Low frequency results using poctave

Now I have time-waveform data with time span of 40s and dt 1e-3 s, and I want the octave spectrum of the waveform under 1 Hz.

However, unfortunately, it seems that using "poctave" function, we can get spectrum only over 3 Hz, according to the instruction of the function.

Can I get low frequency octave spectrum?

Answer 1

[Mathieu NOE]

hello

right, poctave does not go below 3 Hz in the best case and that's already low enough for acoustics (normally)

unless you want to rework / extend some existing code (attached FYI) , my 2 cents suggestion would be simply to "fool" poctave by passing a xx times higher sampling rate argument then divide the resulting octave bands central frequencies by the same factor (I opted for power of 2 factor so that we stay in line with the definition of the octave bands)

Of course there is a bit of extra work to put back everything nicely in a bar plot ..

quick and dirty example below :

% your data (dummy)
dt= 1e-3;
fs = 1/dt;
duration = 40;
t = (0:dt:duration);
freq = 1;
y = sin(2*pi*freq*t)+0.1*randn(size(t));

% shift Fs 
Fs_factor = 32;
fss = fs*Fs_factor;
[p,cf] = poctave(y,fss,'FrequencyLimits',[3 fs/2]);
cf = cf/Fs_factor

cf = 8×1

0.1244 0.2482 0.4953 0.9882 1.9717 3.9341 7.8496 15.6621

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% show the results in dB ref 20 microP
refdB = 20*log10(20e-6);
P_dB = 20*log10(p) - refdB;
plot(cf,P_dB,'-*')
set(gca,'Xscale','log');

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[Mathieu NOE]

and here are the other solution / files Ipromised you , but there is more work to extend the filter banks to even lower frequencies

[Mathieu NOE]

hello again

problem solved ?