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

Re: The scientific “unit” we call the decibel

#171

Post by grues-dinner » Thu, May 22, 2025, 11:14 AM UTC

IsTom wrote: post #95

mikewarot wrote: post #68

Earlier quoted context omitted.

Decibels aren't units... they are ratios. The ratio could be gain, or loss, or compared to the noise floor, or the signal of interest, or a standard unit, such as Watts, milliWatts, or microVolts into 50 ohms. >The fact that decibels work differently for voltage and power is very weird, but understandable in isolation. If you have a given load, increasing the voltage by a ratio of 10:1 (20 dB) is exactly the same as…

> Decibels aren't units... they are ratios. Until they are a ratio to a specific arcane reference level as mentioned in the article.

It's not that arcane, it's referenced to a sound pressure level of 1 pascal. Which, yes, is still arbitrary in that it all ends up back at the arbitrary values of things like metres, seconds, kilograms, etc.

Volts per pascal might make sense in some contexts (like your input buffer power supply), but log volts per pascal makes sense in others, particularly for audio applications where you stack gains and attenuations onto an already-logarithmic domain.

Re: The scientific “unit” we call the decibel

#172

Post by more-nitor » Thu, May 22, 2025, 11:16 AM UTC

sandblast wrote: post #136

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…

Ratios are numbers. They are literally just fractions. No one argues that the numbers don't make sense because you can have different units. But that's what units are for – to know what does the preceding number refer to. Why have a unit that doesn't give you full information?

this

if a TV seller went bonkers and only said "it's 10:16", can you guess the actual size of that TV?

Re: The scientific “unit” we call the decibel

#173

Post by im3w1l » Thu, May 22, 2025, 11:17 AM UTC

atoav wrote: post #44

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…

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 thumb for preventing occasional clipping?

Re: The scientific “unit” we call the decibel

#174

Post by relaxing » Thu, May 22, 2025, 11:21 AM UTC

readthenotes1 wrote: post #4

rebolek wrote: post #3

While the author is technically right, I must argue that in the area of sound work, decibels make sense. Zero is base level, -3db is half loudness, +3dB is double. There may be a better way to describe loudness, but decibel is good enough.

So on my amplifier, 0db is the loudest and -50db, where I usually listen is what? -47db is definitely not twice as loud as -50db, of course.

You should have a legitimate question as to what dB’s the amp is referring to. What brand and model is it?

First, is it actually specified as dB’s? Most amps I’ve seen display volume on an arbitrary scale, no units specified.

Second, the amp has no way to know dB sound pressure, as that depends on the rest of signal chain.

So is the displayed figure referring to dBu, dB mV, something else, or something totally bunk?

Re: The scientific “unit” we call the decibel

#175

Post by rcxdude » Thu, May 22, 2025, 11:25 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…

It's not a case of mathematical purity, it's a case of unnecessary ambiguities. Decibels would be a heck of a lot more comprehensible if a) it was always a ratio or an absolute value with respect to an explicitly written unit, and b) it was always in that unit and not sometimes scaled differently because it's power instead of amplitude, and it would be no less useful to you for exactly the case you're talking about. (RF at least generally does say 'dBm' when talking about absolute power, but it would probably be better to way dBmW, and we could also use dbmV where it made sense without having to go 'oh, but that's divide by 20 just because').

Of course, it's far too late to easily change it now, but it doesn't make the current situation optimal.

Re: The scientific “unit” we call the decibel

#176

Post by ajuc » Thu, May 22, 2025, 11:26 AM UTC

mikewarot wrote: post #68

rocqua wrote: post #51

Earlier quoted context omitted.

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 th…

Decibels aren't units... they are ratios. The ratio could be gain, or loss, or compared to the noise floor, or the signal of interest, or a standard unit, such as Watts, milliWatts, or microVolts into 50 ohms. >The fact that decibels work differently for voltage and power is very weird, but understandable in isolation. If you have a given load, increasing the voltage by a ratio of 10:1 (20 dB) is exactly the same as…

Would you defend using pound for force and mass because "it's often the same"?

Re: The scientific “unit” we call the decibel

#177

Post by megous » Thu, May 22, 2025, 11:26 AM UTC

gwd wrote: post #148

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…

> 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…

Ratio is just a single number. 4:3 can be expressed as 1.3333~ and it just says how much bigger one number is compared to another.

RE: Yes, I was able to read and understand the article. I also have 8 years of EE. Ratio is still a single number in the end. You can have an actual size of a monitor 1600 x 1200 and the ratio of sides is 1600/1200, which is just a single number. You can express it in multiple ways. You still need at least one size + understanding of what the aspect ratio is used to describe in a particular situation (units (mm, px, ...), ...) to be able to calculate the complete dimensions of a monitor screen.

Same issue with % or ppm, or whatever.

You always need a defintition to understand what the numbers are abstracting in any particular situation.

Re: The scientific “unit” we call the decibel

#178

Post by brazzy » Thu, May 22, 2025, 11:27 AM UTC

godsinhisheaven wrote: post #142

taneq wrote: post #94

Earlier quoted context omitted.

Ever have a cheap set of external speakers that got super loud in the first quarter turn of the volume knob but were pretty much the same loudness after that? Yeah, linear pot for the volume knob. No need for an encoder and software, though, logarithmic pots are readily available for precisely this reason. :)

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.

The point is that sound perception is logarithmic. You perceive a 10 times stronger air vibration as twice as loud. So if you have a knob that increases the power that produces the vibrations linearly, you hear a logarithmic increase.

You need a knob that increases power exponentially to hear a linear increase in loudness.

Re: The scientific “unit” we call the decibel

#179

Post by numpad0 » Thu, May 22, 2025, 11:27 AM UTC

I just can't understand why "[specialized domain] uses these stupid wrong units, it should be [unit that no educated smart SMEs use]" types don't do their researches to understand why those are used at all. This type of weird non-SI units appear when means of measurement and unit is related to one another and has downstream dependencies.

Just look at aviation. An airplane's:

  - speed is measured in knots, or minute of angle of latitude per hour, which is measured by ratio of static and dynamic pressure as a proxy.
  - vertical speed or rate of climb is measured in feet per minute, which is a leaky pressure gauge, probably all designed in inches.
  - altitude is measured in feet, through pressure, which scale is corrected by local barometric pressures advertised on radio, with the fallback default of 29.92 inHg. When they say "1000ft" vertical separation, it's more like 1 inHg or 30 hPa of separation.
  - engine power is often measured in "N1 RPM %" in jet engines, which obviously has nothing to do with anything. It's an rev/minutes figure of a windmilling shaft in an engine. Sometimes it's EPR or Engine Pressure Ratio or pressure ratio between intake and exhaust. They could install a force sensor on the engine mount but they don't.
  - tire pressure is psi or pound per square inch, screw tightening torques MAY be N-m, ft-lbs, or in-lbs, even within a same machine.

Sure, you can design a battery charging circuits in Joules, fly an airplane with a GPS speedometer, analog audio-radio circuitry in millivolts. Absolutely no one does. I think that cognitive dissonance should trigger curiosity circuitry, not rant mode.

I mean, just type in "use of decibels[dB] considered harmful" at the box at chatgpt.com. It'll generate basically this article with an armchair version of the top comment here as the conclusion.

Re: The scientific “unit” we call the decibel

#180

Post by msandford » Thu, May 22, 2025, 11:27 AM UTC

godsinhisheaven wrote: post #142

taneq wrote: post #94

Earlier quoted context omitted.

Ever have a cheap set of external speakers that got super loud in the first quarter turn of the volume knob but were pretty much the same loudness after that? Yeah, linear pot for the volume knob. No need for an encoder and software, though, logarithmic pots are readily available for precisely this reason. :)

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.

You're missing a critical piece of information. Human hearing (and vision) are logarithmic sensors.

Ears can register sounds from maybe 20-30 dB upwards of 120ish which isn't a factor of 4-6 in terms of power but rather a factor of 120-30=90 decibels or 9 bels or 10^9 or one billion.

Because your ears have absolutely enormous range you need the potentiometer (pot) to have a logarithmic taper to it. The amplifier has an essentially fixed amount of amplification so that's a fixed sound dB output. Your ears can hear a vast range. A linear pot essentially locks the entire output into the same 10 decibels as the amplifier maximum output through its linearity. Once you've turned it to 10% of the range it has precisely 10 decibels worth of range left. If you want to turn the volume down by 40 decibels you have to do that within the 0-10% part of the pot's range.

A logarithmic pot will give you maybe 40-60 decibels worth of adjustment by dividing things up differently. Every 20% of the range increases the output not by 20% but by a factor of 10 let's say. That gives you a pot with a range of 50 decibels which is enough that it roughly matches the absolutely miraculous range of the ear.