Spiral membrane speaker enclosure

Abstract

Acoustical transmission lines formed into a spiral configuration possess the latent property of having different lengths along adjacent lines when traversing along a radius: for an acoustical spiral, sound waves in adjacent transmission lines will progress from an antiphase relation at the center of the spiral to an in-phase relation at some radius r (λ) from the center. This is a transformation dependent only upon wavelength. When those transmission - line walls that are facing the environment are covered with a low-mass, tightly stretched membrane, radiation from this secondary surface will occur at the appropriate time. Radiation of a transverse wave rotating about the spiral axis occurs when sound waves in adjacent transmission lines are in phase. In this paper, several radiation modes and phasing properties are discussed. Owing to automatic adjustment of aperture size with change in frequency, the spiral membrane enclosure has a broad and constant beamwidth. An acoustical source, such as a loudspeaker cone each side of which is contiguous to the end of an acoustical transmission line, generates within those transmission lines longitudinal waves that are antiphase relative to each other. Constant loading of the source occurs over a wide range of frequencies, providing uniform electrical/acoustical performance. An experrimental model is described. Subject Classification: 85.60; 20.40.