Of course, with this method you do get a reduction in the stereo separation in your audio. Taking this idea into consideration, bass sounds and very high pitched sounds are made mono - because you can't tell the difference. A practical example of this is subwoofer speakers - they can be stuck in the corner of the room away from the other speakers and you still can't really tell that the bass is coming from there. The theory is that we are very bad at telling where very high and very low frequency sounds are coming from. There is a secondary pass to the Joint Stereo formation, which is quite clever and uses another psychoacoustic model. These are only elements however and it is a very useful addition for the reduction of file sizes. This means there will be elements of your wav that are, in effect, mono. It compares the left and right channels and works out how much data it can save by making them identical and encoding the data once. So why bother having twice the data for most of the song when lots of it can be duplicated for each channel? This is where the Joint Stereo idea comes in. Basically the idea is that for the most part, left and right channels are very very similar.

Joint stereo or stereo