Let the loading (per unit area) on the structure be represented by the function f(r0, t), where r0 is the position vector of a loaded point on the body with respect to a ˜xed system of axes, as shown in Figure 11.1. Let the unit impulse response be h(r, r0, t – θ); this is the output at r corresponding to a unit impulse at t = 0 and at location r0.