Based on one tenth of the Bel named in memory of Alexander Graham of telephone fame. Have always understood that Decibels are the logarithmic ratio (base 10) of 'two' or more, power levels.
So am familar that doubling (or halving) a power ratio is approx +3 (minus 3) decibels. etc.
But in day to day parlance one sees (and often hears) that something is loud, at say 90 dBs, very loud at say 105 dBs. or liable to damage ones hearing at say 125 dBs.
Or that 'normal office noise is 76 dBs or something.
But in relation to what?
On other hand have seen auto magazine articles reviewing vehicles that, very carefully, will say, for example, "Interior noise at 'x' mph. is so many dBA".
What is that 'A' ? Ambient or something?????
Also recall doing 'Noise measurements' on communication circuits using dBa (small 'a'). Meaning IIRC 'adjusted'? Those were using a 'Typical weighted circuit response' with zero dBa appearing to be somewhere around minus 90 dBm?
Various 'trades' also seem use dBm, i.e relative to dB Zero being being one milliwatt; dBw, i.e. relative to one watt and others.
And then there are loud speaker measurements; something to do with Sound Pressure Levels at a certain distance from the speaker?
But is there any one single power level or standard that is generally accepted as being a 'Normal' reference. Hence people tending to drop the reference in everyday usage?