Abstract: In a pulse rate modulation type driver device for driving a piezoelectric crystal so as to produce sound, a digitized sound signal input having a sign bit and a plurality of magnitude bits is firstly provided. A driving signal is then generated by varying the pulse density per unit time of a clock pulse according to the value of the magnitude bits. The driving signal is more dense when the value of the magnitude bits is higher and is less dense when the value of the magnitude bits is lower. The driving signal is then applied to the piezoelectric crystal so as to permit the piezoelectric crystal to undergo mechanical strain and thereby produce sound. The driving signal is applied to either a positive terminal or a negative terminal of the piezoelectric crystal depending upon the logic state of the sign bit of the digitized sound signal input.