You can build a two element end-fire sub array using a first-order all-pass instead of pure delay. The on-axis response looks pretty good on paper with about 18 dB rear rejection over a useful passband. It almost seems like a compromise between an end-fire and a cardioid array.
The example graphed below has 4ms of spacing between the speakers and the center frequency of the all-pass, at which it has 90 deg of phase shift, is 62.5 Hz, corresponding to 1/4 wave spacing. Neglecting attenuation due to distance, the front summation at the center frequency is total (6 dB) since the 90 deg from the filter cancels the 90 deg from the spacing. (I found that the first zero in the front response can be pushed up by adding a small amount of electrical delay without messing up the rest of it too badly.)
So does anyone use this? And how does it work in practice? When I get a chance I'll take my TH-Minis out to an open area and try it.
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F is front (audience side)
B is back (stage side)
A is all-pass filter response
Reinventing the wheel instead of cleaning the house.
--Frank
