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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
lowrider57's avatar
lowrider57
5,397 posts
 
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
chervokas
17 posts
 
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
soundsbeyondspecs
132 posts
 
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.

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