Now you've got me obsessed with wave theory!
Ok, since I've spent quite a lot of time on boats of various types (motor and sail-driven), it's a good analogy to help visualize what's happening.
Let's say your mono SPL microphone is in the middle of a pond or lake, and a big wave is generated (a large boulder is dropped into the center of the pond)--that's the low-frequency wave, and it radiates out and hits a flat dock or concrete abutment at the side of the pond. (Think of the latter as a wall in your room.) When the wave hits that, a secondary wave is generated and reflected back and it travels along the pathway of the continuing larger waves coming from the boulder and hits the microphone of the SPL meter. There will be contructive and destructive wave reinforcement and cancellation, respectively, which will cause varying energy levels striking the diaphragm of the SPL meter's microphone, hence the varying levels seen on the meter.
The dock or breakwater is the equivalent of your room's walls. The acoustic wave is generated by the test tone from your subwoofer; the wave motion in the lake could be from a passing motorboat or from the wind. The principle is the same.
(There will be a multiple-choice test following this discussion!)
Alan
