For how FFT work in there, you could refer to this question - Understanding FFT in aurioTouch2
Hi could you please help me with sample code @KenHui
I think you need to use the accelerate framework, inside there is a vDSP API that could do FFT(Fast Fourier Transform). It will convert the data from time domain to frequency domain. According the bin size information, you could extract the magnitude/amplitude after the certain bin size.
P.S. AurioTouch or AurioTouch 1 is not using the vDSP API. I remember before iOS 4 there is an FFT function that could do similar thing but slower. So you may think that vDSP is only available after iOS4.0