Plotting Power Spectrum (Matlab)

Question

I want to make a plot of the power spectrum for a .wav sound file, over the frequency range from -2000 to 2000 Hz.

Attempt:

I used the following code, but the resulting plot is one sided. It doesn't plot the negative frequencies. But the graph I need to produce has to contain negative frequencies. What do I need to do?

Any help would be appreciated.

Here is my code:

[s, Fs] = wavread('chord.wav');
nfft = 2^nextpow2(length(s));
Pxx = abs(fft(s,nfft)).^2/length(s)/Fs;
Hpsd = dspdata.psd(Pxx(1:length(Pxx)/2),'Fs',Fs);  
plot(Hpsd)
axis([-2 2 -150 -35])  %change axis range

Produces:

Answer 1

First of all, wavread has been removed in the 2015 & 2016 versions of MATLAB. So, if you plan on using your code in a newer version of MATLAB, I recommend that you use the audioread function instead.

Now, to answer your question. MATLAB already have a built-in function for plotting the power spectral density (Pxx) of a given input signal - periodogram. I've taken the liberty to copy a piece of sample code from MATLAB's documentation that should help you plot the Pxx you want (it will contain negative frequencies as well).

periodogram(x,[],NFFT,Fs,'centered')

All you need to keep from your code snippet are the input values seen here. The rest, like the axis function, aren't necessary to get the plot you want if you use this periodogram function. The above function will plot the power spectral density of a given input signal x with no specified window ([] which just means the function defaults to a simple rectangular window). The final argument is the one that I presume you are most interested in: freqrange. It can have several different inputs, including twosided and centered. You should try both options and see which output you find more useful.

I hope this helps you!