Horn loaded pleated ribbon high frequency acoustic transducer with substantially uniform coupling

Abstract

An electro-dynamic acoustic transducer has a pleated diaphragm in which an accordion-like alternating movement of the adjacent pleats is perpendicular to the radiation (acoustic) axis of the transducer. Front and rear pole plates cooperate to provide a magnetic field across the active area of the diaphragm. Each pole plate may be a “magnetic throat” constructed from a unitary casting of a ferrous magnetic material (such as an iron silicon alloy) having a number of apertures. In an exemplary embodiment, the open area at the rear of the pole plate may be approximately ⅓ the active area of the diaphragm, thereby providing a compression ratio of 3:1. The apertures are substantially uniformly distributed over the effective area of the diaphragm, to provide uniform acoustical coupling and prevent the occurrence of acoustical resonances and standing waves along the diaphragm's length. Each aperture in the front plate opens up such that the corresponding area at the inlet of the loading horn is substantially open, thereby providing a smooth transition through the magnetic pole plate to the external horn, with the apertures preferably designed with an exponential flare. These apertures may be tapered as part of the magnetic pole plate or the tapered portions may be formed separately from magnetic or non-magnetic materials, such as plastic or aluminum, and joined to a pole plate having corresponding apertures. In a preferred embodiment, each aperture is in the form of a horizontal slit extending across the active width of the diaphragm, and the apertures are arranged in a vertical array to cover this entire active area. Each slit increases in width as it tapers towards the front pole plate, providing the smoothest possible acoustical transition to the external horn.