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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

#211

Post by rightbyte »

FRidh wrote: post #209

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., acousti…

> The problem with using decibels is that since it is a relative unit you need to know what it is relative to

Ye but that is also the point. The author seem to prefer to memorize dozens of absolute values.

Re: The scientific “unit” we call the decibel

#212

Post by dotancohen »

xeonmc wrote: post #206
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

KSP measures funds in some unitless value, it drives me nuts. Everything else is almost (but not quite) perfect.

Re: The scientific “unit” we call the decibel

#214

Post by lamename »

I appreciate the sentiment, but I've found this resource [0] much more direct and comprehensive. It explains all of the nuance regarding dB and related terminology from audio engineering to perception (voltage, power, intensity, volume, loudness, etc.)

The format is a bit circular; just enjoy getting lost in it for half an hour.

https://sengpielaudio.com/TableOfSoundPressureLevels.htm

Re: The scientific “unit” we call the decibel

#215

Post by layer8 »

danadam wrote: post #196
layer8 wrote: post #145

Tangential question: Are there any sound meters that can actually measure down to 0 dB? The best I’ve seen specify 20 dB, which to me is still very audible. (Note that, as the article mentions, 0 dB doesn’t mean “zero sound pressure”, but just “threshold of hearing”.)

Some laboratory-grade equipment probably. > 0 dB doesn’t mean “zero sound pressure” From the decibel definition, zero of anything is -∞ in dB_suffix scale.

> Some laboratory-grade equipment probably.

I wasn’t able to find any listings, that’s why I’m asking.

Re: The scientific “unit” we call the decibel

#216

Post by paipa »

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…

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.

Re: The scientific “unit” we call the decibel

#217

Post by rusk »

sanderjd wrote: post #205
rusk wrote: post #199

Earlier quoted context omitted.

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.

The issue is that it’s not “well defined” that it’s somehow subjective. But the whole point with a logarithmic scale is the small values don’t matter.

Re: The scientific “unit” we call the decibel

#218

Post by rusk »

detourdog wrote: post #162
nomercy400 wrote: post #143

Earlier quoted context omitted.

Does that also work for showers, with mixing hot and cold water. I feel that a 1% change in the knob/balance goes from too cold to too hot.

Nothing to do with decibels but the thermostat on the water heater might be set too high. If one lowers the overall temperature the ratio of cold to hot will balance.

You are bounded by a minimum floor for the hot water. Below a certain point you can get legionnaires.

To me it seems like one of two things: external pressure between hot and cold is mismatched so a small change to one side overwhelms the weaker flow.

Alternatively it might just be a broken or poor quality mixer that isn’t providing the appropriate ‘nuance’ of control, and that may indeed be expressed as some sort of non-linear relationship.

Re: The scientific “unit” we call the decibel

#219

Post by godsinhisheaven »

hansvm wrote: post #144
godsinhisheaven wrote: post #142

Earlier quoted context omitted.

This doesn't make any sense to me. Isn't this completely backwards? Wouldn't this behavior be expected from a logarithmic knob, and not a linear knob? I know what a logarithmic curve looks like, it rises quickly and then it tapers off, exactly the behavior you describe. But then you attribute that to a lineae knob! The parent comment confuses the hell out of me too, I am just really not putting 2 and 2 together here.

"logarithmic" here refers to the number on the scale being logarithmic in the sound pressure level. Restated, power is exponential in the knob value, which roughly matches human perception of a linear increase. An actual linear function is far too slow.

Got it, so the sound pressure is logarithmic, but the sound power is exponential, and you can control both at once with one knob, and they, align, quite well I guess.
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