Originally Posted by beastaudio
I like how most studies just jump right past 3 total subwoofers. I actually don't recall anyone who has effectively tested just three locations. I might get around to this at some point just for fun....and due to the fact that the fourth corner in my room is a no-go since it is the entrance to the room from the staircase.
Originally Posted by rcohen
In theory, 3 isn't ideal because of the asymmetry. Even in a width symmetrical layout, the power would be lopsided front to back.
The study by Harman did add a third subwoofer placed half way between two subs in opposite corners. The third sub was set to +6 dB over the two corner subs to cancel out second order modes:
Low-Frequency Optimization Using Multiple Subwoofers*
TODD WELTI AND ALLAN DEVANTIER
Harman International Industries Inc.
By locating multiple loudspeakers in the listening room standing waves can be reduced by exploiting de- structive interference. The idea was first suggested by Toole . Some have called this “mode canceling.” The concept is depicted in Fig. 2(c), where we have located two loudspeakers such that the odd-order modes are “eliminated.” Each subwoofer excites a series of standing waves, but the acoustical response of the odd-order modes at one side of the room is 180° out of phase with respect to the other side, thus the modes are driven destructively and the subwoofers effectively cancel each other. Unfor- tunately the even-order modes from each subwoofer are in phase and no mode canceling occurs for these standing waves.
In Fig. 2(d) we have added a third subwoofer
at the center of the standing-wave pattern. This subwoofer is at the null of the odd-order modes and hence does not excite these modes. This leaves the two outside woofers to cancel one another just like those in Fig. 2(c). If the gain on the middle subwoofer
is set to +6 dB relative to the other two (since there is only one subwoofer in the center trying to cancel two at the room boundaries) the second-order mode is effectively canceled. As a result the first three modes have been canceled. This is just an example; there are numerous possible configurations for various modes. It is interesting to note that some of the most severe cancellations occur between modes, and some of the most signifi-cant improvement from mode canceling also occurs there. Though modal excitation peaks at the mode’s resonant frequency, there is some excitation at nearby frequencies as well. Where the resonant frequencies of two or more modes are close to each other, they can overlap signifi- cantly. If there is an out-of-phase condition, severe can- cellations can and do occur. These interactions are all subject to optimization.