SEFRAM 7351 Manuel D'utilisation page 37

3. dB and dBm (ACV / ACmV only)
The decibel (dB) is a logarithmic unit of measurement that
expresses the magnitude of a physical quantity relative to a
specified or implied reference level. The dB and dBm are
defined below.
V
AC
dB = 20 log
1
V
AC
dBm = 20 log
0.7746
12