Mark,
Are you sure the mic is rated in "dBm"?
usually mics are rated in relative dB below one volt which is the audio
standard with volt often left off the spec. A mike might be advertised as
having an output of "-60 dB".
1 volt equals 0 dBv. Minus 60 dB is a decrease of a factor of 1000, and is
1 mv.
Now, 6 dB is a factor of 2 voltage wise, so minus 66 dBv would be 1mv minus
1/2 equals 1/2 mv. (0.5mv).
A 5mv mike would be 14 db greater than a 1 mv mike, or -60 plus 14 or -46
dBv.
Mikes are usually rated by voltage output as described above; power in an
audio circuit is rated in watts or mw, thus dBmw would be used in describing
the gains thru telephone amplifiers for example. (dBm is taken to mean dB
below 1 milliwatt.)
Possibly because mikes traditionally were higher impedance, voltage output
devices capable of little power, I have never seen them rated in dBm.
-Stuart
K5KVH
|