Spectrum analyzer...

[bogdan]

I tried to obtain the spectrum of a discrete signal and I had some problems...
If I compute on a PC the discrete fourier transform (FFT or DFT) I obtain valid fourier transforms (the same Excel computes...)...
...but how do I analyze a wave obtained using a microphone ? I don't get the same results professional programs do...I tried using as the "intensity" of a frequency the magnitude...I tried using the real component...the complex one...but still no good...
Can you help ?

[arcnets]

How about
intensity(f) = [squ](R2(f)+I2(f))
where R is the real component, and I is the imaginary one.

[chroot]

You're looking for what is called the "power spectrum." The book "Numerical Recipes in C" is the best place to look (as usual!).

It's available in its entirety, free of charge, at http://www.nr.com

Chapter 12 deals with Fourier transforms and power spectra.

- Warren