>> help db
 DB Convert to decibels.
    DB(X) converts the elements of X to decibel units
    across a 1 Ohm load.  The elements of X are assumed
    to represent voltage measurements.

    DB(X,U) indicates the units of the elements in X,
    and may be 'power', 'voltage' or any portion of
    either unit string.  If omitted, U='voltage'.

    DB(X,R) indicates a measurement reference load of
    R Ohms.  If omitted, R=1 Ohm.  Note that R is only
    needed for the conversion of voltage measurements,
    and is ignored if U is 'power'.

    DB(X,U,R) specifies both a unit string and a
    reference load.

    EXAMPLES:

    % Example 1:Convert 0.1 volt to dB (1 Ohm ref.)
                db(.1)           % -20 dB

    % Example 2:Convert sqrt(.5)=0.7071 volts to dB (50 Ohm ref.)
                db(sqrt(.5),50)  % -20 dB

    % Example 3:Convert 1 mW to dB
                db(1e-3,'power') % -30 dB

    See also abs, angle.