A gas flow generator comprising: an ultrasonic driver comprising a piezoelectric or electrostrictive transducer mounted on a substrate, operation of the transducer being arranged to cause the driver to bend; a first membrane disposed on or formed integrally with the transducer or the substrate; and a second membrane mounted substantially parallel with the driver and spaced a given distance therefrom, one of the membranes being perforate, whereby ultrasonic bending of the driver on actuation of the transducer causes a gas flow through the perforate membrane.