Chapter Nine
Horn loudspeakers
Abstract
The chapter begins by treating the drive units for horn loudspeakers, including circuits, frequency response, and examples. The properties of horns come next, including acoustic impedances at the throat. Then, other types of horns are treated: parabolic, hyperbolic, and conical, both infinite and finite in length and Klipsch type. The details include effects of bends, cross-sectional shapes, materials, and examples.
Keywords
Acoustic impedance; Direct-radiator loudspeaker; Horn-loaded drive unit; Mechanical admittance; Nonlinear distortion; Throat admittance
Part XXVIII: Horn drive units
9.1. Introduction
Horn loudspeakers usually consist of an electrodynamic drive unit coupled to a horn. When well designed, the large end of the horn, called the “mouth,” has an area sufficiently large to radiate sound efficiently at the lowest frequency desired. The small end of the horn, called the “throat,” has an area selected to match the acoustic impedance of the drive unit and to produce as little nonlinear distortion of the acoustic signal as possible.
Horn loudspeakers are in widespread use in cinemas, theaters, concert halls, stadiums, and arenas where large acoustic powers must be radiated and where control of the direction of sound radiation is desired. The efficiency of radiation of sound from one side of a well-designed direct-radiator loudspeaker was shown in Chapters 6 and 7 to be typically less than 1%. By comparison, the efficiency of radiation from a horn loudspeaker usually lies between 10% and 50%.
The principal disadvantages of horn loudspeakers compared with the direct-radiator loudspeakers are higher cost and larger size.
Before proceeding with an analysis of the horn loudspeaker, it should be mentioned again that the radiating efficiency of a direct-radiator loudspeaker can be increased at low frequencies by mounting several units side by side in a single baffle. The mutual interaction among the radiating units serves to increase the radiation resistance of each unit substantially. For example, two identical direct-radiator loudspeakers very close to each other in an infinitely large plane baffle, and vibrating in phase, will produce four times the intensity on the principal axis as will one of them alone.
Direct-radiator loudspeakers used in an array often are not as satisfactory at high frequencies as one horn loudspeaker because of the difficulty of obtaining uniform phase conditions from different direct-radiator diaphragms. That is to say, the conditions of vibration of a loudspeaker cone are complex, so that normal variations in the uniformity of cones result in substantial differences in the phases of the radiated signals of different cones at high frequencies. A very irregular and unpredictable response curve and directivity pattern result.
This problem does not arise with a horn where only a single drive unit is employed. When two or more drive units are used to drive a single horn, the frequency range in which the response curve is not adversely affected by the multiplicity of drive units is that where the diaphragms vibrate in one phase.
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