Spark or Shadow photographs, developed by Harvard professor, Paul E. Sabine, show the progress of a single sound wave as it travels through a model space. Sabine used them to make pioneering studies of architectural acoustics. In this photograph, the sharp snap of an electric spark, located on the other side of the black circle, has created a single sound wave, much like the circular wave produced by dropping a stone in a pond. A small fraction of a second later, a distant second spark illuminates the entire scene and creates an image of the position of the wave. In the time between the two sparks, the wave has reflected off the back and sides of the box. The tangled mass around the black circle is acoustic noise from the spark apparatus. While these Spark photographs are generally presented as images of sound waves, the image is actually caused by a change of density in the air as the sound wave travels through it. Much like a lens, this dense air refracts the light and produces a band of light and shadow that reveals the presence and position of the wave. Because of the wave’s optical effect, this type of photography did not require a camera. The model was simply placed on a photographic plate in a dark room and the illuminating spark created an image by projection.