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What is the meaning of the unit that is written between parentheses alongside the dB unit? For example 2 dB (mW) or 2 dB (uW)? What exactly does this unit refer to? Does it refers to the unit of the reference value, the measured value (P2), or both?

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    \$\begingroup\$ Unusual notation but I would interpret it as being 2 dB referenced to the unit in brackets. \$\endgroup\$ – Peter Smith Feb 15 '19 at 14:51
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    \$\begingroup\$ related and might already answer the question: electronics.stackexchange.com/questions/194755/… but not exactly a duplicate \$\endgroup\$ – Arsenal Feb 15 '19 at 14:51
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    \$\begingroup\$ I would interpret 2 dB (mW) as 2 dB over the 1 mW base unit = 1.585 mW \$\endgroup\$ – winny Feb 15 '19 at 14:52
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    \$\begingroup\$ According to ISO 80000, such an addition of a postscript to the unit symbol dB is incorrect. This information should be carried by the quantity symbol. \$\endgroup\$ – Loong Feb 15 '19 at 18:48
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    \$\begingroup\$ And here I expected (from the question title) that this would be about dB(A) ... \$\endgroup\$ – Hagen von Eitzen Feb 15 '19 at 20:40
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dB is a measure of ratio, nothing more.

As such, if an absolute measurement is expressed in dB, then we need to know what the denominator of that ratio is.

For frequently used units, like dBm for instance, the string 'dBm' is a recognised standard way of expressing 'dB with respect to 1mW'.

Some writers baulk at combining dB directly with a unit, and so the practice has grown up of putting the reference level in brackets, for clarity, giving us dB(unit). Some people write dB(mW), it means exactly the same as dBm.

As a ratio, dB is dimensionless. The quantity expressed by dB(unit) will therefore have the same dimensions as the unit.

In some fields, it's not uncommon to see the unit being left off, which is very naughty, but unfortunately very common. I often read reports of sound having a level of so many dBs. You can usually guess what the default reference level is.

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    \$\begingroup\$ Sound is usually dB relative to the threshold of human hearing, right? Something like dBmPa or dBμPa or whatever. Edit: looked it up, I suppose that would be dB (20 μPa). dB(SPL) seems to be a more common term. \$\endgroup\$ – Hearth Feb 15 '19 at 15:53
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    \$\begingroup\$ @Hearth I most often see "dB (SPL)". Properly, a weighting should be indicated, but I think A weighting is assumed if none is specified. In the US, laws about sound pressure levels are all with reference to A-weighted values, so at least in The States that's the default. \$\endgroup\$ – Todd Wilcox Feb 15 '19 at 17:03
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    \$\begingroup\$ @ToddWilcox for A weighting (or B and C) I would write dB A while dB SPL generally means a flat frequency weighting and a 20 $\mu$Pa reference. dB HL would be referenced to the norms of hearing and the corresponding dB SPL depends on the frequency and headphones/transducer. dB SL (sensation level) is referenced to the hearing of the individual. \$\endgroup\$ – StrongBad Feb 15 '19 at 18:33
  • \$\begingroup\$ dB is always a measure of power ratio. \$\endgroup\$ – Mike Waters Feb 15 '19 at 20:55

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