getting a binary image from a spectrogram image that shows frequency only above certain amplitude (in percent).

Here is what I have done so far.

[W,fs]=wavread(FileName); %this reads a wave file

[~,~,~,P]=spectrogram(W(:,end),tres,tres/2,fres,fs); %tres is time resolution and %fres is frequency resolution.

I=flipud(-log(P)); %here 'I' gives the spectrogram image in a matrix.

imshow(I,[]); %this line will display the image.

now say I want only the top 10% of the amplitude in binary image what would be the efficient way to do it? Please look into my code. I think I calculate my spectrogram image slightly different.