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The scientific “unit” we call the decibel

lcamtuf.substack.com

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Re: The scientific “unit” we call the decibel

#501

Post by kragen »

hatthew wrote: post #500
kragen wrote: post #223

This is an incredibly valuable article for anyone who's trying to make sense of decibels in some context. Michał clearly explains almost all of the gotchas you have to understand. I realized recently, after years of doing it for signal powers, that dB are a pretty convenient way to do mental logarithmic estimates for things that have nothing to do with power or signals, with only a small amount of memorization. Logar…

Does this have anything to do with decibels specifically or is it just general log-space arithmetic?

Decibels specifically are just general log-space arithmetic. (Usually.)

Re: The scientific “unit” we call the decibel

#502

Post by foxglacier »

Ekaros wrote: post #482
foxglacier wrote: post #481

Earlier quoted context omitted.

Moles still work without knowing that atoms exist. The quantity being called "amount of substance" not "number of things" reflects that. However, they are still a redundant over-complicated legacy that doesn't need to exist.

Moles are used because they are useful abstraction. When you want to get from reaction equation to actual real word having some fixed weight is useful. It becomes useful to have fixed relatively usable quantity to work with. They are relatively decent size for actually weighing out things. And honestly mapping number of elementary particles directly to grams is not unreasonable. Even if because physics of world are t…

They're not necessary, just a complicated way of doing what SI prefixes already do. Chemists don't notice this because they're intertwined with a whole network of conventions that would all have to be discarded at the same time. That's the legacy effect - it's too hard to untangle all at once.

Inches are a relatively decent size for measuring lengths too, but people manage without.

Re: The scientific “unit” we call the decibel

#503

Post by kragen »

kragen wrote: post #223

This is an incredibly valuable article for anyone who's trying to make sense of decibels in some context. Michał clearly explains almost all of the gotchas you have to understand. I realized recently, after years of doing it for signal powers, that dB are a pretty convenient way to do mental logarithmic estimates for things that have nothing to do with power or signals, with only a small amount of memorization. Logar…

It occurs to me that 100 is 2% more than 7×7×2, so you could reasonably approximate 7× (if you didn't know it) as follows. Since 1dB is about 23%, 2% is about 0.09dB, so 98× is about 19.91dB, 49× is about 16.90dB, and so 7× has to be half that: 8.45dB. And so it is.

Re: The scientific “unit” we call the decibel

#504

Post by relaxing »

creeble wrote: post #457
relaxing wrote: post #432

Earlier quoted context omitted.

That’s true but not that helpful for determining what your ears actually hear.

For music, they hear about 10dB is half/twice as loud.

That’s true but not really helpful for understanding what the amplifier is reading (thanks to nonlinearities down the line.)

Re: The scientific “unit” we call the decibel

#505

Post by bombela »

nayuki wrote: post #470
cb321 wrote: post #186

Abbreviation confusability is relative to { in fact one might say measured by ;-) - number? entropy? etc. } the listener/reader's knowledge/exposure, much as sound levels need a reference distance. I have heard "bare K" refer to a great many different things, not just kilobits (transmission) or kilobytes (storage) or kilograms (drug trade) or kilometers (foot races) and on & on, but pages or items or etc. The fundame…

> I have heard "bare K" refer to a great many different things The worst of the worst is when it refers to kilometres. Like, "I'm selling my car on the used market and its odometer is 150k", where they mean "150k km", but stacked prefixes are not allowed in metric, so the correct notation is "150 Mm" or "150 megametres". I get immense pushback whenever I point this out - the response is usually along the lines of "bu…

In 3D printing, I often catch people talking acceleration in kilo-milli-meter/second². Written and pronounced "16k accel". Instead of writing "16m/s²" and pronouncing "16 accel".

I have nightmares of 150km meaning kilo-miles instead of kilo-metre. And when I say that my car has 260 mega-meter on the odometer, even people born metric look at me funny.

And don't get me started on MB vs MiB. I have seen so many stupid production outages because of people miss-allocating resources by confusing SI and IEC bytes.

/rant

Re: The scientific “unit” we call the decibel

#506

Post by kazinator »

globular-toast wrote: post #64
kazinator wrote: post #26

Earlier quoted context omitted.

But, no! -3dB is half power, not half loudness. -3dB down is just slightly less loud.

Exactly. There's no way +3dB is perceptibly twice as loud. That's easy to test for yourself. +10dB is roughly twice as loud.

+10dB is twice of something: it is an increase in loudness which is twice compared to a +5 dB increase.

If +10dB is absolutely twice as loud, it means that 110 dB is 32 times as loud as 60 dB. We are then essentially denying that perception of loudness is logarithmic!

Re: The scientific “unit” we call the decibel

#507

Post by badlibrarian »

marcosdumay wrote: post #389
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.

I suppose if you don't count the engineers who work on engines. Or those expensive dynos that use the big font to report horsepower then plot it on the chart along with torque.

Re: The scientific “unit” we call the decibel

#508

Post by badlibrarian »

thaumasiotes wrote: post #444
badlibrarian wrote: post #98

Earlier quoted context omitted.

That a unit of power has a dozen metric abbreviations and converts to watts at different values depending on what it is measuring (and when) is precisely the point. Decibels aren't ridicluous or a unit of measure. Horsepower, however...

> That a unit of power has a dozen metric abbreviations and converts to watts at different values depending on what it is measuring (and when) is precisely the point. How is that point illustrated by an extended digression on the words for the unit in various different languages?

The "extended digression" (single sentence) is a quote that shows the dozen metric abbreviations. A link to the source was provided, with a suggestion to read it in a critical context. And it worked: it taught you to fish. And you promptly quoted the first screenful of text from the link. Let's call this a win-win and move on, shall we?

Re: The scientific “unit” we call the decibel

#509

Post by nayuki »

bombela wrote: post #505
nayuki wrote: post #470

Earlier quoted context omitted.

> I have heard "bare K" refer to a great many different things The worst of the worst is when it refers to kilometres. Like, "I'm selling my car on the used market and its odometer is 150k", where they mean "150k km", but stacked prefixes are not allowed in metric, so the correct notation is "150 Mm" or "150 megametres". I get immense pushback whenever I point this out - the response is usually along the lines of "bu…

In 3D printing, I often catch people talking acceleration in kilo-milli-meter/second². Written and pronounced "16k accel". Instead of writing "16m/s²" and pronouncing "16 accel". I have nightmares of 150km meaning kilo-miles instead of kilo-metre. And when I say that my car has 260 mega-meter on the odometer, even people born metric look at me funny. And don't get me started on MB vs MiB. I have seen so many stupid p…

> acceleration in kilo-milli-meter/second²

Oh dear. I haven't looked into 3D printer specifications, but I definitely understand how it is a problem now that you mentioned it.

Relatedly, most pocket-sized power banks have their charge capacity quoted in milliamp-hours, like 10,000 mAh. There is a high temptation for the layman to call it "10k mAh" instead of the proper and shorter "10 A⋅h". I'm disappointed that people don't think critically when applying the metric system, and just blindly follow certain patterns.

> I have nightmares of 150km meaning kilo-miles instead of kilo-metre.

Yeah, that would be bad. On the other hand, if you bought a used car in America with "150k" on the odometer, I don't have a problem with that meaning "150,000 miles" because rules don't apply to imperial, and "150k" isn't legal metric notation anyway.

> when I say that my car has 260 mega-meter on the odometer, even people born metric look at me funny

I understand your frustration, and I educate people on the existence of the megametre even to an ostensibly metric audience in Canada. (Our road distances are quoted in kilometres, speeds in kilometres per hour, car maintenance intervals in kilometres.)

I pitch megametres as a way to reduce words, imply less accuracy, and all around improve efficiency. For example, if your car calls for an oil change every 8000 km, I just say 8 Mm (megametres) because it's not like the engine will destroy itself if you wait until 9 Mm. And "eight thousand kilometres" is more syllables than "eight megametres".

Similarly, I talk about my yearly bicycling distance as 3 megametres, not 3000 kilometres or "3k k". I mention that chain waxing can support a chain lifetime of over 10 Mm rather than the more verbose 10,000 km.

> And don't get me started on MB vs MiB.

Agreed. And in online debates, I see way too many people who defend the abusive notation where 1 KB = 1024 bytes, and they think that HDD marketers screwed everyone over by using the smaller unit so that their capacities look bigger, and they think that RAM manufacturers have the true claim to prefixes. All of that is nonsense and doesn't hold up to the slightest scrutiny. e.g. https://news.ycombinator.com/item?id=44060099

Re: The scientific “unit” we call the decibel

#510

Post by oddthink »

kragen wrote: post #497
oddthink wrote: post #494

Earlier quoted context omitted.

I've been using decibels as a scale for log-odds for a while when reporting results. It works pretty well, since the relevant part of the scale is wide, unlike using nepers, which I had never heard of until I started noodling around with this.

Like, how many decibels of evidence does this observation provide for this hypothesis? That seems potentially pretty workable, as long as you can ensure conditional independence.

Yes. For like a simple naive Bayes, you can say we start at 0, a user vote is worth +3, a merchant vote is +5, we decay by 0.1 every month, publish if we hit +13 (95%), un-publish at +10 (90%), etc. etc.

It's the same math as logits, but the scale's a bit nicer.

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