How to calibrate dual subwoofers?

[Chris D]

This is something that comes up from time to time, and I've seen people with various opinions. Thought this was interesting, which comes striaght from a newsletter from SVS:

Dual subs for large spaces, or larger bass appetites, has been “the way” for years. Got duals? Get your SPL meter and get busy!

Keep the subs co-located, or within 4 to 5 feet (say the 1/3 and 2/3 wall points on the front stage).

Set the Phase Control to “0” on both subs to start. Also, set the AV receiver’s subwoofer channel level to “+1db” for now.

Play the AVR or Avia test tones, and with one sub at a time playing, adjust gain on each until achieving normal calibration level.

Finally, fire-up both subs together. The combined result should be 6dB higher than one individual sub. If yes? Lower the subwoofer channel level to “ - 5dB” and you’re done!

If the subs are more than 4 or 5 feet apart, or you aren’t getting the full 6 dB increase in measured output, then adjust the phase on one sub until you approach that 6dB of gain.

I thought this was interesting, and probably the best input I've read so far as to how phase should be used during the calibration. But why would there be an additional 6dB with a second sub? Logarithmically, I guess I'm not seeing why that works out to "double".

[Hdale85]

I think I've heard before that a second sub adds 6db? How accurate this is I'm not sure of but probably not very accurate.

[Hamblis]

Chris, I've been reading the Musical Fidelity web site recently, prompted by their view of the need for higher powered amplifiers. They say that "....each dB step in sound pressure level requires a 25% increase (approx) in power. A 3dB increase requires double the power."

Perhaps it's 6dB with a second sw due to the impact of low frequencies?

Steve

[chrispy35]

Check out this paper: http://www.harman.com/wp/pdf/multsubs.pdf

Slides 46-52 seem to indicate that the best placement (least deviation in frequency response over the listening area) for dual subs is on opposing walls...

[Chris D]

Huh... but if dual subs (or any two speakers putting out IDENTICAL signals) are placed symmetrically, won't they cause a big cancellation node right down the middle of the room and in patches around the room?

[Chris D]

chrispy- I just took the time to read through that whole paper. HOLY COW! That was so valuable! I had no idea... so I see what you're referring to, that with two subs, the "ideal" placement is for them to be at the middle of opposing walls. (i.e. either middle of the left and right walls, or front and back) I never would have thought that would be better than opposing corners. I thought you were supposed to do corners for bass output. I might just see if I have room in my theater to do left and right walls.

[John Holmes]

chrispy- I just took the time to read through that whole paper. HOLY COW! That was so valuable! I had no idea... so I see what you're referring to, that with two subs, the "ideal" placement is for them to be at the middle of opposing walls. (i.e. either middle of the left and right walls, or front and back) I never would have thought that would be better than opposing corners. I thought you were supposed to do corners for bass output. I might just see if I have room in my theater to do left and right walls.

This is also the THX suggested setup of dual subs up as well (in the middle of the walls and opposite).