|source from http://www.svsound.com/questions-faqs-rscomp.cfm
other discussion in AVS
Maybe you already know how powerful a tool a simple Radio Shack Sound Pressure Meter is. If you use some of the popular test disks out there to check your room response there are some things you need to know about the meter. Some of these disks, like the popular "Autosound 2000 Bass CD #101", have sweeps or even chapter stops that show how even (or not) the response of your system is down low, below 20Hz even.
Subwoofer One" MS Excel worksheet