An electrostatic loudspeaker comprises a multi-layer panel (1) incorporating an electrically insulating middle layer (2) sandwiched between first and second electrically conducting outer layers (3, 4). A signal generator is provided for applying an alternating electrical voltage across the outer layers (3, 4) to initiate vibration due to variation of the electrostatic forces acting between the layers, thereby serving as a loudspeaker. Furthermore at least one of the outer layers (3, 4) is permeable to air displaced by such vibration. Such a loudspeaker can serve as a low cost audio loudspeaker which can be made lightweight and flexible or large-area so as to render it suitable for a wide range of applications, for example to provide sound reproduction in a home environment without requiring any bulky enclosure, public-address systems, or in a notebook computer or mobile telephone.