Maximum efficiency in a horn unit can be achieved only if a conjugate match exists between driver and horn. This match is possible only if the unloaded resonance of the driver is greater than horn cutoff frequency. Since the throat resistance of a finite horn at cutoff is a small fraction of its asymptotic value, the point of reactance annulling is chosen at this frequency. The use of hyperbolic-exponential horns makes possible practically any desired resistance or reactance characteristic near cutoff. The hyperbolic-exponential horn allows much more uniform transmission down nearer to cutoff than is possible with the exponential type.
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