I have been working on a fun project to make an exhibit for my local makerspace (The Gizmo Dojo in Broomfield, CO) to take to events. I had some spare ultrasonic sensors lying around so I decided to experiment with turning them into a high quality musical instrument using a MIDI software synth.
Here's a demo of the Ultrasonic Pi Piano configured to play a different instrument for each sensor. The distance from the sensor determines the note that gets played.
I wanted the ability to change instruments without adding additional components so I implemented some basic gesture controls where the instrument can be changed by holding your hands over two of the sensors for a few seconds. Here's a video with a full demonstration.
If you want to re-create this project yourself, I have written up a detailed instructable here: