What is spectral whitening?
What is meant by "spectral whitening" in DSP?


Spectral whitening is usually an attempt to make the spectrum of the signal "more uniform".  One reason this might be a good thing to do is that it can have the effect of making the autocorrelation of the signal "narrower" (and closer to a Dirac delta, for discrete-time signals). This can help localize in time. 


  What effect does spectral whitening have when used in image processing? (visually or otherwise...)


It's generally not pretty.  Most images are "low pass" (most of the information is in the low frequency part of the spectrum).  One simplistic approach to whitening in images is to do a column-wise (or row-wise) difference (i.e. diff in matlab).

This will mean negative pixel values, which generally do not map to anything sensible with standard images.

This example shows how prewhitening can improve localization in image processing template matching. The picture from that link is:




  Where might spectral whitening be useful in audio processing or analysis? 


If you are trying to localize (in time) the onset of a sound, then it's possible that spectral whitening can improve this.  It's also possible that it can reduce (disimprove) the SNR.


  What would a spectrally whitened audio signal sound like?


For audio of speech or music, it will tend to bring in more higher frequencies.