Abstract: A unidirectional speaker enclosure has a cylindrical cabinet supporting a speaker at its front end and having a closed bottom at its rear end. The front end constitutes a main aperture surrounded concentrically by a secondary aperture communicating with the rear face of the speaker membrane through two relatively large resonance chambers formed by two cylinders disposed concentrically within the cylindrical cabinet. The arrangement is such that a lag corresponding to a wavelength of the desired critical frequency is obtained between the backward wave and the forward wave produced by the speaker membrane.