In the real world, we often encounter sounds from multiple sources at once - speech against a background of noise, or the telephone ringing while some music is playing, where the music itself may be made up of the sound of several instruments playing at once, as well as the voices of singers. Physically the sound waves of all these sources all become superimposed, mixed together into a jumble which, in strict, mathematical terms, cannot be 'unmixed'. Nevertheless our brains are often able to separate such complex auditory scenes into foregrounds and backgrounds, allowing us to listen selectively to only one source out of many. This "auditory scene analysis" is perhaps one of the most complex and impressive achievements of the auditory brain. The neural processes underlying auditory scene analysis are discussed in chapter 6 of "Auditory Neuroscience" . The following web pages provide further information about this topic.