Wednesday, October 21, 2009

Calculate the power of audio in frequency block

Here is the matlab code, it shows the power between frequency 200-2000. This is the power of all audio data instead of a small window block (like 256 samples):

>> [snd samplefreq] = wavread('crash.wav');
>> num_samples = length(snd);
>> minfreq = 200; maxfreq = 2000;
>> f = fft(snd) / (num_samples/2);
>> seconds = num_samples / samplefreq;
>> low = floor(seconds * minfreq);
>> high = floor (seconds * maxfreq);
>> p = abs(f(low:high)) .^ 2;
>> figure; plot(p);
If you want to show the fft result directly, use ‘fftshift’ to produce a more standard plot with the low frequencies in the center of plot:

The x-axis is plotting the number of samples. It is more useful to plot the single-sided spectrum with frequency in x-axis. For example, we want to display 200-9000Hz spectrum:

Ns = length(p);
plot([201:2*(9000-200)/Ns:9000],p(1:Ns/2-1)); %make sure the size are the same

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