To control the parameters of the sound I use intensity of light. So I need to have several small sources of light. I use bright LED`s. As sensor I use light sensitiv resistors. The source and the sensor are placed, facing each other in the vertical axis. Anything that is passing between source and sensor is changing the amount of light receivd by the sensor.
As medium, I use transparent disks. Obsolet CD-R`s. I put them into water for some days and then scratch off the metal foil that is containing the data. A stepper motor turns these disk, similar to a record player. The speed of the steppe motor can be controlled manualy. The disk rotates rectangular to sources and sensors. If you paint on that disk, the color is shading the sensors and so changing the parameters of the sound. Each of the two voices has four parameters.
volume
pitch
cutoff frecuency of a lowpassfilter
speed of an amplitude modulation