Marcell Marosvolgvi made an audio analyzer using a Teensy 3.2 and Audio Shield.
This project uses a Teensy 3.2 to generate a sine wave and send the output to an audio shield. Using a external loop, the data is fed back into the input of the shield and read by the Teensy and analyzed. The data is then sent to a Raspberry Pi for graphing and displayed on a 7″ TFT display.
Code for the project is published on GitHub.