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redneck mothers

Posted by Duke on February 2, 2006 at 13:18:04:
I designed a speaker very much like the one we're talking about here, but haven't gotten around to bulding it yet. It's a floor-stander intended to be placed right up against the wall, and I was going to call it the "Redneck Mother" after the Jerry Jeff Walker song ("well, it's up against the wall, redneck mother..."). The DCM Timewindows of yore also used a splayed array, but they weren't intended for against-the-wall placement.
A horizontally splayed array would (as an example) have one Fostex aimed 30 degrees to the left of center, and the other aimed 30 degrees to the right of center. I'm not sure what would be the best angle to use - 30 degrees is just a guess derived from eyeballing the published curves. If you look at the middle (dotted line) curve at the link below, that's the 30 degee curve. From straight ahead, you'd be 30 degrees off-axis to both the left and right driver, so the outputs would sum and pull that 30 degree line up a bit. From on-axis of one driver, the other driver is 60 degrees off-axis (the bottommost published curve) so it reinforces in the lower octaves much more than in the upper ones. The net effect is to tilt the frequency response slightly downward as compared to the normal on-axis curve. This is an improvement, as the driver is a bit on the bright side otherwise.
The drawback is that, to either side of the centerline, the output from the farther driver is delayed in time relative to that of the near driver. This will introduce comb filter effects, or narrow-band peaks and dips in the response. The ear is pretty forgiving of comb filter effects - every sound you've ever heard in a room with a non-carpeted floor was comb-filtered by the floor bounce. If the goal was ultra high fidelity I probably wouldn't recommend a splayed array, but since the goal is good sound from a mono source across a large area it's what I'd do.
So now you have improved the tonal balance and are getting a fairly uniform response over a much wider area than you would have with a single driver. If it works, should be nice.
You can drive each Fostex with a separate amplifier channel, but make sure each is getting a MONO signal. You don't want the right channel to be way down in volume and lacking in highs from off to the left of the speaker, and vice versa.
Note that this speaker is definitely going to need boundary reinforcement to get much output south of 100 Hz.
I must admit that I still have slight reservations about the lack of energy above 10 kHz. Adding a top-octave enhancing tweeter to each splayed baffle (could be a much cheaper unit than the FT17H) might be worthwile, so I'd say leave room for the tweeters on the front baffle in case you want to add them later. Just be careful that the supertweeters don't brighten up the tonal balance too much - you want to cross over first order around 15 kHz (it will still be helping out some to about an octave below crossover). The idea is to just enhance a little without overdoing it, kind of like a few sprinkles on top of a cupcake.
Duke