How Can I Measure Relative Amplitudes of Specific Frequencies in a Noise Field?

[Bert Rackett]

I would like to measure the relative amplitudes (energy levels) of several specific
frequencies in a noise field. I thought of attaching capacitive microphones to
tubes that would resonate at those frequencies. I've visited hundreds of web sites
that invariably give equations for frequencies and resonant points, but say nothing
about the amplitude domain unless they're talking about musical instruments.
How much larger will my response be in my tube? How large are the harmonic
responses? I have several texts, but they speak qualitatively about resonances and
not quantitatively. can someone point out a text with the math?
Thank you.
Bert Rackett

 

[billiards]

As long as you're sampling at more than twice the highest frequency you want to detect, can't you just put the signal through a fast Fourier transform? You could use something like MatLab to do this and plot the Power Spectrum quite easily.

It would be interesting to see how the power spectrum changes in relation to the position of your microphone. You might expect to see notches at frequencies with wavelengths that destructively interfere with reflections off of the surrounding walls.