[dave7]

The subs can interact with each other relative to the listening/measurement position. If you adjust the phase of them both equally through the processor than you are correct, you won’t likely see any change when you measure them without the mains being active. But, adjusting the subs’ phase individually can alter your response.

I feel this is important because your sub(s) don't share the same main baffle as you mains so they need to be timed correctly in order for their output to match the other drivers in you room properly. I think of phase as time aligning the subs to each other and to the mains. (BTW – I know there is an inherent problem with this logic since often the subs are farther away than the mains…but I solder on anyway)

To begin, I adjust the phase while playing one sub with one main to get the smoothest response. You will see using REW or RplusD how phase alters the response mostly at the crossover point where the two speakers are outputting the same frequencies. FWIW, I was never successful doing this by ear or even using a rat-shack meter. The graphical display in REW/RplusD makes it painfully easy to do though. Then I adjust the second sub's phase to get a good response with the first sub.

As an alternative, you might get the two subs working well together without the mains by using their on-board phase adjustment, and then alter the phase through your processor to match the mains.

Here's another surprising illustration - I did all the above and got things adjusted really nicely between both mains and both subs in all kinds of combinations. Then, I measured the FR of my center along with the subs. I had a huge null around the crossover frequencies. I thought I simply had the XO set too high, but that wasn't the case. I thought to myself "****!" because everything was so good with the mains, what the heck was going on. It was Terry Montlick of TM Labs (whom I had hired for acoustical consulting) who said it could be a simple placement issue with my center.

He ended up being correct. I simply moved my center rearward a little bit and that null filled right in perfectly. Incidentally it ended up being equidistant to my listening position as the mains. The lesson here is that just because I had it all in phase and level adjusted for the mains didn't mean squat until I tested each speaker individually (not easy or even possible for most people). The resulting improvements were stunning. It is not hard to do all this adjusting, but it does invite all sorts of obsessive-compulsive jabs/comments/jokes from our better halves.

I am far from an expert on these matters. I learned this when I worked with Terry. This is just my experience. I get the idea that most folks don’t set up their single sub correctly and two is even more of a challenge. But the in-room results are usually better with two...at least in my room they are, and I am not referring to SPL…I get a much smoother response.

Sorry about the length of this post...