One important use of the concept of inverse systems is to remove distortions of some type. A good example is the problem of removing echoes from acoustic signals. For example, if an auditorium has a perceptible echo, then an initial acoustic impulse is followed by attenuated versions of the sound at regularly spaced intervals. Consequently, a common model for this phenomenon is a linear, timeinvariant system with an impulse response consisting of a train of impulses: