How do you set ACV DB function in Model DMM6500?

In the SCPI command set mode, the following commands will get you there:

:SENSE:FUNC 'VOLT:AC'

:VOLT:AC:UNIT DB

:SENSE:VOLT:AC:DET:BAND 300

:SENSE:VOLT:AC:DB:REF 5

This will set the unit to ACV, db and reference of 5.

If the unit is set to TSP, then use these commands:

dmm.measure.func = dmm.FUNC_AC_VOLTAGE

dmm.measure.unit = dmm.UNIT_DBM

dmm.measure.detectorbandwidth = dmm.DETECTBW_300HZ

dmm.measure.dbreference 5