The PWM signal in question is fed through a piezo speaker in parallel with a resistor. The output from the speaker is then sent to an FFT (fast fourier transform) app for Android devices ...

This simple STM setup utilises a piezoelectric actuator constructed by deliberately cutting a piezo speaker into four quadrants. With individual drive wires attached to the four quadrants.

An amp emulation pedal can transform your acoustic amp into an electric combo, giving you the best of both worlds ...