Well i guess you might want to look up Fourier Transformation. Thats a transform from time to frequency domain. So you will get the spectrum of frequencies present in the signal. Here is what wikipedia says:
Fourier transform - Wikipedia, the free encyclopedia. Basically you need to compute the Fourier-coeffs. for all the frequencies that might be present (depends on the quality of your data), replace integration by summation since your data is in discrete form anyway and remember what time duration corresponds to the length of your buffer so you get the correct Hertzs in the end. Im pretty sure youll find a lot of code on the net, search for "FFT" or sth. (... Ive never done this
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Regards