Sound is just vibration moving through air or some other physical medium, and the pitch of a musical note is just the frequency of that vibration. Given those facts, it’s pretty remarkable how many different kinds of instruments we’ve invented throughout human history to make pleasing vibrations in predictable ways. One of the most basic kinds of instrument is the lithophone, which is made up of rocks that are struck directly in order to produce individual notes, much like a xylophone. Jay Harrison built his own room-sized robotic lithophone and used an Arduino to do it.
Like all lithophones, this one uses rocks that are sized specifically to produce specific notes. In this case, those rocks were made from North Welsh green slate. There are a total of 24 of those rock tone bars covering two full octaves. The tone bars are struck by small mallets that are actuated by motors. Each tone bar, actuator, and mallet combination is setup as a subassembly, and those are arranged in a circular pattern with their cables running to the controller in the center where the user sits.
The robotic lithophone is controlled by an Arduino Mega board, which was chosen because it has more GPIO pins than most other development boards. It can be played in real-time with a keyboard, in which case the keys on the keyboard are passed to the Arduino which actuates the corresponding mallets. It can also play MIDI files. That’s handled by Maxuino which runs on the Arduino and can accept data from Max audio software, which in turn can accept MIDI files via a patch. This robotic lithophone was designed for Harrison's “Electromechanical Lithophone” art installation. While it’s not technically a purely electromechanical device, this robotic lithophone is quite hypnotic to listen to.