In a piezoelectric acoustic alarm piezoelements (2a, 2b) each excited by a square-wave voltage are arranged opposite each other in a resonance cavity (1) and each oscillate in opposing directions. The two square-wave voltages (Va, Vb) controlled by a microprocessor (8) are phase displaced by a square-wave pulse length and are positive or negative. A piezo-sound source designed and excited in this way is characterised by a high sound volume with low current consumption and small size.