Quote:
brucek wrote:
 Absolutely. This where REW can help show the best sub phase and distance setting to get that rise down. |
I'm not sure that is the approach I'd pursue...
Generally speaking, you're trying to optimize acoustic summation through the crossover frequency so that you can also maintain a flat power response. I would be willing to wager that the rise might be the result of processing inside the receiver. For example, the difference between "Direct" and "Stereo" on my Denon AVR is a 3dB rise at the crossover frequency. I've seen this show up on other systems too.
Nevertheless, if the rise is in fact the result of acoustic summation, then the best approach would be to implement a cascaded filter on the subwoofer, which would increase the steepness of the crossover, thus reducing the polar lobing that occurs from the subs' interaction with the mains. In other words, this should improve the power response of the system.
In order to know what kinds of interactions are going on, you'll need to measure each speaker separately, then do a final combined system response. If your phase and time-alignment is correct, you should see a 6dB rise at the crossover intersection, which indicates perfect summation.
Rane has an interesting article that addresses some of the concerns for crossovers:
http://www.rane.com/note160.html
Of interest here is the notion that mis-aligning the phase (like what occurs with a butterworth filter)
results in the main lobe being redirected - not attenuated. This means that somewhere else in the room you have more bass, which is rarely ever going to be conducive when trying to optimize the sound over various listening positions.
Likewise, you should think of the subs' positions in the room as a line-array, where the physical offset between drivers changes the shape of the lobe (note that it is also frequency dependent). Here's a picture that describes the change in shape of the polar lobes, which is shown as a ratio of wavelength so that you can calculate the different lobe at each frequency for a fixed distance between sources:
