My SISO project is a ultrasonic theremin, capable of playing (somewhat electronically) notes in a single octave, indicated by different distance zones on the cardboard backing, through its onboard speaker. A simple cardboard paddle was used to more accurately hit the notes, but a pencil or hand works just as well. The arduino is programmed not to emit a tone when the range detected by the ultrasonic sensor exceeds the length of the cardboard backing, and to emit tones of increasing pitch as the distance from the sensor increases. However, I suspect my pitch values are somewhat flat, resulting in the performance not sounding quite as good as I had hoped.