Secret Base Design's spectrum analysis apps, Spectral Eye, have been updated with Audiobus. The free version of Spectral Eye is still free, but the new Pro version is on sale for a buck! Both have Audiobus, but the free version can only be used as an input.
Spectral Eye Pro iTunes Description:
Spectral Eye Pro 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.
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 Pro 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.
The Pro version supports both input, filter, and output slots on Audiobus, so that you can directly analyze the freqencies produced by other synthesizer apps. For synthesists looking to create new sounds, or understand what they are hearing, Spectral Eye is an invaluable too.
The pro version also supports Virtual and Core MIDI; you can see the frequencies of sounds around you, and then play along with them using the novel Spectral Eye interface. If you don't have perfect pitch, you can use Spectral Eye to transcribe melodies that you hear or compose on your own.
Here is a new demo video for the 1.2 version of Spectral Eye, and an introduction to Spectral Eye Pro.