DBrn

Note 1: Weighted noise power in dB is referred to 1.0 picowatt. Thus, 0dBrn = -90 dBm. Use of 144-line, 144-receiver, or C-message weighting[?], or flat weighting, must be indicated in parentheses as required.

Note 2: With C-message weighting, a one-milliwatt, 1000-Hz tone will read +90 dBrn, but the same power as white noise, randomly distributed over a 3-kHz band will read approximately +88.5 dBrn (rounded off to +88 dBrn), because of the frequency weighting.

Note 3: With 144 weightings, a one-milliwatt, 1000-Hz white noise tone will also read +90 dBrn, but the same 3-kHz power will only read +82 dBrn, because of the different frequency weighting.

Source: from Federal Standard 1037C and from MIL-STD-188