Calibration to THX reference level

After my initial enthusiasm with my new 990 and Odyssey Stratos Plus amp, I finally decided to calibrate the front two channels and sub to THX reference levels (I'm only running a 2.1 mostly music setup now). I wonder if I did so correctly.

Since this is a 2.1 setup, I didn't think the 990 could set the levels automatically well and I chose to set them manually.

I have a test CD with pink noise at -15db from peak as well as various test tones at -15 dB from peak: 40 Hz, 100 Hz, etc. I have another CD with more extensive tones as well, but haven't used it yet to measure the overall room response (it has many sub tones and sub sweeps as well, to pick out room resonances).

At my primary listening/viewing position, using the unbiquitous Radio Shack SPL meter, I obtained a 75 dB average signal from the front mains with the 990 set at -23 dB, and I then dialed in the corner-loaded sub on a 40 Hz tone to also produce a 75 dB reading. Figuring, these tones are 15 dB down from peak, I'd get 90 dB peaks with the 990 at -23 dB and thus 105 dB THX reference level peaks with the 990 at -8 dB. So, I set the Left, Right, and sub channels -8 dB down. Remeasuring at 0dB (with hearing protection) verified that I was close to THX reference levels (well, the sub wouldn't hit a 115 dB peak 14 feet away at 20 Hz, but I expected as much: I'd need 127 dB one meter from the sub. I can hit 130 dB at one metre with it not bottoming out, but not at 20 Hz -- at 32 Hz. This is within it's design parameters.

So, have I overlooked anything?

Oh, and yes, THX reference level is VERY LOUD!. At most, I watch/listen 12 to 16 dB down from it.

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Thanks, Wayne.

I was a bit worried about setting the 990 to 0 dB and using it's internal test tone: I've read that it's loud, as in scary loud, as in bottom out one's sub loud (though mine has a level control).

I suppose I can try it with a monitor speaker before I run it through the mains and sub to see just how loud it is.

The 990's test tone is intended to be used with the volume set to 0dB. Yes, it is loud, just as with other processors and receivers (it seemed just as loud as my 950's test tones did), but it should not risk speaker damage.

Gonk are you sure? The 990 test tone for the sub went into sustained clipping on my passive sub amp. I know woofers are mean't to handle current, but isn't there a chance that the voice coils could be damaged?

Driving my Radia 520i amp (an Odyssey Stratos Plus) into clipping would destroy the 520i. To be precise, the ribbons would catch on fire. Granted, that's different than the sub, but if the sub test tone is so loud, I'm concerened about the others.

I had followed Outlaw's recommended manual procedure for balancing the speakers, using my own test tones and noise sources (not trusting the level of the 990s), and calibrated the absolute level to THX levels at 0 dB on the 990 (padded down 8 dB actually for L and R).

The sub test tone isn't louder than any other tone, but unlike most subwoofer test tones it is full range, which means the sub is getting a much different sort of signal than it is used to. That's the reason that only the sub test tone seems louder than folks are used to.

...but unlike most subwoofer test tones it is full range...

Ah, then I'd have no need to worry, since my sub has the ability to filter the input (up to about 150 Hz, IIRC) and passthrough output.

Good thread-
My only question is why does my Avia Guide to Home Theater DVD suggest setting THX reference levels at 85db? They actually knock setting it at 75db since this can cause softness in the center and surround channels during movie playback. They also state that 85db is the movie industry standard reference setting for theaters.

I have set mine at 80db to split the difference and I did notice an improvment with surround / center track on movies.

Comments?

The Avia Guide's tones are 20db below reference, hence the 85db you need to use. The Outlaw's tones are 30db below reference: thus you use 75db. Setting it at 80 just means you haven't achieved reference at 0db. It won't hurt anything for sure.

Reference = 105db max at the listening position at 0 db when properly calibrated (115db from the LFE). Very few subs can hit the LFE reference.

cool, thanks tonygeno-
I agree that very few subs can hit that level, but we all want one don't we???

My sub can hit 115 db@20 Hz, but only one metre away. My listening/viewing position is 4 metres away from the sub, so it's 12 dB down from that, or around 103 dB.

If I'm not mistaken, it will hit 132 dB at 32 Hz at one metre, though.

Of course, these figures are with corner reinforcement -- that tends to pick the response up below around 30 to 40 Hz. Anechoic response is nowhere near as impressive. And these figures are achieve close to Dmax -- needless to say, I don't make a point of driving it that hard.

When I try auto-calibrating my new 990 for 7.1 set up, I get a msg. saying that since my rear speakers are large and fronts are small, I should switch the speakers and try again

So I tried manual calibration using my Avia disc, which gave poor results. Then I tried again using the 990 test tones which resulted in level differences of -4 to -5dB for the rears and backs.

Why are the levels so different? My guess is that when I calibrated using the Avia disc, I had the 990 in the wrong mode. What mode should the 990 be in for external source calibration?

Better yet, should I expect that the levels using the 990 internal calibration test tone will ever yield the same results as using an external (DVD produced) tone?

I've encountered the same front-rear speaker difficulties with auto calibration, but I assumed it was because I had the powered woofers on my front towers ( Polk RT2000s ) set too low to compete at the louder than comfortable volume levels of calibration. I found that if I crank up the woofer volume, auto calibration works fine.

reference levels are just that, a "reference", that doesnt necessarily mean that they will be what you prefer.

the experience is about getting what you want to out of it. be it thx reference levels or just the sound that you like the best.