The buzzer module uses a 4kHz fixed frequency, piezoelectric DC buzzer which operates in one direction. An internal oscillator that produces a 4kHz alternating current creates the pitch. As current flows in either direction across the piezoelectric material, it changes shape and creates the sound. An increased voltage will increase the amount of deformation of the piezoelectric material, increasing the amplitude of the sound wave being produced, and the volume of the sound we hear.
Contents and Images credit: Circuit Scribe Website