There are two basic methods for improving uniformity. These may be used in combination.
Reduce the frequency of low-amplitude elements. This applies mainly to slotted bar horns and somewhat to slotted block horns. The frequency of low-amplitude elements can be reduced by adding risers, adjusting the nodal radii, adding nodal holes, or contouring the slots. Secondarily, for slotted horns the uniformity will be affected by the slot parameters: lateral locations, widths, and web lengths.
Remove material behind low-amplitude regions. The reduced support causes these regions to "flap", thereby increasing their amplitude relative to high-amplitude regions. The effect is somewhat localized near the removed material. This method can be primary (as with spool horns) or secondary (as with flanges added to the ends of bar horns).
Additional methods have been proposed but do not seem to have been widely used because of limitations.
Increase the gain of low-amplitude regions. This method has been applied to bar horns (see Holze (2) patent) but may also have limited application to block horns.
Add additional half-wave resonators on the primary resonator's stud surface. See Scotto patent.
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