The scientific “unit” we call the decibel (dB)
184 comments
·May 22, 2025kristjank
svara
> makes me really doubt that the OP has any practical experience about the things they're talking about.
Maybe not, but you can get used to many odd things given enough experience.
I totally share the authors view. I don't usually have trouble grasping the definition of a unit, but dBs are just hilariously overloaded.
The same symbol can literally mean one of two dimensionless numbers, or one of who knows how many physical units.
That's not normal, something as basic as units is usually very cleanly defined in physics.
Someone in this comment section said it's not a problem because there's usually going to be a suffix that is unambiguous. If that were actually the case, you wouldn't see these types of complaints.
TheOtherHobbes
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 useful in their own way, none of which are directly comparable, all of which are understandable in context.
dB is the same. It's a ratio split into convenient steps - more convenient than Bels would be - that compares two quantities. The quantities can be measured in different units. The units are implied by the context.
The only mild confusion is the relationship between voltage and power ratios. But that's a minor wrinkle, not a showstopping intellectual challenge.
gwd
> 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 you also just have to know in context.
svara
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 potential in Volt.
And then you've got the cases where it really just is a ratio (one of two possibilities).
You'll see all of these called "decibels".
You see why people are irritated?
pwdisswordfishz
Why, you're right: aspect ratios are stupid because people often don't bother distinguishing between SAR, PAR and DAR, leading to lots of confusion. And sometimes even mistake the ratio of pixel densities for the pixel aspect ratio:
sandblast
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?
rusk
dB for sound in particular aligns with human experience. The (10 -) 1-10 on a volume knob typically aligns with a logarithmic scale because we hear differences in loudness at an order of magnitude.
A linear volume knob would be frustratingly useless as you would have to crank it many many many times the higher up you want to go. Presumably hundreds of times. A traditional pot couldn’t do that of course but maybe you could satisfy your curiosity with a rotary encoder?
losvedir
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.)
IsTom
You could have log Watts or something, it doesn't have to be dB to be logarithmic.
nomercy400
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.
null
ajuc
Logarithmic scale aligns with human experience.
OP isn't criticizing logarithmic scale in general but dB in particular.
If dB in particular aligned well with human experience - volume knobs would be labeled with dB values instead of 1-11.
taneq
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. :)
mapt
Having a logarithmic scale is a very different feature than having this one symbol to express all logarithmic scales with contextual meanings that sometimes need to be incorporated as two or three separate variations in a conversation.
Why wouldn't this work better broken out as half a dozen different units, with objective zero points and mathematical convertibility?
holowoodman
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...).
lifthrasiir
I do see needs for something like decibels. It is a useful way to communicate an inherently logarithmic quantity. The real and IMHO only mistake is that it was described as a typical unit, while it should have been a unit constructor and specially treated in the SI unlike others. (Not in the SI, but the [milli]meter in mmHg etc. and `p` in pH---really p[H+] [1]---are other popular examples of unit constructor.)
[1] To be clear, I'm aware that pH and p[H+] are technically different. But that's orthogonal here.
rcxdude
I work in a related field (lots of signal processing, but not necessarily RF-y or audio-y), and it's a constant source of confusion. I actively avoid using them in technical communication because they will be misinterpreted by someone, and when we have customers who use them they usually don't actually know enough context to disambiguate them.
rocqua
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.
mikewarot
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 increasing the power by a ratio of 100:1 (20 dB) (because increasing the voltage ALSO increases the current, and the power is the product of the two)
margalabargala
> If you have a given load, increasing the voltage by a ratio of 10:1 (20 dB) is exactly the same as increasing the power by a ratio of 100:1 (20 dB) (because increasing the voltage ALSO increases the current, and the power is the product of the two)
It's not that we don't understand this. We do understand this, and simply think it's ludicrous that the same nominal "unit" is used to refer to both, rather than calling the voltage one, say, "hemidecibels". Because we're not talking about power always, we're talking about, as you say, ratios.
IsTom
> Decibels aren't units... they are ratios.
Until they are a ratio to a specific arcane reference level as mentioned in the article.
ajuc
Would you defend using pound for force and mass because "it's often the same"?
baxuz
You haven't addressed any of the points the author made. You only show habituation and status quo bias.
rusk
[flagged]
jvanderbot
The fact that the numbers on the scale align with intuition at a few points does not mean that the scale is a good one. dBs are jargon encoded with false rigor. No other "unit" has flexible meaning based on the background of the speaker, but jargon does.
neuroticnews25
I think the "whining" is just a stylistic choice and an excuse to talk about the things he finds worth noting. I didn't perceive the tone of the article as negative.
tim333
The author gives the downsides but not the upside of why it is like that.
It's basically so it describes sound levels on an understandable scale with 0db being just audible and 100dB being very loud.
It also corresponds to the energy carried by the sound - 0 dB is 1 pW per square meter so it is kind of a scientific unit. It's probably easier to have a measure that is understandable by the public and let engineers do conversion calculations for signal levels in networks than the other way around.
lifthrasiir
The only thing you should know is that any use of bel and thus decibel should ideally have the reference level suffixed (usually in parentheses or subscript), not implied. The absolute sound pressure level is dB(SPL). The human-perceived loudness level is dB(A) and similar. The RMS voltage expressed in power is dB(u) (formerly dB(v), not same as capital dB(V)). And so on. And then each different instance of dB unit is simply distinct, only connected by the fact that it represents some ratio in the logarithmic fashion. Treat any new dB unit you haven't seen as an alien.
hashhar
This is exactly it. The people who get confused by decibels are treating it a unit in it's own right when it's really just a ratio of some unit.
margalabargala
Disagree.
The people who get confused by decibels, are exposed to other people treating it like it's a unit in its own right.
I agree that what the parent described, should be done. If it was what was done, this article wouldn't exist.
lifthrasiir
As I've said in the other comment, I believe this should be ultimately addressed by the SI.
agos
people are often confused by decibels because the necessary disambiguation is more often than not absent (see: spec sheets of some kind of appliance talking about noise)
cesaref
Generally speaking, the db scale is very useful for many practical situations, and this is overlooked in this critique.
As have been pointed out, it's just a power ratio on a logarithmic scale, but this has many benefits, the main one being that chaining gain/attenuation in a system is just a case of adding the db values together. 'We're loosing 4db in this cable, and the gain through this amp is 6db, so the output is 2db hotter than the input'. Talk to any sound engineer and you'll do this sort of thing successfully without necessarily understanding the science, so that's a massive win.
severusdd
While I thoroughly enjoyed reading this piece of internet-rant, I've to argue that dB is still probably the best we have on this!
In RF engineering, expressing signal levels in dBm or gains in dB means you can add values instead of multiplying, which definitely appeared like a huge convenience for my college assignments! A filter with -3 dB loss and an amplifier with +20 dB gain? Just add. You can also use this short notation to represent a variety of things, such as power, gain, attenuation, SPL, etc.
I guess, engineers don’t use dB because they’re masochists (though many of them surely are). They use it because in the messy world of signals, it works. And because nobody knows anything that might work better!
fouronnes3
When I worked on a radar project, my fellow radar engineers (I'm software) used dB a lot. A lot of them would actually agree with the article, but historical sometimes wins even when you're aware of its shortcomings. Aren't we the same in software anyway? The email protocol, terminal escape sequences, the UX of git command line, etc... Each of those could have an "X is ridiculous" blog post (and I would enjoy every single one).
One upside of dB not touched in the article is that it changes multiplication into addition. So you can do math of gains and attenuations in your head a bit more conveniently. Why this would be useful in the age of computers is confusing, but on some radio projects both gains and losses are actually enormous exponents when expressed linearly, so I sort of see why you would switch to logs (aka decibels). Kinda like how you switch to adding logs instead of multiplying a lot of small floats for numerical computing.
svara
A pet peeve I share! An expanded version of this article should become the article on decibels on Wikipedia.
I've read that article many times over my life and for the first couple times came back thinking I was too dim to understand.
Transparently leading it with "Here's something ridiculously overcomplicated that makes no sense whatsoever..." wouldn't fit Wikipedia's serious voice but actually be pedagogically very helpful.
esperent
There's often a Criticism of... section in Wikipedia pages.
Maybe this blog post could work as a source, although it would be better to find something more established.
ggm
Do a deep dive into audio vu Meters and how they got calibrated. Without being 100% sure, it's basically a totally subjective model, where back in the 1920s the BBC and some US company decided to assert "like us" and two models persist which have been retconned into some BIPM acceptable ground truth but it basically was "test it against the one we made which works"
The hysteresis in the coil-magnet meter response turned out to be a feature, not a bug.
kazinator
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.
mousethatroared
If they're identical and in phase, the addition is obviously a multiplication by 2 -> 6dB.
If they're unrelated the signals can also cancel out. So not 6bB. If not 6 dB then what is it? An integral that has already been solved for us :D
cycomanic
That has nothing to do with decibels at all, that's the fundamental physics (or mathematics if you want) of adding waves, i.e. interference.
numpad0
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.
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.
kazinator
> 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 soon as you mention bels or decibels, you're talking about the power aspect of the signal.
If volume were measured in meters, which were understood to be the length of one edge of a cube whose volume is being given, then one millimeter (1/1000th of distance) would have to be interpreted as one billionth (1/1,000,000,000) of the volume.
When you use voltage to convey the amplitude of a signal, it's like giving an area in meters, where it is understood that 100x more meters is 10,000x the area.
There could exist a logarithmic scale in which +3 units represents a doubling of voltage. We just wouldn't be able to call those units decibels.
dj3l4l
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 meaning is incomplete until the denominator of the ratio within the logarithm is known.
This is the gripe that is being conveyed in this article. Mathematically, the Bel is unitless. It is only by additional context that one can understand the value of the denominator in the logarithm.
leoedin
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.
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 your head, and still preserve a big degree of accuracy.
Whining about it makes me really doubt that the OP has any practical experience about the things they're talking about.