Every system has a particular frequency that it tends to vibrate at. For instance, if a string on a guitar is plucked that string will vibrate very near or at its resonant frequency. By driving a system at its resonant frequency, very large displacements relative to the input signal strength can be achieved. Driving a buzzer with an input signal that has the same frequency as the buzzer’s resonant frequency will create the greatest SPL with the least input power.