the 817Hz notch is caused by the distance


between the drivers centers being about 16.7 inches. If you move them farther apart, you will still have a notch, BUT AT A LOWER FREQ., where it will be even more audible(?).
Some other jbl dual woofer format speakers roll off the lower woofer about an octave sooner than the top one for a smoother response in the xover region, FWIW.
I'm not sure what would be gained by using dual woofers above and below the horn as far as efficiency is concerned, compared to the 4648a-8 configuration, which I find to work rather well.
I can see building new bass enclosures SOMEDAY, that are a bit shorter, match the 511 Altec's in width, and maybe slightly less depth, to arrive at a net of 5 cu.ft. Vb. But the work would be for cosmetic benefit only in my eyes, aside from getting the woofs into a Pi align tuning configuration.


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