spenav OP
I would prove I needed to bother covering it up, it may not make much difference.
Measure Room Frequencies from Seated Ear Height Listening position(s)1st, no covering, drape a blanket over it, measure again, differences?
this CD has 29 1/3 octave test tones. (I could help you find an inexpensive copy)
SPL Meter with tripod mounting hole (many do not have the tripod hole)
You just need answers relative to the next, not perfect calibration, these inexpensive meters work well.