A well defined and spatially rooted center image is the hallmark of a properly set up stereo system. The listening room and seat location precludes good stereo imaging. Specifically, left/right symmetry.
Lop-sided Iayouts create asymmetric direct and reflected sound energy or uneven sound fields. Equally important, the distance differences affects signal arrival and consequently timing related psychoacoustics. These aberrations when combined will result in poor imaging across the front soundstage in general and a smeared center image.
From personal experience, lot of this can be addressed using a pre-pro with distance and volume trim settings, albeit, not fixed entirely. (Soap box moment alert: People with less than perfect listening environments have much to gain by using the lowly AVR for 2.x listening when compared with expensive stereo pre-amps sans distance and volume correction. /r)
I am curious to try time-intensity trading with constant directivity speakers. Would this arrangement adequately compensate for less than perfect left/right room symmetry and seat not exactly on center axis. Too many questions, not enough room.
Which is why I have my system setup entirely symmetric, each speaker pair is exactly the same distance from the mlp, toed in exactly to the mid point, and the center is directly between the l/r. All speakers are time aligned down to 1/10th of a foot, if I play a mono track routed to all 7 speakers in a 5.1.2 setup, the image appears to float directly in the middle of the room.
I believe time intensity trading requires a more narrow dispersion pattern than one would find in most home audio speakers designed for controlled directivity. In order for time intensity trading to work, the horn or waveguide must have tight pattern control from 1600hz on up, since 1600hz is the frequency where localization shifts from interaural time differences to interaural level differences, due to the wavelengths being smaller than the head.
The RP-150m I’m currently using for my l/r has pretty tightly controlled dispersion starting at 1800hz and continuing to 14khz, (the woofer is crossed over to the horn at 1500hz), with a uniform dispersion pattern of 90 degrees (+-45 degrees), each additional 15 degrees off axis gives a 2dB drop in level from 1.8khz-14khz, with 45 degrees off axis being -6dB.
I may have to try some steeper toe in angles to see if I can get the time intensity trading to work, I believe you’re supposed to toe them in so that they’re aimed at a point about 2’ in front of the mlp, so that the “lines” intersect with the opposite ear.
Earl Geddes has written much about controlled dispersion speakers and time intensity trading, but recommends dispersion patterns that I personally feel are much too narrow for home theater use, something like 20-30 degrees horizontally. Since we must inevitably place speakers into rooms and have to deal with the reflections from the walls, floor, and ceiling, it’s important that the sound that bounces off those surfaces at early reflection spots is very similar to the direct sound. A wider dispersion pattern such as 90 degrees ensures that in a majority of rooms and placement situations (outside of very wide rooms where the speakers are placed very far from the side walls), the reflected sound has the same frequency response as the direct sound, but is slightly attenuated.
A narrow dispersion pattern might work for a single listener seated relatively close to the speakers, but for home theater, people often sit as far as 10-12 ft away, and multiple seats are off axis, the sofa along the side wall in my living room is only about 35 degrees off axis, which means even the farthest seat falls in the coverage pattern, and every seat in the house receives uniform frequency response. Toeing them in at a steep angle, say, 40 degrees may offer good time intensity trading for seats nearer the middle of the room, but places the side wall seats outside of the speakers coverage pattern.
