It is conceivable that a relatively small cone in a low Fc horn with a proper mouth size would provide enough pathway length to achieve a considerable amount of acoustic resistance applied to the cone.

As the cone diameter and the corresponding throat size increase, the horn pathway would shorten for a given Fc and mouthsize, and the acoustic resistance would be reduced as a matter of course. The resulting response due to increased impedance/reaactance caused by a too-short-horn would be more "peaky" in such a case. The typical design response is to raise the Fc of the horn in question.

The matter seems to be related to overall pathway length, mouth size, and Fc, and the diaphragm/throat size is subjective in relation to that consideration.

DM

If employed in a less-than 1:1 diaphragm-to-throat ratio, the front chamber would act as a low-pass filter by attenuating higher frequencies. In essence, the front chamber is an acoustic filter by its physical dimensions and could give a -6db rolloff on the upper frequency bandpass.

A horn that lacks a front-chamber (1:1) PER SE would be limited in its upper band pass as a result of foldings (if any), and the mass rolloff, electrical impedance, etc., of the driver, amongst other considerations.

DM

On the other hand, though, the 1978 EV midrange horn (Keele) US patent # 4,071,112 has 3 separate flare rates for purposes of bandwidth and coverage pattern. So there is no real limitation as to what you can do if you can achieve a balance of all the variables involved to arrive at your particular goals.

DM

The use of changing wall angle to increase uniformity of a constant directivity horn is a different matter, because it addresses the loss of pattern control. What CuppaJoe is talking about is widening a very small area near the throat. This will not widen or narrow the pattern at any frequency. It might cause a reflection at high frequency, hopefully, out of band. But the pattern would not be set by this feature because the rest of the wall angle constricts radiation to a narrower pattern.