An apparatus which is an array of multiple series connected ultrasound transducers forming assemblies having positive and negative reactive impedance frequency regions that resonate and that are driven by an efficient electronic circuit at or sweeping around the resonance.

A system (1) for generating hydrogen gas comprises a reaction vessel (101) containing an aqueous solution (102) and a cathode (105) and an anode (107) each positioned at least partly in the reaction vessel (101). The system (1) comprises first and second ultrasonic transducers (215-220) which emit ultrasonic waves in the direction of the cathode (105) and the anode (107) respectively. Each ultrasonic transducer (215-220) is driven by a respective transducer driver (202) to optimise the operation of the system (1) for generating hydrogen gas by sonoelectrolysis.