Sale has ended but you can still get this app.
Download SPL and Spectrum Analyser Now

Official Description

SPLMeter shows SPL (deciBels) of the Android's mic signals, and their frequency spectrum. The meter has three speeds: Fast, Medium and Slow. Shows peak envelope and frequency of maximum intensity, and clipping. ISO 1/3 Octave bars and Spectrogram.

(If you need a more fully featured RTA, with 1/1, 1/3 and 1/6 octave display, RT60, Noise Criteria, A&C Weighting etc. combined with a signal generator, please take a look at my "AudioTool" app in Android Market.)