A method and system for controlling an array of piezoelectric transducers (11, 12, 13). Respective driving signals (Vn) are applied to the transducers. The driving signals (Vn) comprise an alternating component (A) oscillating at one or more driving frequencies to cause corresponding vibrations in the transducers for generating acoustic waves (Wn). One or more of the driving signals (Vn) are offset by a respective bias voltage (Bn). The bias voltage (Bn) is controlled to reduce a difference in resonance frequencies between the transducers. To eliminate any remaining difference, the alternating component (A) to at least a subset of the transducers (11,12) is periodically reset. In this way the phases of the resulting acoustic waves (W1,W2) can be synchronized.