"listenjous"
The plotter is driven by a single belt looped around two motors, which must work in coordination to move the pen in two dimensions. For example, to make a horizontal straight line, they move at the same speed in the same direction, while a vertical line requires them to move in opposite directions. Curves require one motor to speed up while the other slows down, changing directions. In this experiment, the sound of each motor is picked up by an induction loop “telephone tap” and analyzed for its frequency content. The estimated frequency is fed into a sine or triangle wave oscillator. The two oscillators are plotted against each other as a lissajous curve (a graph useful for visualizing complex harmonic motion). As the sound of each motor is fed back to control its speed, the system approaches equilibrium, creating increasingly simple geometric shapes from the harmonically-related speed of the motors. Eventually the imperfect process of digital sampling and frequency analysis causes the pen to wander off course, starting the process over again elsewhere on the paper.
