A horn loudspeaker, in particular for reproducing bass sound in public address systems, in which the horn (23) is mounted directly to the driver element (21) without any intervening compression chamber. The back side of the driver element (21) is covered by a back chamber (24) designed with walls of a semi-permeable material. The walls may be perforated or made of a “leaky” material such as cell foam with open structure, or a fibrous material. The “leaky” back chamber will prevent the build up of high pressures in the back chamber at large cone excursions. The horn loudspeaker is suitable for stacking in groups of two or more, so as to produce a sound source scalable for reproducing any frequency range heard by humans in public address systems or hi-fi systems. Due to the back chamber design, the horn loudspeaker, for high frequencies especially in conical horn shaped versions, can be stacked close together. Thus, a sound source including a number of such closely stacked loudspeakers can provide a homogeneous sound field covering a wide area even at high audio frequencies.