Video of recording:
1 – Recording of tree bark – the very textured and rough surface produces a small audio signal consisting mostly of static. 2 – Walking across grass – another rough and textured surface which creates a bit more variation in sound due to shadows created by trees above. 3 – Recording of window – produces a more complex sound due to the transparent and reflective surface. 4 – Recording of car headlights and hood – produces an even more complex sound due to the transparent, reflective, and contoured headlights and hood.
This recording method requires that the two photocells be in motion in close proximity to a surface. If these conditions are not met the sound produced is minimal to nonexistent in natural light and constant in artificial light (depending on the frequency of the light source). Changing the material through which the artificial light travels causes minor changes to the sounds volume, while the frequency remains constant.
The limitations above (proximity to surface and required motion of the device) point to a device that is either hand held or put into motion be some other means. This may not be the most suitable way to map a site.
(more to follow)