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The scientific “unit” we call the decibel

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351–360 of 511 posts

Re: The scientific “unit” we call the decibel

#351

Earlier quoted context omitted.

> In every context where decibels are used, either the unit they qualify is explicitly specified, or the unit is implicity known from the context. The author's whole point is that this is not true. To adapt your analogy, it's not like being mad at the number three, it's like being mad about people not attaching any units to the number three, arguing that it's clear in context. It isn't!

Sure, but the author is wrong. “How old is your son?” “He’s 3.” Clear in context. People write things like dB SPL (A-weighted) in spec sheets because spec sheets benefit from being unambiguous. Most of the time it’s really clear, like you’re talking about insertion loss or amplifier gain and there’s only one reasonable way to interpret it.

When you say 35 it would be strange for it to be anything but years.

3 days, weeks, months or years are ironically all common units when someone is "3".

Re: The scientific “unit” we call the decibel

#352

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…

> The units are implied by the context. This is the absurd part. There do exist other ratios that masquerade as units, e.g. specific gravity, and its meaning also changes depending on what you’re using it for - liquids are compared to pure water at 4 C, gases are compared to air at 20 C. As the parent comment points out, you can get used to things with experience. That doesn’t make them any less absurd. Look at Fahre…

> Look at Fahrenheit, for example. I’m American, and I still think it’s absurd, but because I’m extremely used to it, it feels natural.

What do you find absurd about Fahrenheit?

If it is that its 0 point is not absolute 0 then I think you can make a good case, at least for scientific work. It's a bit harder to make the case for absolute 0 being the 0 point a scale for ordinary day to day use since all temperatures most people deal with will be 3 digit numbers (and making you degree large won't help because people will still need 3 digit number--they just won't be integers any more).

If you find it absurd compared to Celsius then I think it is hard to make a convincing case. They are both scales with a 0 point way above absolute 0, differing only on where they put their 0 point and the size of the degree. (They originally differed on direction, with Celsius putting 0 at the boiling point of water and setting the degree size so that water froze at 100, but Celsius soon came to his sense and flipped so the numbers went up as it got hotter).

Fahrenheit set 0 at the coldest temperature he could make in his lab and tried to set 100 at body temperature. Celsius (once he got the direction fixed) set 0 at water freezing and 100 at water boiling.

That gives Fahrenheit a smaller degree and puts the range of temperatures most people deal with most of the time above 0.

Celsius made it easier to memorize two temperatures that are very significant in many human activities, namely the freezing point of water and the boiling point of water (although the latter is probably less important...generally most people only deal with boiling water when they are trying to boil water and don't need to care about the temperature. It's not like freezing which can happen naturally and so people often need to monitor temperature to find out if there is danger of freezing).

But that 0 point in Celsius means that a lot of people have to regularly deal with negative temperature which is a little annoying.

The metric system chose Celsius, but I've not been able to find any compelling technical reason for that. A metric system with Fahrenheit would have fine too.

Note that unlike mass, length, area, and volume units pre-metric systems generally only had one temperature unit. There was nothing in temperature like miles, yards, inches, feet, furlongs, etc. for length and gallons, pints, cups, etc. for volume. A system that went with one single length unit (the meter) and one single volume unit (the liter) and then derived larger and smaller units from those using consistent ratios and prefixes that were the same across different types of units was a massive simplification.

I asked an LLM why metric went with Celsius and got a lot of circular reasons. For example it cited that various thermodynamic forumals would not work with F degrees because the Boltzman constant is defined in the SI system using K. But the Boltzman constant is defined that way because SI uses the metric system. In an F based metric system the Boltzman constant would be defined in R and everything would work fine.

The non-circular reasons it suggested were also not satisfactory. One was that C was more common than F in Europe at the time the metric system was created, which technically does answer the question I asked but then raises the question of why C became more common pre-metric.

It also suggested that having water freeze at 0 and boil at 100 fits in better with a decimal system which doesn't really make a lot of sense.

As for why C became more popular than F pre-metric it suggests that the 0 and 100 points were easier to reproduce. Fahrenheit's choice of body temperature for the 100 point was definitely a mistake as it is too fuzzy (it was even dumber than metric's initial choice for the meter as 1/10000000th of the distance from the distance from the North Pole to the equator along the meridian passing through Paris).

Freezing and boiling of water do take some care to use (you need to control pressure and contaminants) but are going to be more consistent that body temperature.

But there is no reason I can see that the fuzziness in Fahrenheit's 100 point couldn't have been fixed by simply changing the defining points from 0 and 100 to water freezes at 32 and boils at 212. Yes, it is not as easy to memorize as 0 and 100 but does let us have a scale where most temperatures dealt with by most people most of the time are 2 or 3 digit positive integers.

Re: The scientific “unit” we call the decibel

#353

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…

The same can be said of any unit. "If you just learn to work with it regardless of its weirdness, it's completely normal" doesn't mean the standard isn't ridiculous. I don't think anyone has stated that a logarithmic scale is bad. The type of logarithmic scale changing depending on the field it's being used in, and the non-standard notation (dBm being used instead of dBmW for instance), is just inconsistent for no re…

> I don't see a practical reason why dB's normal use can't have been covered by normal prefixes

Because you can easily add dB values that are on greatly different scales in your head, especially when the values aren't exactly powers of ten. If I have 86 dBW of Effective Isotropic Radiated Power and -162 dB of free space loss to some distance, the power flux density at that distance is -76 dBW/m^2.

Re: The scientific “unit” we call the decibel

#354
post #337
post #230

Earlier quoted context omitted.

The problem stated in the article is that the unitless quantity of 1 Bel is effectively applied only to power ratios. It is of course true that one can transfer a scaling of powers into the base of the logarithm when we are trying to figure out what effective scaling of voltages corresponds to 1 Bel of power scaling, but it is ultimately more meaningful to state that a Bel is "only a scaling of power", which is a sta…

The puzzling thing is why they chose to adopt a term used to describe "only a scaling of power" to talk about log10 values. If 1 Bel simply meant 10x, I'd get it, but Bel has baggage and also means sqrt(10)x. Is odds a power-like or amplitude-like quantity? If you can't tell, dB isn't the most fortunate choice. It's not like mathematicians need fake units to talk about unitless ratios and their logarithms.

The useage of dB in the case of probability is exactly as the 1 Bel -> 10x meaning, but that is the technical meaning, without further context of the unit in the logarithm. There is no need to conceptualise this further as involving power or amplitude (which does not apply in probability).

I think that the unit having been popularised in the telecommunications industry just meant that every other instance of a log_10 ratio in physics lead to a realisation that it was a Bel. For Bayesian odds, this was probably because even the development of Bayesian probability was largely advanced by physicists (E.T. Jaynes being a famous example), who also were trained, and often worked, in signal processing of some kind or another. But I doubt they would have thought about this "power-ratios-only" adherence that is more the conception of telecommunications engineers, as opposed to physicists.

Re: The scientific “unit” we call the decibel

#355

Earlier quoted context omitted.

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

Though that's still incomplete, because (more of the context stuff that you just have to know already) the "A" here refers to the frequency-weighting scheme used in the measurement, and not to the reference level (which is SPL). It should probably be given as: dB(A) SPL, or dB SPL (A-weighted).

Though that's still incomplete, because (more of the context stuff that you just have to know already) the "SPL" here refers to the reference level in air (20 µPa) and not water or oil (1 µPa).

It should probably be given as: dB(A) SPL (1=20 µPa).

Though that's still incomplete, because (more of the context stuff that you just have to know already) the 20 µPa only applies at the standard temperature and pressure.

It should probably be given as: dB(A) SPL (1=20 µPa, T=293 K, P=101.3 kPa, in standard air).

Though that's still incomplete, because (more of the context stuff that you just have to know already) the (A) here is actually the A-weighting curve specified in IEC 61672:2003.

It should probably be given as: dB A-weighted (IEC 61672:2003) SPL (1=20 µPa, T=293 K, P=101.3 kPa, in standard air).

Though that's still incomplete, because (more of the context stuff that you just have to know already) ...

...

The point of communication is to transport information, not pointless pedantry (except for a small subset of the population). Nobody is confused as to what 85 dBA refers to.

Re: The scientific “unit” we call the decibel

#356

Earlier quoted context omitted.

> In every context where decibels are used, either the unit they qualify is explicitly specified, or the unit is implicity known from the context. The author's whole point is that this is not true. To adapt your analogy, it's not like being mad at the number three, it's like being mad about people not attaching any units to the number three, arguing that it's clear in context. It isn't!

Just because the author is ignorant of the context doesn’t mean that the engineers working in those fields are. They use it because it all makes sense in context.

Yeah hence his comment “If you know you know.” The author is far from ignorant on the subject, he’s pointing out the unit is often used without context.

Re: The scientific “unit” we call the decibel

#358

Speaking of which. Does anyone know why, in Physics, we only use the operators +, * and exponentiation? And why higher operators in the hyperoperation sequence are never used? https://en.wikipedia.org/wiki/Hyperoperation

You would use it if it was useful or convenient

Re: The scientific “unit” we call the decibel

#359
post #168
post #84

Earlier quoted context omitted.

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…

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.

A properly designed volume control affects the sound power output logarithmically, but is labeled linearly (with 1-11 or 0-1 or something like that) to reflect how humans perceive the effect.

People in the field get this wrong all the time — for instance, the volume control on ChromeOS appears to be a linear multiplier, yielding a control with huge perceived steps in the output between 0 and 3, and negligible perceived change in the output between 7 and 10.

I suspect that the confusing design of the dB contributes towards how often such mistakes get made.

Re: The scientific “unit” we call the decibel

#360

Earlier quoted context omitted.

Just because the author is ignorant of the context doesn’t mean that the engineers working in those fields are. They use it because it all makes sense in context.

Yeah hence his comment “If you know you know.” The author is far from ignorant on the subject, he’s pointing out the unit is often used without context.

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