I tried a method of crosstalk elimination with my four Sony SSCS-5 speakers last night and it really did a great job of making the speakers "disappear." See crude diagram below. I like this arrangement because you can do it with just four bookshelf speakers and a simple 2 channel setup. No DSP or EQ is necessary for good results, although I think a subwoofer is best otherwise you might cancel out the bass too much. I was delighted at the effectiveness of this arrangement in terms of the apparent purity and cleanness of the sound, and the very good width and depth of the sound stage, the ease of keeping my head in the right location by resting it up against the divider wall, and the repositioning of the divider baffle behind my head instead of right smack in my face. Everything seemed to be coming from around and behind the speakers, so even center panned vocals coming straight from the position of the speakers seemed to be singing from behind the speakers, as if the speakers had nothing to do with it. This is even more surprising because the speakers were quite close to my face as I didn't have the means to make this setup very large in this quick and dirty experiment. I could easily reach out and touch the speakers. The sound stage with my eyes closed was wonderfully deep, wide, spacious. I'm reluctant to use the word enveloping but it at least seemed I was on the edge of a very wide open space right ahead of me. One interesting thing I discovered on accident while getting the arrangement dialed in was that even when I was set up wrong so that the speakers behind me were too close, I still perceived the sound as coming from the front. On initial tests using left and right channel test tones the channels sounded reversed! I was hearing the rear channels louder and sooner than the front channels but my brain still perceived the sound as coming from ahead of me. If I got too close to the rear channels it would then become apparent that the sound was coming from behind me, but it seems the precedent effect with forward to back sounds has a strong preference for forward as the perceived direction, at least in this setup. Once I moved the front speakers close enough to me, the left right channels developed nice, wide separation in the correct directions, and the music was really wonderful. I especially like the effect for big orchestral recordings.
So why do this instead of the Polk style arrangement or recursive DSP? To my ears, it's much cleaner sounding. Because of the divider wall it's a pretty clean elimination at each ear with very little residual side effects and colorations. With the Polk system I can get a very wide sound stage but then the crosstalk signal also reaches the ear it isn't supposed to. Fortunately the ear is hearing only the correct channel signal but it's out of phase and time delayed so now there's new comb filtering. With the DSP it loads up the system with signal and costs headroom. It can sound quite good but not as clean and pure, at least not the dsp methods I've tried so far.
I'm sure that with some EQ applied to the rear channels to compensate for HRT and possibly effects from the divider baffle the effect could be enhanced further. But it really sounds good without the DSP! Using front divider baffle arrangements with the baffle right in my face I have succeeded in getting enough separation that hard panned test sounds seemed to be coming from a full 90 degrees to the sides. This rear divider baffle setup only got me to about 70 degrees or so, but that's enough to really sound good.
It is interesting to me that the speakers seem to "disappear" once the direct crosstalk is eliminated. All the cabinet diffraction issues, the grills, early reflections from the walls behind the speakers, the ceiling and floor are still there but don't seem to matter much. The speakers reveal their location when the crosstalk is not eliminated. They disappear when it is eliminated, or at least attenuated to a substantial degree. So it could be that the crosstalk gives our ears a clue to the true source of the sound even though we are also noticing stereo effects. Or it's possible that the rear channels are adding enough confusion in some other way to force my brain in to perceiving the sound as coming from other than the direction of the speaker.
One other thing I'll say about this setup is that it never gave me any perception that the sound sources were actually happening in the room with me, but instead it gave me a sense of hearing into another space. So this was definitely more of a "I am there" experience, or perhaps more like my listening space has developed an opening into that recorded space. I can still hear my space, but I can hear that space really well too.
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