Posted March 26 2003 - 02:44 PM
As you probably already know, the crossover control on the sub itself controls the frequency cutoff on the high end that will be reproduced by the sub, with some slope and overlap involved. Recievers or separate processors allow you to select Small or Large as speaker size for the mains, center, and surround speakers. This selection can usually be made individually for the center, the 2 mains, and the 2 surrounds. When you select Small the bass frequencies below a certain point (typically 80-100hz, and on many receivers adjustable) are not sent to the speakers so designated, but are redirected to the subwoofer.
This applies to the bass in the 5 (or 6 in the case of DDEX and DTSES systems) regular channels and not the separate LFE (Low Frequency Effects--the .1 in 5.1) channel. There is much more bass in these channels than many people realize, the LFE channel by no means handles all the bass in a movie soundtrack.
Since these very low frequencies require the most power to reproduce at high volumes, and are for the most part can't be localized by the human ear (you can't tell where they're coming from as opposed to higher frequencies like birds chirping in the surrounds), it makes sense to let the powered sub's amplifier take the load off your receiver's amplifier thus increasing it's "headroom" and helping to avoid overloading it's amps.
This is called bass management and allows one to get very good sound without having to purchase huge full range speakers for the mains, center, and surrounds.
Basically you want to set the crossover on the sub's controls significantly higher than the crossover on your reciever. This way there will be no gap in the frequencies being reproduced by your sub in combination with the other speakers. Since little will come out of the receiver's sub output terminal above it's crossover setting, you can set the sub's crossover setting at it's highest setting and still not have much if anything above the reciever's setting come out of the sub, so you aren't going to get localization of bass.
The level knob on the sub is a different matter, as it's basically a volume control and does the same thing as the sub level setting on your receiver.
Most receivers have a limited range of available settings for the sub output. The lower settings may not produce enough voltage at the output to the sub to trigger the sub's auto-on function, and the higher settings on some receivers will sometimes produce distortion. A good idea, I think, is to balance the sub and receiver level settings so that a setting on the receiver about in the middle of it's range produces sub levels even with or a bit higher than main speaker levels.
I start with a midrange setting on the sub's control and try adjusting for a proper level with the receiver's control. If I get a good level with the receiver's control in about the center of it's range, I'm good to go. If I need to turn receiver sub level significantly lower than midrange I turn down the knob on the sub and try again, until I reach a balance between the sub knob and the receiver's sub level that gives me the correct sub level with the receiver's level near the center of it's available range (-10db to +10db in the case of my current receiver).
The MCACC (auto calibration) on my Pioneer originally came up with a sub level of -8.5 db, and the sub didn't turn on as often as I liked. I turned down the knob on the sub and reran MCACC until I got an MCACC sub setting of -2.5. Then, simply because I like a bit more bass than spec, I turned the receiver's sub level up manually to 0db.
I hope I haven't totally confused you, and keep in mind that this is just what's worked best for me. Others may have equally or even more valid opinions on how this adjustement should be done.
There are also some other factors involved--some receivers have separate sub and LFE level adjustments for example, and if one has no sub one can direct LFE to mains or even all 5 or 6 regular speakers and have both LFE and non LFE bass reproduced by them if they're capable of doing so and the receiver or amps can handle the load.
I prefer not to push the subwoofers until they're properly run in.