The invention relates to underwater acoustic transducers for emitting a low-frequency broadband acoustic signal. The inventive transducer consists of a cylinder-shaped cap (105) which is closed at one end and open at the other. A piezoelectric plate (101) which operates by bending is fixed on the base of this cylinder, outside of the same. The inner cavity (106) defined by the cylinder is freely open towards the outer medium in which the acoustic waves are emitted and makes it possible to reduce the central frequency and increase the bandwidth while maintaining identical dimensions.