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).
#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!
Why 90dB? Try measuring at 75 to 80dB.
If you do a Google search for "Radio Shack SPL meter corrections" you find links to lots of discussions about this topic on AVS forum. I've got both the Stereophile and Rives Audio test cds (the Rives cd has tracks that are supposedly corrected for the anomalies inherent in the meter.) My JL Audio Fathom has a calibration mode that purportedly corrects and equalizes in room response. If I accept the measurements I make with the Stereophile cd, my room is plus/minus 1.5 db from 50-25 hz and down 3 db. at 20 hz. The Rives cd tests have such a wide variance from this I wonder if it is calibrated to the model db tester I have (There are analog and at least two digital versions.) I wrote Rives and asked what model meter they are using for their calibrations, but they never replied to my question. Bottom line is, it's hard to know how accurate any given RS meter is (but its measurements are consistent and are good for seeing the comparative results of adding room treatments, moving subs to new positions, etc. ) If one wants to do better, I guess you have to step up to the next level and buy something like the calibrated Dayton Omnimic measurement system Parts Express sells.
Oh, and supposedly the RS meter requires adding a +7.5 db correction @ 20hz and 5 db @ 25 hz.
To Onhwy61,
I used 90 db because this is the average SPL when I listen to music. I don't know if this is the correct way to begin taking measurements, or not.
90 db is pretty loud for constant listening. I hope you do not ruin your hearing or listen for long periods at that level. Although if your in room response is as bad as the measurements say..... your real levels may be quite a bit lower.
I think the microphone is designed to be pointed up rather than directed at the sound source. Slow trajectory is commonly used to get a steadier reading. Measurement with sine waves might be fraught with artifacts.

While the meter can be of some use, since it is not in most cases correctly calibrated, the readings can be very misleading. Its mike, and some freestanding mikes also, can have a wide error rate, best to look to rent a calibrated mike and use some of the free computer programs to do a better test.
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.
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!
I actually have found what Ethan Winer has reported here -- -- 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
From the RS manual...
The meterʼs built-in microphone works best when you point it directly at a sound source.
Here’s how I did my home system: I’ve set-up a pair of front speakers (15’s, 7’s and tweets) using the older analog model with Ranes DEQ60 and the Alan Parsons SoundCheck CD. The SLM was on a tripod at ear level centered between the speakers in a large sweet spot. I pointed the mike towards each speaker separately.

I used a 1000z tone on the test CD to set my 80 dB reference volume. Next, ran 30 CD tones independently to each speaker while adjusting the playback band levels to 80dB’s from 25-20K Hz with the DEQ60. WIth little further adjustment by ear, the separation and balance is nice at all listening levels. I knew of the cabinets 50Hz roll-off prior to this set-up, and the settings on the DEQ60 shows a similar pattern.

I pointed the mike upwards to adjust volumes for a 5.1 configuration using a Proceed AVP2+6. I set a SVS Ultra-13 sub in a sealed mode to 15Hz and crossed over at 50Hz with a 24dB roll-off to match the fronts. Once set-up, test tones and sweeps sounded very level to me and show a very linear pattern on the SLM (+/- 2dB above 50Hz without the sub) . I’ve used a Behringer 2496 RTA w/matching mike in the Dual Mono mode and enjoy better musicality using the SLM/DEQ60. Other listeners don’t hold back positive comments - if I thought it was musically off after I fine tune it by ear, I’d go for a higher end SLM that can be routinely calibrated.