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Join Date: Oct 2003
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tony, that quote is roughly right.
the basic rule of thumb is when a wavelength is about the width of the driver (the actual cone), the driver will be beaming with the sound -6db at 45 degrees off axis.
standard driver sizes are 15, 12, 10, and 8 inches nominal, with cone diameters of roughly 13, 10, 8, and 6 inches.
those actual widths correspond very roughly to beaming frequencies of 1,000hz, 1333hz, 1666hz, and 2000hz. so that is about as high as you'd want to run them when crossing over to a 90 degree horn.
for good directivity control, you want a smooth transition from the horn to the woofer through the crossover region.
so take a horn, which is called 90 degree horizontal, that means that if you go off axis 45 degree to the left or right the sound will be down -6db.
the basic rule of thumb for a 90 degree horn is roughly the same as that of a driver. when the horn is about 1 wavelength wide, it will be starting to lose it pattern control.
what this means is that you roughly want a horn that is about the same size or one size larger than your woofer, at least in some sense.
so working backwards from the cone frequencies of 1,000hz, 1333hz, 1666hz, and 2000hz, and you'd want horns that are at least 12, 9, 7, and 5 inches in diameter.
there are lots of other factors and there is also some wiggle room, but that is roughly the idea.