Spectrolite

Spectrolite visualizes sound as spectrograms in real time.
Spectrolite supports constant-Q spectrograms, which use a logarithmic frequency scale for a display where musical notes are evenly spaced. 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. Gain, Q, and other parameters can be adjusted in real time and instantly apply to the display of past signals as well as future ones.
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.