Frequency counter based on the microphone input. Counts when the input rises or falls past a set level and converts into into frequency or a time period. FOR INDICATION ONLY. Results depend on your device and its hardware. If you just want to know the frequency of a sound with harmonics (e.g. mucial instrument) a FFT based app such as keuwlsofts spectrum analyser or guitar tuner (both free) will be better. This app can provide more accurate frequency measurement for single frequency input signals. Features include:

Display of triggered event count and frequency or time period.
Graph of input signal 2.5 ms/div up to 640 ms/div.
Gate time of 0.1s 1s 10s or 100s.
Gain from x1 to x1000.
Trigger on rise or fall.
AC or DC coupling.
Set a noise level so new event is not triggered until the signal has first passed this level.

More details can be found on the website.