Ian, GM3SEK wrote:
"Decibels are always used for comparing power levels."

True.

The decibel or dB is a power ratio. The dB alone says nothing about the
quantity of power, only the ratio between powers. Thus, the dB is useful
to express the gain of a linear amplifier or the loss of a resistance
pad.

The dB can be used to express a power level by reference to some known
quantity of power. In this case, the ratio is said to be so many dB
above or below the reference level.

In the telephone industry, the most common reference level is one
milliwatt. Signal loss is common. Cables fave loss per foot or mile.
Attenuation is a division process. Amplification is a multiplication
process. It is convenient to express the amount of power at a particular
point in a system as being so many dB above or below a reference power
of one milliwatt because adding and subtracting decibels gives the same
answers as multiplying and dividing powers. Decibels above or below one
milliwatt is usually abbreviated as + or - dBm.

DBa and dBrn indicate the interfering effect of noise in a
communications channel. DBx is used to indicate crosstalk coupling into
telephone circuits.

VU is a volume unit. Reference is to one milliwatt, same as the dBm, but
the ballistics of the meter are specified so that a standard response is
made to varying signal level.

Other dB units are easily contrived. There are dBw (dB referenced to one
watt), dBk (dB referenced to one kilowatt), dBRAP (dB above reference
acoustical power which is defined at 10 ro the minus 16 watts), etc,
etc, etc.

Best regards, Richard Harrison, KB5WZI