Turn your phone into a Spectrum Analyzer. Speccy is an Audio Spectrum Analysis tool, which visualizes the spread of frequencies detected by your microphone. It uses an FFT signal processing algorithm and supports the most languages of any similar app. It's also the only app of its kind allowing you to export audio snapshot data to the clipboard, for plotting in Excel, etc.
It has a wide range of uses, such as: assessing environmental noise, tuning speaker systems, identifying audio signals obscured to the human ear by noise or simply testing the microphone on your device. You could use Speccy to compare the quality of different types of headphones, test signal generators or measure audio signals just outside human perception. It apparently has also been used successfully to find gas leaks.
Speccy provides the most 'analysis window functions' (the 13 most popular for audio engineers) and, for your added peace of mind, the only permission Speccy needs is microphone access.