There are two conceptually distinct challenges for a listener surrounded by concurrent sounds (though much of the time they are closely related), and the term ‘cocktail party problem’ is often used in reference to both of them. The first is the problem of sound segregation. The sounds in an auditory scene all sum together to generate the signal that enters the ear. The second challenge is that of directing attention to the sound source of interest while ignoring the others, and of switching attention between sources, as when intermittently following two conversations.