AFAIK, the reference of the 0 dB value is the voltage level which delivers 1 mW of power in a 600 ohm resistor (old standard reference impedance in telephone circuits). It translates into some 0.775V RMS
This being the "regular" dB, not dBm or dBa or any other.
What you describe is exaktly what dbm is.
There is no "regular" dB. Well there is, but that means nothing more then a logarytmic relation between two things. It is usualy then named as DB or DBr (r=relativ)
A absolute DB scale use to have a referenz level. The Problem is, that there are so many different references...
-DBspl (sound preassure level)
Ref: 0.000002 pa (pascal) = 0 DBspl
Acoustic loudness meassurement
-DBm
Ref: 1mW (into a 600 Ohm terrmination--> 0.775 Volt)
ITU standart in telefone circuits. It was the best
Compromise to delifer the most power ouver Long distances.
-DBV (Capital V, --> Volt)
Ref: 1 Volt = 0 DBV
Consumer audio level.
DBu
Ref: 0.775 Volts = 0 DBu
Professional Studio and lifesound level.
If such a circuit is terrminated with 600 Ohm, it corresponds with DBm
DB A/B/C curve
Ref: None !!
It is just a a filter added in the frequency Domain.
There are many many more...
So it is really important to know what reference is used. Otherwise you compare Apples and oranges.