Create Audio Transfer function in Android & IOS app and output it as sound

Create FFT spectrum of left channel audio out and subtract the FFT frequency spectrum of Microphone IN - the result is is what is called transferfunction - then turn this frequency spectrum again in audio time domain and output it on right channel - this is a therapeutic app where the patient gets to hear a sound on the left ear - tries to reproduce this sound as best as he can by humming into the microphone - and then gets to hear the transferfunction on his right ear which is then a combination of all frequencies that were missing in his humming relative to the played sound in his left ear