The purpose of this paper is to study numerically the problem of identifying a rigid 3D finite body Ω imbedded in an infinite acoustical medium Ω_e = R³ - Ω (wave velocity c). The cavity is subjected to a known harmonic incident pressure p^I(x)exp(-iωt), which satisfies Helmholtz’ equation (Δ+k²)p^I = 0 inside Ω_e with k = ω/c.