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The scientific “unit” we call the decibel

lcamtuf.substack.com

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Re: The scientific “unit” we call the decibel

#61

Post by atoav »

eru wrote: post #32
neepi wrote: post #22

[flagged]

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.

There are better ways, if you optimize for ease of understanding and not ease of use.

dB-based units make working between parts of a circuit, different devices etc. a breeze because adding 6dB in one part and removing it in another will result in no change in level. This is something the totally uninitated use every day in audio applications (typically that would be dBFS).

What other system do you propose? One where clipping audio has a level of 100% (0dBFS) and the dialog of a movie has 10% (-20dBFS) while the background atmo has 0.003% (-50dBFS) and the noise of the recorder has 0.000001% (-120dBFS)?

You can probably see why we use a log scale here..

Re: The scientific “unit” we call the decibel

#62

Post by svara »

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…

> 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.

Re: The scientific “unit” we call the decibel

#63

Post by rocqua »

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.

That's great, but only if 'Base level' is clear to everyone involved. Besides the base level, you also need to define what exactly you are measuring. Including the frequency weighting.

This makes a spec sheet that says "this machine produces X dB of sound" effectively useless.

Re: The scientific “unit” we call the decibel

#64

Post by globular-toast »

kazinator wrote: post #26
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.

But, no! -3dB is half power, not half loudness. -3dB down is just slightly less loud.

Exactly. There's no way +3dB is perceptibly twice as loud. That's easy to test for yourself. +10dB is roughly twice as loud.

Re: The scientific “unit” we call the decibel

#65

Post by calmbonsai »

If you're specifying decibels in written form you always include the basis or you're simply being incomplete with your units. I don't understand the complaint there.

In casual conversation, the context implies the basis.

Dealing with decibels is also another shorthand to know the domain has a wide enough value gamut such that logarithmic values (where addition is multiplication) makes sense. See also, the Richter scale.

Re: The scientific “unit” we call the decibel

#66

Post by rocqua »

atoav wrote: post #21

But dB is not a unit, it is a multiplier. dB on its own is unitless and if we say "X is reduced by 6 dB" you know that the value of X is half of what it was before (×0.5) if something is amplified by 6 dB it is double of what it was before (×2.0)¹ Note that the unit only starts to play a role when you reference your dB value to some absolute maximum, e.g.: dBV which is referenced to 1V RMS dBu which is referenced to…

> But dB is not a unit, it is a multiplier. dB on its own is unitless.

The point of the article is exactly that this should be the case. But it ends up not being the case. Mostly because people use dB with reference to some assumed baseline. But also because a 20db change could be a 10x change conpared to baseline, or a 100x change compared to baseline, depending on what unit you are measuring in.

Re: The scientific “unit” we call the decibel

#67

Post by lifthrasiir »

holowoodman wrote: post #56
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…

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…

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.

Re: The scientific “unit” we call the decibel

#68

Post by mikewarot »

rocqua wrote: post #51
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…

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 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)

Re: The scientific “unit” we call the decibel

#69

Post by rocqua »

hock_ads_ad_hoc wrote: post #20

The author seems confused. Decibels aren’t units. They’re a way to express a ratio between a reference value and some other value. They’re used where they are useful.

dBs are so often used as units. That's 80% of the complaint in the article.

The fact that dBs aren't units, but are used like units, is the point being made.

Re: The scientific “unit” we call the decibel

#70

Post by jeroenhd »

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…

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 reason.

For a unit of immense scale, I rarely see it used outside of the -100 to 100 range, though. That puts its daily use square in the middle of SIs giga/nano range. I'm sure the formulae are a bit easier by not having to include exponents, but I don't see a practical reason why dB's normal use can't have been covered by normal prefixes.

What sets the dB* aside from other American units is that this one is very close to following standard units. If it weren't for the deci- prefix and the usage of standard units like Watts and Volts ("Bell-horsepowers"), the inconsistencies in practical use would probably have been expected, making learning about the weird intricacies of each field a lot less infuriating.

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