Spectral Eye is a free, universal app that analyzes incoming audio signals to extract the underlying frequencies. Most sounds are built up from a variety of sine waves; a simple whistle might be a single wave, while a guitar, piano, or the human voice have many waves added together. The free version of the app can also generate tone, and can be placed in the input slot of Audiobus. There's also Spectral Eye Pro, which supports MIDI, and can be placed in the Audiobus output slot -- allowing you to see exactly what frequencies are present in a synthesizer tone.
By using a Fast Fourier Transform, sound can be decomposed into each frequency. Normally, this information is displayed on a chart, and there are number of excellent apps that do exactly this. Spectral Eye rearranges the FFT into a spiral, so that frequencies that are an octave apart are lined up as rays from the center of the display. The strength of each frequency is represented by the size of a red circle.
Play music, and Spectral Eye will reveal the frequencies. For a guitar, it's easy to see that in addition to the root frequency of a note, there are also a number of harmonic frequencies -- and these change based on the strings used, the pickups, and the amplification system. It's the frequencies present in the notes that make a Telecaster sound different from a Les Paul, and a piano different from a trumpet. Play an instrument, music from your iTunes library, or have it listen to whatever sounds are nearby -- the display is fun to watch.
If you're interested in understanding the nature of sound, use Spectral Eye to get a better view. The app was created as a utility used in the development in other Secret Base Design apps; we're making it freely available, because we found it to be useful, and thought others might like it too.
The app will remain free, and will not have ads. If you find it useful, we would very much appreciate a review in the app store, and you can get in touch with us through our support site, our Facebook page, or Twitter.
Have fun with the app, and we hope you enjoy opening your Spectral Eye!