Finding the strongest beat in a sound wave using autocorrelation

I have a sound wave representing a piano piece played at a steady tempo, and would like to get a graph of the saliency of each beat. I understand that this is done by plotting the autocorrelation function, however I don't quite understand why a graph of r coefficients against each possible lag value (which is, as far as I udnerstand, the deffinition of an autocorrelogram) would have anything to do with beats.

The following code produces a graph that doesn't in any way suggest anything to do with the actual steady beat of the piece (60 BPM):

[y,Fs] = wavread('d:\bach.wav');
[r,lags]=xcorr(y,'coeff');  
plot(lags,r)

Clearly I'm understanding autocorrelation wrongly. For instance, in this very simple example http://upload.wikimedia.org/wikipedia/commons/e/ed/Acf_new.svg, the frequency of the sine hidden in noise is nowhere visible from the autocorrelation graph - or is it? Furthermore, that frequency would actually be the pitch of the sound, and not any rhythm-related measure!

Anticipated thanks for any clarifications!!

[Daniel Shub]

I am closing since this doesn't seem to be a MATLAB question, but rather an audio processing theory question.

[AwedBy Matlab]

Thanks for your input Daniel, however I do want to get this problem solved in Matlab (with or without clarification on the theoretical aspect), so if you don't mind could you leave the question open, I appreciate it.

Answer 1

[AwedBy Matlab]

Still haven't figured this out, would really appreciate any thoughts from more experienced scientists..