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Re: when horn expansion and mouth size exceed the 1/4 lambda

The requirement of a horn pathway length being 1/4 wavelength is rather a waste of time. More of an "old" rule of thumb.

The actual horn pathway length is determined by the throat size, the flare rate, and the mouth size, where the mouth size is a relative constant. A general accepted "rule" on mouth size is that mouth circumference = 1/3 wavelength. This is the "relative constant" part, if you want a reasonable match to the environment, and that is then dependent on the radiation angle that the horn exhausts into.

Since the requirements of the mouth size are relatively "fixed", that leaves the flare rate (Fc) of the horn, and the throat size as variables. Altering either or both of those will automatically have an effect on the horn pathway length. Throat size is a matter of efficiency, desired SPL, and band width. Limiting that to an excessively small size (as demanded by the 1/4 wavelength "rule") is limiting the potential of the design.

See why you can't just apply the 1/4 wavelength "rule" out of hand - it has no real meaning... we all know that MOST horns are quite a bit shorter than 1/4 wavelength at Fc and still perform quite well. In general, a very short horn path would, in theory, require fairly accurate annulling to make the driver in question fairly insensitive to variances in reactance.

DM

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