Structure-borne sound converter for bass frequencies. The units are attached to a surface which is able to resonate, which causes it to oscillate (e.g. a panel of a car body, the base construction of a settee etc.). No enclosed cabinet is required. Using the BS 130 - 4 Ohm, this enables the lowest bass frequencies, such as those used for effects in films, to be reproduced in such a way that they are felt through the body rather than heard.