stereo vs mono sourced pink noise

In running pink noise with an SPL meter, I'm trying to get all 5 speakers to calibrate at the same 85db level. However, I'm noticing that pink noise reads much louder in the center channel, DEPENDING on whether the pink noise SOURCE is a MONO or STEREO protools track. Inotherwords, I pan the pink noise from the stereo track around the surround field to get this result.



Inotherwords ~ A protools stereo track (both channels locked together) panned to the center reads double loud than panned to either the left or right channel. I am generally leaving the C % control at 100 percent. Can anyone explain what I am doing wrong?



thx



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