The center meter is receiving audio from the ear bud in read dba sound power.
I set the reference voltage and set the meters to read db rel.
Then I dropped the signal and compared the two Fluke meters. Then I set the left meter to read voltage as I scrolled through the signal range. If you scroll to the last reading you will find less than 2 mv produces 60dba which is 'normal conversation'. If you can't hear 60 dba you are likely hearing impaired. My wife is watching TV in the room down the hall and I read 40dba which is load enough for me to hear.