Using a RS SPL meter


What is the proper way to use a RS SPL meter in order to measure the room response. What I have done is set the meter to "C" weighting, "fast response". In particular, I wanted to see what was happening in my room from 200 Hz down (my room is 14'5" wide x 19' long with coffered ceiling).
This is the part I'm not sure I did correctly. Using a Sterophile Test CD, I played 200Hz track, and adjusted the volume of my preamp in my room until the RS SPL measured 90 db. I then played each of the following tracks (with lower freqs), and recorded the db reading from the SPL. I put the RS SPL on a tripod, at ear level, from my listening position, and pointed the meter directly forward (aimed between the two speakers).
Wow, the response dropped significantly below 60 Hz? I'm talking about a drop of 18db by the time i got down to 25 Hz. (My speakers are Acoustic Zen Crescendos, which are full range). I understand (from another thread here on A'gon) that the RS SPL is deficient in the lower frequencies, and that I need to add db's to the reading I get (however, even after db's to the response, the response below 50 would still be down over 12 db).
So,
#1 Am I using the meter correctly?
#2 If not, what is the correct way to measure room response using the RS SPL meter?
#3. Any thing else you want, to add or need to know?
In advance, thanks!
Louis
louisl
Iirc I used to set the meter to 0 dB for the average sound level in the room. Then when you move around the room you can see the peaks and the dips in dB relative to the average level.
Good advice from Geoffkait and Tom 6897 (an ear is a terrible thing to waste). But also you should read the J. Gordon Holt article in the Stereophile archives on flat frequency response and consider that along w the impact of the measures you take to achieve it.
Geoffkait,
What do you mean, "I used to set the meter to 0 db"?
Also, which way do I point the RS SPL meter,
straight up, like the above suggestion?
Or At the speaker, point it between the speakers?
I assume I make the measurement from my listening position?
Obviously, from these comments above regarding how poorly calibrated the RS SPL meter is calibrated, I should only use it as a ROUGH guide?
Any suggestions as to what software I can download to use to get better measurements of my room?
Thanks to all!
From the RS manual...
MICROPHONE
The meterʼs built-in microphone works best when you point it directly at a sound source.

http://support.radioshack.com/support_audio/8712.htm
I actually have found what Ethan Winer has reported here --http://realtraps.com/art_spl.htm -- to be largely true. The Radio Shack SPL meter is pretty accurate for the bass frequencies, not so accurate for anything above around 1kHz and no correction table is actually all that meaningful because of the wide variety of frequency response between samples. Also, the mic is an omni so it doesn't much matter how exactly you point it. Straight up is the preferred method for surround sound but straight along the listening position line is better for two-channel since pointing at the source is likely to give you the flattest frequency response. Yeah, the device is inexpensive and not calibrated, but -18dB is steep. Are you talking about a steady decline or are you finding nulls and resonances at certain frequencies? It's also possible you have a bit room resonance at 200Hz and that may not be the best reference tone, try setting a reference level with white noise. I just used an older analog Radio Shack SPL meter to set up my subwoofer xover and level and managed to get pretty flat response down to 50Hz, where there was a big room null followed by a resonance at 40Hz, and room reinforced output down to 20Hz