ABSTRACT
It is well known that the acoustic wave equation, in a non-dissipative heterogeneous medium, is invariant under a time-reversal operation. Indeed, it contains only a second-order time derivative operator. Therefore, for every burst of sound p(-r , t) diverging from a source-and possibly reflected, refracted or scattered by any heterogeneous media-there exists, in theory, a set of waves p(-r ,−t) that precisely retrace all of these complex paths and converge in synchrony at the original source as if time were going backwards. Here, p(-r , t) ∈ R is a function of space and time. In this chapter, we take r ∈ R3 , but the time-reversal symmetry is still valid if the space dimension is one or two. This gives the basic idea of time-reversal acoustics.
