Disclosed is a loudspeaker assembly incorporating innovations resulting in an enclosure of very small volume to which is integrated a large area, shallow and lightweight acoustic diaphragm assembly capable of a natural resonant frequency of a few Hertz. This is achieved by incorporating a vacuum chamber in conjunction with a chamber containing compressed gas or vapor that acts against a movable pressure boundary of changeable area being mechanically coupled with the acoustic radiating diaphragm. In an alternative operating mode, the apparatus also serves as a passive low frequency acoustic attenuator.