a circuit is provided for supplying a transmission oscillator voltage suitable for a piezoelectric sound source transducer in which the transducer also serves as a receiving transducer . the circuit contains an electrical series resonant circuit having a capacitor c and an inductor l with l · c =. sup . 2 , wherein the electrical series resonant circuit is matched to the series resonant frequency f 0 of the transducer and is connected parallel to the transducer . the circuit further contains a pair of diodes , connected anti - parallel to one another , and connected in parallel to either the inductor l or to the capacitor c , with the output of the receiver being connected to the capacitor c or to the inductor l .