Waveforms generator to test a Hi Fi system
You connect the headphone wire to the AUX input of the Hi Fi.
Available waveforms: Sine, Triangle, Ramp-Up, Ramp-Down, Square Wave (duty selectable 10,20,25,30,40,50%), white and pink noise
Frequencies range from 16Hz to the maximum possible for the device, selectable in steps of 1,10,100,1000,10000 Hz by pick number.
The actual audible frequency depends on your Android device.
The app analyzes, capturing the sound with the microphone, showing:

Frequency determined by FFT (Fast Fourier Transform)
Frequency detected by the method of the zeroes count (if the waveform is not overly distorted)

PCM input to the DAC
PCM recorded by microphone
FFT with the peak in red of maximum amplitude

Caution: It is not a professional tool, it can not be used to test or do professional certifications.