Spectrolite

Spectrolite visualizes the frequency content of sound in real time in the form of spectrograms.
Spectrolite specializes in constant-Q spectrograms, which use a logarithmic frequency scale for a perceptually and musically meaningful display where musical notes appear evenly spaced. As of version 2.0, spectrograms based on linear and mel frequency scales are also supported.
The display covers the full frequency range of human hearing. Intuitive touch panning and zooming lets you dive in and explore the spectrograms in detail.
The Pro in-app purchase adds grids and scales for time, frequency, and amplitude, as well as a spectrum view and a tap-to-measure function. This expands Spectrolite into an audio measurement tool that you can use to measure frequencies, musical pitches, amplitudes, and durations.