They are units too. When you have regulations relating to sound they will say how many dB the limit is.

No. dB is not a unit. If it's quoted as a unit, then the person using it doesn't know what they are doing. dBx where x is some reference value is a unit and that means dB relative to the reference unit. So dBm/dBv etc are units. dB is not.

Mathematically, dB is the ratio between two unit values and for example if you divide metres by metres you cancel the units.

	▲	qmmmur 7 months ago \| parent [-]
		Exactly. If someone says that "this plane is about 130dB" what they mean is in reference to 0dB given 0dB is the threshold of human hearing. It seems most people are confused by dB because they don't understand it provides a logarithmically scaled unit compared to a reference which happens to be quite useful for something like audio where our perceptual models are stacked with these types of curves.

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jononor 7 months ago | parent | prev | next [-]

No they will specify something like dB SPL, A-weighted with Fast time integration. Sometimes referred to as dB(A) SPL. And measured with a particular method, which specifies things like distance from sources, environmental conditions, etc.

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formerly_proven 7 months ago | parent | prev [-]

I've never seen dB being used like that in anything remotely official.

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coolcase 7 months ago | parent [-]

https://www.osha.gov/noise

Maybe the reference is implied though?

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formerly_proven 7 months ago | parent [-]

That's hilarious, though they do seem to get it right on the other, more in-depth pages.

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throwaway290 7 months ago | parent [-]

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

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radiowave 7 months ago | parent [-]

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).

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formerly_proven 7 months ago | parent [-]

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.

	▲	radiowave 7 months ago \| parent [-]
		Granted, I should have been clearer about my intended point, which is just the hazard of assuming that the letter(s) after the dB tell you the reference, because sometimes they don't.