Hi o,
Testing SPL in your room at different frequencies will give you an idea of where in the frequency spectrum your room is accentuating sound pressure levels (SPL) and where it is attenuating them, so that you can:
1) know which frequencies to target for attenuation
2) see the effects of placing absorption panels at different locations.
As much as it pains me to link this site, this is a good tutorial on measuring your room:
http://realtraps.com/art_spl.htmAll you need is media that generates different known frequencies through your speakers, a radio shack analog (one with a needle meter) sound pressure meter, and a lot of time.
http://www.ebay.com/itm/REALISTIC-SOUND-LEVEL-METER-33-2050-RADIO-SHACK-/201727489735?hash=item2ef7e...The Stereophile Test CD is a good one, but only available through them, so may take too long to procure. I found this download if you are computer savvy and have a way for your pc to play through your system:
http://decibelcar.com/apps/199-download-test-tones-a-frequency-sweeps-mp3-cd-format.htmlOnce you develop a plot like in the link for each speaker, then you will know where to begin experimenting with your absorption panels and have a way to measure the results vs pure trial and error.
Best to you o,
Dave