Arbitrary 3D obstacle: Acoustically hard boundary

Arbitrary 3D obstacle: Acoustically hard boundary Let us derive a general low›frequency form of an integral equation in the 3D case up to the linear (with respect to the small parameter k) small terms. To be more specic, we will consider a plane wave incident on an obstacle D with acoustically hard boundary, i.e., we will treat Eq. (2.62). If the direction of propagation of the plane coincides, for instance, with the axis x3, then we have pinc(x) = exp(iky0 3) in Eq. (2.62), or pinc(x) = 1+iky0 3 +O(k2) for small k.