Plotting Average Spectra Plot using MATLAB

Question

I have four 1xN sound signals and I want to view the average spectra plot like the one given in the link below: http://i1233.photobucket.com/albums/ff396/sakurayen/Plot/AMaximumLikelihoodApproachtoSinglechannelSourceseparationpdf-AdobeReader.jpg

I've tried to use the MATLAB function , PSD, to plot the spectral but I am getting a different plot instead. Note that the data used for both the plots are the same.

plot obtained using PSD function in MATLAB: http://i1233.photobucket.com/albums/ff396/sakurayen/Plot/PowerSpectralDensityofRJMF.png

Thanks!