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The scientific “unit” we call the decibel

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191–200 of 511 posts

Re: The scientific “unit” we call the decibel

#191
post #148

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…

4:3 only makes sense to you because you know which is length and width a priori. I for example, always have to recheck that. So if it was written as 1.33 or 4/3 it makes the same difference to me, and is similar in that way to dB

Re: The scientific “unit” we call the decibel

#192
Love this kind of articles but I wished the author suggested solutions.

I go even further than this author : sometimes decibels are computed using logarithms and what we put inside the logarithm has a physical unit. But I can prove mathematically that this is wrong and that whats given to a log function has to be dimensionless. Hence a lot of dB calculus is mathematically wrong and physically meaningless

Re: The scientific “unit” we call the decibel

#193

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…

"decibels are simply a dimensionless ratio, used as a multiplier for some known value of some known quantity."

Except they are not. 1 dB can sometimes mean a ratio of ~ 1.26 and other times it can mean a ratio of ~ 1.12.

"In every context where decibels are used, either the unit they qualify is explicitly specified, or the unit is implicity known from the context."

Maybe in university, but certainly not in the real world.

Re: The scientific “unit” we call the decibel

#194
post #185

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…

I assume you mean dB(A) here, because the sound level at dB(SPL) depends on the reference pressure chosen (20 mPa for db(A) with a calibrated microphone) and the constants for dB(A) are applicable to a large part of human sound perception. Though, if you use human perception as a reference, you'll also need to take dB(B) and dB(C) into account, depending on the sound you're playing and where you're playing that sound.

You can't use "dB" standalone. You need to specify what kind of dB you're talking about, because every field has different dBs that measure different units or are adjusted to different constants.

10dB is like 10k. 10dB is a tiny number if you're talking about volume, but blows your audio equipment if we're talking dB(u) or dB(v) (not to be confused with dB(V). In the same sense, 10k is a low number if it's the price for a new electric car, or an election changing amount of voters when talking about a population.

Re: The scientific “unit” we call the decibel

#195
post #185

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…

I assume you mean dB(A) here, because the sound level at dB(SPL) depends on the reference pressure chosen (20 mPa for db(A) with a calibrated microphone) and the constants for dB(A) are applicable to a large part of human sound perception. Though, if you use human perception as a reference, you'll also need to take dB(B) and dB(C) into account, depending on the sound you're playing and where you're playing that sound…

Ah - it's complicated I guess. I was looking at this stuff https://en.wikipedia.org/wiki/Sound_intensity#Sound_intensit...

Re: The scientific “unit” we call the decibel

#196
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.

Re: The scientific “unit” we call the decibel

#197

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…

"decibels are simply a dimensionless ratio, used as a multiplier for some known value of some known quantity." Except they are not. 1 dB can sometimes mean a ratio of ~ 1.26 and other times it can mean a ratio of ~ 1.12. "In every context where decibels are used, either the unit they qualify is explicitly specified, or the unit is implicity known from the context." Maybe in university, but certainly not in the real w…

I have a feeling that a lot of people here haven't read the article. There's lots of "Its just a dimensionless ratio" comments

Re: The scientific “unit” we call the decibel

#198
Decibels are not ridiculous, but very frequently the notations for quantities expressed in decibels are ridiculous.

The decibel is an arbitrary unit for the quantity named "logarithmic ratio".

Logarithmic ratio, plane angle and solid angle are 3 quantities for which arbitrary units must be chosen by a mathematical convention and these 3 units are base units, i.e. units that cannot be derived from other units. For a complete system of base units for the physical quantities, there are other 3 base units for dynamic quantities that must be chosen arbitrarily by choosing some physical object characterized by those quantities, i.e. a physical standard (originally the 3 dynamic quantities were length, time and mass, but in the present SI the reality is that mass has been replaced by electric voltage, despite the fact that the text of the SI specification hides this fact, for the purpose of backward compatibility), and there also are other 2 base units for discrete quantities (amount of substance and electric charge) which must be established by convention.

Like for the plane angle one may choose various arbitrary units, e.g. right angles, cycles, degrees, centesimal degrees, radians, or any other plane angles, for the logarithmic ratio one may choose various arbitrary units, e.g. octave, neper, bel, decibel.

So if we choose decibel all is OK. Decibels have the advantage that for those used to them it is very easy to convert in mind between a logarithmic ratio expressed in decibels and the corresponding linear ratio, so it is very easy to make very approximate computations in mind, but good enough for many engineering debugging tasks, in order to replace multiplications, divisions and exponentiations with additions, subtractions and rare simple multiplications, for a quick estimate of what should be seen in a measurement in a lab or in the field.

The problem is that whenever a logarithmic ratio is specified in decibels, it must be accompanied by 2 quantities, what kind of physical quantities have been divided and which is the reference value. Humans are lazy, so they usually do not bother to write these things, assuming that the reader will guess them from the context, but frequently the context is lost and guessing becomes difficult or impossible.

An additional complication is that one never uses logarithmic ratios for electric voltages or currents, but only for powers. When it is said that a logarithmic ratio refers to a voltage or a current, what is meant is that the logarithmic ratio refers to the power that would be generated by that voltage or current into an 1 ohm resistor. A similar problem exists for sound pressures, because logarithmic ratios are used only for sound intensities, so where sound pressure is mentioned, actually the corresponding sound intensity is meant.

This complication has appeared because voltages, currents and sound pressures are what are actually measured, but powers and sound intensities are frequently needed and using logarithmic ratios with different values for related quantities, while omitting frequently to mention the reference value, would have caused even more confusion than the current practice.

Re: The scientific “unit” we call the decibel

#199
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…

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.)

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).

Re: The scientific “unit” we call the decibel

#200

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…

> 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!

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