When people speak inside a building, the acoustic wave generated causes the windows and other objects in the room to oscillate with the same waveform as the spoken sounds. A laser beam can be bounced off of these objects in order to encode the beam with the sound vibrations. Those vibrations can then be recovered from the reflected beam as shown in Figure below.

As the laser propagates from the aperture to the target object, the beam expands due to the divergence of the beam. The beam then bounces off the reflective object and travels back to the receiver telescope all the while it continues to spread out due to divergence. As the window vibrates back and forth the distance between it and the detector telescope is increased or decreased slightly, which in turn, increases or decreases the spot diameter on the receiver. As the spot diameter changes so does the irradiance within the spot. The detector measures this as an increase or decrease in signal amplitude, which reproduces the audio signal.