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The scientific “unit” we call the decibel

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Re: The scientific “unit” we call the decibel

#151

Post by stephen_g »

nabla9 wrote: post #109

Decibel is ridiculously overloaded concept. It can be used to express and calculate relative change in power, amplitude ratios, and absolute change. All of these are different units and should always use different notation, but sometimes it's skipped.

But a lot of them can be combined - like you can take dBm of a microwave signal going into a filter, subtract the dB figure of loss through a filter, add the dB figure of gain through an amplifier, then add the dBi of antenna gain, and get a meaningful number (equivalent isotopic radiated power or EIRP, in dBm) out the end! It’s quite amazing!

Re: The scientific “unit” we call the decibel

#152

Post by verzali »

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…

Yeah that was my thought as well. Decibels make it much easier to do link budgets as you can just add or subtract instead of multiplying or dividing.

Re: The scientific “unit” we call the decibel

#153

Post by foxglacier »

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]

It's like telling a chemist that moles are a useless complication. They'll swear up and down how important they but have no reasons except that everyone in the field uses them so they must be important. Sometimes conventions live on past the original need they met and experts are blind to that because they're only experts in using the convention, not evaluating conventions. Moles might have made sense when we didn't even know atoms existed and perhaps dB are similar, though I wouldn't know about that one.

Re: The scientific “unit” we call the decibel

#154

Post by emilfihlman »

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…

You are completely missing the point, which is that decibels are opaque and carry hidden field specific meaning that is not possible to know from the "unit".

This is not a criticism of how useful they are in calculations.

Re: The scientific “unit” we call the decibel

#156

Post by camtarn »

margalabargala wrote: post #125
Ekaros wrote: post #110

Earlier quoted context omitted.

Well 10 millimetres of rain is different that 10 millimetres thickness for say branch...

Could you elaborate on this? If 10mm rain falls, and you have an open collection container, then the depth of the water in that container would be more, less, or the same as the thickness of the branch?

...as long as the container has straight sides. The area of the opening needs to be the same as the area of the container all the way down, otherwise the two areas don't cancel out.

Re: The scientific “unit” we call the decibel

#157

Post by pwdisswordfishz »

TheOtherHobbes wrote: post #119
svara wrote: post #62

Earlier quoted context omitted.

> 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 ph…

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…

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:

https://trac.ffmpeg.org/ticket/11279

https://trac.ffmpeg.org/ticket/11284

Re: The scientific “unit” we call the decibel

#158

Post by jfengel »

KeplerBoy wrote: post #108
IsTom wrote: post #93

Earlier quoted context omitted.

You could have log Watts or something, it doesn't have to be dB to be logarithmic.

We have a unit for that. It's dBm and very easy to grasp. 0 dBm is 1 mW, every 10 dBm is an order of magnitude more (10 dBm = 10 mW). dB is only confusing if people omit which quantities they are relating. If it's clear like in the case of dBm which relate to 1 mW, it's an awesome tool.

Unfortunately, people omit the quantities all the time. Domain experts assume it when talking to each other, and non-experts repeat it without knowing that it refers to anything at all. (I still don't really know what it means for a sound to have "decibels".)

Re: The scientific “unit” we call the decibel

#159

Post by lifthrasiir »

margalabargala wrote: post #141
lifthrasiir wrote: post #130

Earlier quoted context omitted.

As I've said in the other comment, I believe this should be ultimately addressed by the SI.

I would agree. Right now what we've got is basically "millis", and you just have to know whether the speaker is talking about length or mass. I like your proposal.

I actually want those suffixes mandatory, because there may be multiple plausible suffixes for each use. For example the loudness might be dB(A), dB(B), dB(C), dB(D) depending on the exact curve or even dB(SPL) if the sound pressure level is used as a proxy. So it is much more confusable than, say, "millis" when suffixes are implied.

Re: The scientific “unit” we call the decibel

#160

Post by foxglacier »

IsTom wrote: post #93
rusk wrote: 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…

You could have log Watts or something, it doesn't have to be dB to be logarithmic.

You can't take the log of a quantity with units like watts. It would be log of some ratio of powers, and then it doesn't matter what unit of power you use because they cancel out. Instead, it matters what the denominator in the ratio is so we're back at needing something confusing like dB :(
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