US Patent 7155021 Loudspeaker having an acoustic panel and an electrical driver

A panel-shaped loudspeaker comprising a panel (1) having a first main surface (1A) and a second main surface (1B) parallel to the first main surface and further comprising an exciter (3) positioned at the first main surface. When the exciter is energized the panel is capable of sustaining bending waves and has a distribution of resonant modes of its natural bending wave vibration, which results in an acoustic radiation. The loudspeaker has a tuning element (17) extending near the second main surface of the panel and positioned opposite the exciter so as to form a resonant cavity (18) between the panel and the exciter.