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.