Everybody is focused on the tubes... What about the diffuser with pyramid shaped lenses? the highly directional light projects a beam (flourescent lights are known for awfully harsh shadows) through the thin, lightweight 24x24" or 24x48" lens(diffuser). as the light is projected down a minimum of 8 feet the tiny deflection from sound hitting the diffuser is amplified. Then, at an interface of light and shadow on the floor, the moving projection becomes a brightness signal that has audio encoded in it. they could use a lens and a photodiode. Didn't exactly have 5000fps cameras falling off trees 40 years ago.