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.)