that the quantity is A-weighted, `L_{p, A} = 80 dB` for example to express an A-weighted sound pressure level of 80 dB.

Regarding sound pressure and sound power. Sound power is not expressed using A-weighting because it does not make sense. Sound power is a property of the source. A-weighting is a property of the receiver, that is, the human listener.

Re: The scientific “unit” we call the decibel

#210

Post by mark-r »

im3w1l wrote: post #173
atoav wrote: post #44

Earlier quoted context omitted.

As someone who teaches this concept to.. art students I have to say that this complaint sounds a lot like the typical misconceptions and confusions a beginner would have. Yes, dB is a weird and unintuitive concept and it takes a moment to understand it, but it is also extremely useful once you get it. The fact that people don't write out the reference values does not help either, people will bounce out that audio mix…

So one thing I always wondered about with audio mixing is that clipping should be random? Like if the sine waves somehow all line up the momentary amplitude could be much greater than the typical amplitude. And this would mean that there isn't a fixed volume above which you get clipping and below which you don't get clipping. Playing tricks with phases could prevent/cause clipping. Is -20 db then simply a rule of thu…

You're talking about the difference between Average amplitude and Peak amplitude. Yes, the peaks will be higher than the average, but not by a constant amount unless you're working with a pure constant sine wave. It will depend on the sound you're working with. The phase is part of that sound so generally you can't mess with it.

-20 db would just be a starting point, you could adjust up or down from there.

Post reply on HN