Introduction

Coincident source loudspeakers have the benefit that they avoid the response and directivity irregularities seen with conventional spaced drivers in the crossover region. For coincident-source loudspeakers, where the high-frequency driver is positioned at the apex of the midrange diaphragm, the directivity of both drivers is determined by the shape of the midrange diaphragm. This results in both drivers having the same directivity at crossover. [1]

However, design of such loudspeaker units is a challenge. Compared to conventional drivers, space is constrained and the acoustic design must allow both drivers to function correctly over a wide bandwidth. Additionally, today’s new digital formats require further extension of high frequency performance. Reproducing these very high frequencies necessitates an absence of undesirable modes in the waveguide throat since these Dodd Optimum Diaphragm and Waveguide Geometry cause an ‘acoustic short circuit’ resulting in response dips and directivity irregularities.

In this paper we begin by considering some well known properties of spherical waves and ideal spherical wave sources. Boundary conditions allowing spherical wave propagation will be discussed. The simulated behavior of some less-simple sources on a planar baffle is presented and contrasted similar sources loaded by an infinite conical waveguide. Finally the results from a finite waveguide with optimal shaped dome are presented and discussed.

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