I just thought I'd throw out some alternate thinking on level calibration of a home theater speaker system.  These issues are not really dealt with in any manuals or other papers associated with home theater that I'm aware of.

Some background:

On a movie dubbing stage (and in all professional work) full range pink noise is used for speaker calibration.  This is used because it exercises the entire audio spectrum from deep bass to the highest treble at once, and provides a more accurate "average" sound energy measurement of the speaker system.  Analog SPL meters are used since generally an analog meter movement is easier to interpret as an average level than the changing digits of a digital display.  The lowly RadioShack analog meter is used daily for checking the calibration of dubbing stage systems, although the much more costly Bruel & Kjaer meters are used for absolute calibration.  These systems are calibrated for 85db SPL, which is the average SPL of dialogue in a movie.

On the home theater front, manufacturers have standardized on a psudo-pink noise test tone source.  This tone is basically filtered pink noise with the center of the spectrum centered about 1Khz and falling off steeply at higher and lower frequencies.  It does NOT exercise the bass or treble at all, and as would be expected, it relies on the flatness of your speaker system in the area of 1Khz for the accuracy of the calibration.  Generally home speakers are pretty flat in this region, but this is not always the case.  If your front speakers are flat at 1Khz and your surrounds have a peak of 3db at this frequency, after you calibrate your system, your surrounds will end up 3db too low in level.

So, let's adapt the professional model and use it for home calibration.

For home theater use, aside from increased accuracy, there is an important benefit of using full range pink noise to calibrate your speakers.  That is the fact that with this signal, TIMBRE changes are VERY audible from speaker to speaker.  By ear, you can readily hear any changes in tone caused by room influences or non-identical speaker systems as the signal is switched from speaker to speaker.  The pink noise should sound identical in every speaker (except for the sub).  You can make tweeter level adjustment, or speaker position adjustments by ear and get very uniform results, and better imaging as a result.

As a calibration signal source, you can take advantage of the greater accuracy of full bandwidth pink noise.  First of all, I would turn off your subwoofer for calibration of the other speakers. This is because the bass management in your receiver or pre/pro will re-direct the bass from "small" speakers to the sub, and make measurements on the other speakers more difficult than necessary.  The bass coming from the sub will just cause the meter to jump around needlessly.  Then, as a comprimise between absolute full range accuracy and practical ability to follow the needle on your SPL meter, set the "weighting" to "A", rather than the more common "C".  This "A" weighting makes the meter more sensitive to the mid range frequencies and less so to the  bass and treble, but nowhere near as extreme as the bandwidth limited test tone built into receivers, pre/pros, and on test DVDs.  After you get all the main and surround speakers to the same level, swtich to "C" weighting and note any differences:  this will be mainly due to the greater sensitivity in the bass region of the meter.  If the needle is still relatively easy to read, you might want to do any final touch-ups in the level of any speakers that give largely different readings from the "A" weighting reading.  Finally, switch on your subwoofer, direct the test tone to your sub, and use the "C" weighting position of your SPL meter to get a level setting.  Note however that the needle will swing more with this deep bass, and it may be difficult to get a good "eyeball average" from the meter. Some by-ear level setting will probably need to be done in conjunction with the readings from the meter.

The Avia home theater DVD test disc has 5.1 full range pink noise test signals, and this is all you need to do the above calibrations.  Note however that the pink noise on the Avia disc is recroded at -20db below digital full scale verses -30db on other consumer discs (and from receivers and pre/pros).  You have two choices if you are calibrating to a "reference" level on your receiver or pre/pro. You can either calibrate to 85db SPL verses 75db with the volume control at "reference" level, or if the pink noise's volume is too bothersome, reduce the volume control on your receiver or pre/pro to -10db and calibrate your system for 75db SPL.


[This message has been edited by soundhound (edited May 29, 2003).]

SoundHound,

Interestesting suggestions. So for my system, I would:

Disable Logic 7, etc., and go to plain vanilla DD decoding

Set my volume control at +0dB.

Play the Avia tones.

And calibrate to 85dB? Is that what you're suggesting?

Would I set the volume to -5dB and then calibrate to 80dB, seeing as how the de facto standard Radio Shack SPL meter is more accurate in the middle?

Jeff

Jeff:

Yes, you would set your pre/pro for DD 5.1 and calibrate as in the instructions.

Generally analog meters are most accurate at the full scale position. If you have heard anything specific about the RadioShack meter to the contrary, of course take that into consideration and go for mid-scale.

There is a website that provided the frequency response curves and alternate settings for the RS meter than can be used for more finite adjustments.
It has been a while, and I can not find it at the moment, but when I do will post it. It may be of some value in this discussion.

Yes, I'd be very interested in seeing that.

I'm about to equalize my system tonight: I got the Aerials a while ago, and I haven't equalized since. (I just set everything to flat in the interim.)

Since I'll probably be using Avia to equalize, I might as well finish the job by level setting with it. If I do, I'll post my results here.

Jeff

Is this what you are referring to regarding the RadioShack meter?

http://www.svsubwoofers.com/faq_rscomp.htm

[This message has been edited by soundhound (edited June 08, 2003).]

That is not the same URL, but it looks like some of the same info.

SoundHound,

During equalization, I noticed a huge notch for my right speaker in the 100 Hz - 200 Hz range. Any suggestions what might be causing that?

My first guess is that out-of-phase reflections may be cancelling out the sound in that range. Any thoughts?

Jeff

Jeff:

That is my thought also. You might move the speaker just to see if the notch goes away. Don't get too carried away "over correcting" every hiccup in the response, or you might end up with worse sound than before.

SH,

Yeah, I learned that the hard way way back when. Now, I mostly cut away some of the worst peaks, and, rarely, give a tiny bit of boost. In this case, the 125 Hz band was way down (around -12dB) so I added a very gentle (+2dB) gain to that band just to put it back onto the map.

But other than that, I leave well enough alone.

Thanks for the thoughts. It's tough to do room treatments when you live in an apartment, but I'll have to see what I can do.

Jeff