What is the plot when analyzing audio signals in Matlab?

[SIP]

Hi,

I am trying to get a plot of the sound's magnitude vs. frequency. I have managed to read in a wav file as a matrix. I then compute the fft of this matrix. When I plot the absolute value of this, I get a symmetric plot with peaks at the start and the end. What is this a plot of? Is it magnitude vs. frequency? Someone please help, I'm really confused! Any help would be much appreciated!

 

[Proton Soup]

when you do the fft of a real-valued signal, you get a series of complex-valued frequencies where the real and "imaginary" parts are vectors containing the phase information (that is, how much those sinusoids are shifted along the x-axis).

because of the mathematical properties of ffts of real signals, the result is also a series of complex conjugates symmetric about zero. when you take the magnitude of the conjugates, they are obviously equal. so... when you're only interested in magnitude, you only look at half of the result.

if you want details, you could download the course materials here:
http://see.stanford.edu/SEE/courseinfo.aspx?coll=84d174c2-d74f-493d-92ae-c3f45c0ee091