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

Re: The scientific “unit” we call the decibel

#51

Post by rocqua » Thu, May 22, 2025, 7:55 AM UTC

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…

Decibels in gain, those are fine. Though they use a silly base. dBm makes a decent amount of sense, given the RF background. The fact that decibels work differently for voltage and power is very weird, but understandable in isolation.

But audio decibels are horribly underspecified. And any other use of a decibel as a dimensionful unit is horrible. I think the RF people know, and that's why they use dBm. Any system that uses decibels as dimensional units needs to make their baseline clear.

I recently saw a fan advertising a low decibel noise "at 3 meters". And it's nice that they advertise (part of) the baseline, but it sweeps a ~10db difference in pressure under the rug, comparee to the standard 1m reference.

Re: The scientific “unit” we call the decibel

#52

Post by throwaway290 » Thu, May 22, 2025, 7:55 AM UTC

formerly_proven wrote: post #47

coolcase wrote: post #46

Earlier quoted context omitted.

https://www.osha.gov/noise Maybe the reference is implied though?

That's hilarious, though they do seem to get it right on the other, more in-depth pages.

Yep, the NIOSH meter app they recommend uses dB(A).

Re: The scientific “unit” we call the decibel

#53

Post by fouronnes3 » Thu, May 22, 2025, 7:55 AM UTC

neepi wrote: post #43

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…

[flagged]

However you must concede that the perspective of an outsider can be refreshingly eye opening sometimes, even to experts. Especially when it reveals the arcane practices that make a field difficult to learn - for example here the point about the base being too implicit is very valid. The article is perhaps ignorant of a few things, but its criticism shouldn't be dismissed outright IMO. Accepting constructive criticism from someone less experienced at something is a good exercise in humility, and can often help you improve. It's far from a "curse in any field".

Re: The scientific “unit” we call the decibel

#54

Post by leoedin » Thu, May 22, 2025, 7:58 AM UTC

I worked in RF (radar) for a while and the dB/dBm is an incredibly useful tool there. It makes reasoning about amplifier chains and insertion loss so much more straightforward. It also means you can talk about transmitters and receivers in a comparable unit - in reality the signals are many many orders of magnitude apart.

Re: The scientific “unit” we call the decibel

#55

Post by rocqua » Thu, May 22, 2025, 7:58 AM UTC

jeremyscanvic wrote: post #35

Another surprising place decibels pop up, pretty far from loudness related things, is in image comparison metrics. Peak signal-to-noise ratio is mean squared error normalized by a certain peak intensity and it is generally expressed in dB, i.e. as 10 log10(normalized mse). https://en.m.wikipedia.org/wiki/Peak_signal-to-noise_ratio Edit: typo

Decibels make a lot of sense for ratios in any kind of signal processing.

They are not that suited to sound, but sound is generally hard to quantify.

Re: The scientific “unit” we call the decibel

#56

Post by holowoodman » Thu, May 22, 2025, 7:59 AM UTC

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…

Decibels are completely ridiculous. They are only useful if you are a cable monkey just adding and subtracting amplification/dampening factors. As soon as you need to do any kind of non-trivial conversion or computation, dB* is more of a hindrance to understanding and simplicity, because you will always need to look up in some strange table of dB weirdness. Integrate over a spectral power density in dB/Hz? You better convert that to real metric first... Need to solder in a capacitor/resistor/coil as a filter? Better convert to real metric first, because only some pre-made filters are specified in dB (and there are quite a few weird conventions, so you better convert to real metric and check first...).

Re: The scientific “unit” we call the decibel

#57

Post by kazinator » Thu, May 22, 2025, 7:59 AM UTC

The confusing thing about decibels is not the Watts versus Volts thing.

It is the following.

If you mix two identical signals (same shape and amplitude) which are in phase, you double the voltage, and so quadruple the power, which is +6 dB.

But if you mix two unrelated signals which are about the same in amplitude, their power levels merely add, doubling the power: +3 dB.

Re: The scientific “unit” we call the decibel

#58

Post by rocqua » Thu, May 22, 2025, 8:01 AM UTC

smat wrote: post #2

Great post, the fusion of scale with unit is a mess. When using it as a factor, for example when describing attenuation or amplification it is fine and can be used similar to percent. Though the author is right - it would be even more elegant to use scientific notation like 1e-4 in this case. For using it as a unit it would really help to have a common notation for the reference quantity (e.g. 1mW). But I guess there…

Scientific notation would be worse, because you can't add them to get the combined gain. That's where dB's really shine.

Re: The scientific “unit” we call the decibel

#59

Post by Dylan16807 » Thu, May 22, 2025, 8:01 AM UTC

neepi wrote: post #37

eru wrote: post #32

Earlier quoted context omitted.

Obviously there are better ways. The author mentioned several. Eg you could clean up the naming of the variants of the unit, and make it no less useful for RF work, and strictly better in general.

Sorry but no. Absolutely no one cares about the mathematical purity of a unit like the decibel. It's a hammer that turns log stuff into easy to work with linear stuff. It's x10 because that turns into reasonably easy to use numbers with enough precision to use casually. Otherwise it's just a log ratio. Any suffix is a log ratio to a reference value for the entire system (dBv / dBm etc). Say in a 50 ohm system I have…

They're not complaining about the basic nature of it being a log ratio to a reference value. Your use case would be pretty much the same if all the contradictory and confusing details of decibel were smoothed out.

Source: I'm pretty sure I understand the actual complaint better than you do, and you're being dismissive of something nobody said.

Re: The scientific “unit” we call the decibel

#60

Post by neepi » Thu, May 22, 2025, 8:01 AM UTC

fouronnes3 wrote: post #53

neepi wrote: post #43

Earlier quoted context omitted.

[flagged]

However you must concede that the perspective of an outsider can be refreshingly eye opening sometimes, even to experts. Especially when it reveals the arcane practices that make a field difficult to learn - for example here the point about the base being too implicit is very valid. The article is perhaps ignorant of a few things, but its criticism shouldn't be dismissed outright IMO. Accepting constructive criticism…

That would assume that the entire world hasn't already tried this a thousand times over.

Measurements are standardised communication tools. If you start changing the definitions, things fall out of the sky and on your head.