How do you correlate the SPL with db

SPL is a unit of measure - Sound Pressure Level

db is a particular practical logarithmic scale for plotting SPL's named in honor of Alex Graham Bell - decibel

SPL db = 20 Log (Measured Sound Pressure / Ref Sound Pressure)

For your purposes it is the same thing...

There is no absolute way unless you calibrate the Behringer since dB is a relative measure. That said, the RTA should be scaled in dB as well although probably not the same dB.

However, all this is relative and that may be all you need. Filter magnitude is referred to a base line and the RTA should indicate how many dB +/- from baseline the peaks/nulls are. Sloppy but possible.

Kal

The SPL you referred to is the absolute db value, 0db represents the max value the display can show. The scale on the eq is relative, 0db means flat or no deviation.

Lets say you have a 1000Hz signal and you turn the volume up such that the mic records an 80db signal level. Assuming the RTA max value is set at 90db, you should see -10db (80db is 10db lower than 90db) at 1000Hz showing on the RTA display. At this time -10db equals 0db on the eq because there is no deviation. Now if you raise the eq at 1000Hz to +3db, you should see -7db signal level on the RTA display because you have just added 3db to the signal strength (83db is 7db below 90db).