Abstract: A horn comprising a diaphragm, an electromagnet, a transducer to sense the vibrations of the diaphragm and generate a vibration-dependent electrical signal, and a feedback circuit which controls a power supply to the electromagnet. The feedback circuit includes an electronic power circuit (E, IEP) controlled by a control circuit (.mu., F, CCS) arranged to adapt, condition and process the electrical signal from the transducer (S) in such a manner as to automatically determine the frequency and duty cycle for controlling the electronic power circuit (IEP) under the various environmental, electrical feed and constructional tolerance conditions of the horn.