This app brings to your phone a spectrogram viewer that features time frequency analysis.

Processing is customizable. You can decide on the FFT resolution yourself - which begins from a minimum of 32 to a maximum of 65536. You can even select your preferred window type, color scale type, as well as interface modes.

To lower spectrum leakage, you will be supplied with nine window algorithms.

This app also features a fast pause and resume button, the option to keep the screen on (overwrites your phone's screen timeout setting).

Please do note that the amplitude value is unavailable as there's potential for inaccuracy without device calibration.