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. musicial instrument), a FFT based app such as keuwlsofts spectrum analyser or guitar tuner 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.