The terminal rating of electronic communication equipment (amplifiers, microphones,
etc.) is commonly in decibels. Equation (9.11) indicates clearly, however, that the decibel
rating is a measure of the difference in magnitude between two power levels. For a specified
terminal (output) power ( P 2 ) there must be a reference power level ( P 1 ). The reference level
is generally accepted to be 1 mW, although on occasion, the 6-mW standard of earlier years is
applied. The resistance associated with the 1-mW power level is 600 , chosen because it is
the characteristic impedance of audio transmission lines. When the 1-mW level is employed
as the reference level, the decibel symbol frequently appears as dBm. In equation form