A panel form loudspeaker comprises a resonant multi-mode radiator (11) which in turn comprises a plurality of substantially concentric sub-panels (20, 21, 22, 23). A plurality of analogue drivers (10) drive the radiator (11), one or more of the drivers being operational at any time. A signal level measured at the input to the loudspeaker determines the operational state of each of the drivers (10). The concentric sub-panels may take various shapes and have different areas.