[TenTec] 5 mV = -67 dBm

[Ken Brown]

There is a standard used in telephony and analog multiplexing wherin the 
value of an audio level in dB(with a suffix I cannot remember) is 
expressed as it would be read on an AC RMS voltmeter with a scale 
calibrated in dBm at 600 ohms, regardless of the actual impedance. In 
other words an AC RMS voltmeter which has a scale that is calibrated in 
decibles relative to 1 milliwatt at 600 ohms is used. A reading is taken 
and the level is called dB(something). The impedance of the source or 
load may be thousands of ohms or only a few ohms. So the dB(something) 
value is really not equal to dBm, except in the special case where the 
impedance happens to be 600 ohms. Nevertheless this way of specifiing a 
level is often used. I wish I could remember what letter they follow dB 
with.

Ken N6KB