How to convert narrow band to third octave band

[michefabb]

I have a pressure signal p(t), I made the FFT of (p/p0), now I want to get the sound pressure level of every third octave band between 16 and 20000Hz (by this way I can add logarithmically the 32 values and I have the dB of the noise, so that the sum doesn't depend by the frequency discretization). If I'm right I should convert the amplitude of FFT from narrow band to third octave band, but how could I do? Please let me know something

 

[GregLocock]

Assign each frequency line in your FFT to a 1/3 octave bin. add all the dB values in each bin, using dB addition.



then check by adding all the 1/3 octaves up to see if you get the same overall level.





Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[michefabb]

Thanks for your answer.
I don't understand: if I do so I could sum directly all the values (it gives the same result) and it depend by how many points I take. How could I do?

 

[GregLocock]

You need to divide your fft's magnitueds by a claibration factor that is dependent on the number of lines in it. Different fft algorithms use different scaling factors. the only reliable way is to check that the rms of the input signal is the same as the rms of the spectrum.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.