Stefan Andrei Chelariu, the creator of Kanzen Field Coil Loudspeakers, discusses the technique for measuring and observing the directivity of loudspeakers. An interesting observation is that the wavelengths of sound waves and light waves are relatively similar, which means that diffraction lenses essentially function in the same way. This concept is the basis behind old JBL lenses like the 2395.
In this setup, the neon lamp is driven by the signal from the microphone. When the microphone picks up a strong signal, the lamp becomes brighter. By using a long-exposure camera, all the areas where the lamp appears bright can be recorded, based on the sound dispersion. It’s truly a brilliant approach.
The pictures are from W.E. Kock’s work on sound waves and light waves.