Geometric Music is a free app (download here, or fiddle online) that takes sounds recorded from your environment and assigns them on-screen geometric shapes. Manipulating the shapes and arranging them relative to one another allows the user to build looping, layered musical pieces.
The app works like ultra-simple sample-production software, allowing you to create layered beats. But while complex programs will drown you in buttons and knobs, Geometric Music only uses shapes to dictate how something sounds. Every noise you make is translated into one of four shapes (circle, triangle, square or hexagon), and you manipulate those shapes to change the rhythm, speed and volume. Think of it as tapping into a digitally-enabled form of shaped-based synesthesia.
I downloaded the app for my iPhone and spent about a half hour tinkering before my looming work schedule forced me to put it down. The image below should give you a basic feel for the interface. I could go into more detail describing the various ways that shapes can be manipulated to modulate sound, but, like most good hands on experiences, the best way for you to get a feel for the app is to try it for yourself.
I will say this: I was skeptical going in that shapes could provide an intuitive way to communicate musical characteristics. Don't get me wrong, I think most people – whether they identify as syntesthetes or not – can and do talk about music in terms of shape, or temperature, or color; but the thought of bridging these subjective interpretations and a hands-on experience with a smartphone app struck me as a bit fanciful. Having toyed around with it myself, I'm a little less wary, but still far from convinced. The app seems less about simulating synesthesia, and more about depicting music and more conceptual terms.