I am building a pair of Peter Morris's PM90 PA speakers, but don't fret, this is a general horn question! The sandwich construction walls which form the two horn mouths of the 2 x 12" drivers are 28mm thick, and the range covered by these is 100-700Hz. Given that I have a little more thickness to play with than most horns, is there any beneficial profile of the horn edges, or does it remain irrelevant given the shortest wavelength of the sound trying to get out is nearly half a metre?
Right, you'll need at least a 100 Hz radius and ideally whatever its cutoff is, even a so-so one would be the mean = [100*700]^0.5 = ~264.6 Hz = ~8.15"/20.7 cm radius.
Actually, considering this is part of our critical midrange hearing BW [500-2500 Hz where Altec spec'd their horns], so recommend doing at least a 500 Hz round over to at least 180 deg and preferably back to 270+ deg.
To someone with only a small room available and corners not an option or other limiting scenario, so best they can do IME is my usual recommendation to make bulky foam surrounds to damp them, but still require a significant effort if WAF, etc., is required.
Such treatment can completely transform this two piece cast pot metal 'ring - a - ding' Altec 500 Hz XO horn: Tom Danley measured
Such treatment can completely transform this two piece cast pot metal 'ring - a - ding' Altec 500 Hz XO horn: Tom Danley measured
'Sound is round' as one DIYer [accurately] opined, so 500 Hz radius = ~13543.31"_sec SoS/2/pi/500 = ~4.31" radius.