Developer 4Pockets put together this demo video of Pocket RTA - a realtime spectrum sound analyzer for the iPad.
PocketRTA samples sounds using either internal or external microphones and then applies a Fourier Transform to the samples to obtain a frequency spectrum. The result is an accurate representation of the sampled sound broken down into its frequency components.
The video outlines some of the features specific to the HD version of the software.
See our previous PocketRTA post for app details.