How to calculate PWM duty cycle to compensate for different piezo sounder response at different frequencies?
simulate this circuit &ndash; Schematic created using CircuitLab

This is a variable clock or an Astable Multivibrator or a Relaxation Oscillator.  Choose any buffer to suit the voltage or power level required.  CMOS has an impedance of ~300 Ohms for 74HC and ~50 Ohms for  74ALCxx ~5V