Two boxes are the two systems. We do 2 things. 1. Send audio signal to device via modulated laser, amplitude modulation. Device outputs the signal received to audio device. 2. Laser light bounces back based on piezo driver, which is driven by microphone or audio input. The vibration is much less than the laser wavelength, and creates an interference pattern on a point source on photodiode. That signal goes through high pass filter to oscilloscope or headphone.