In the interaction of an acoustic field with a moving airframe, a canonical initial value problem for an acoustic field induced by an unsteady source distributionq(t,x) withq≡0 fort≤0, in a medium moving with a uniform unsteady velocityU(t)î in the coordinate systemxfixed on the airframe, is encountered. Signals issued from a source pointSin the domain of dependenceDof an observation pointPat timetwill arrive at pointPmore than once corresponding to different retarded times τ in the interval [0,t]. The number of arrivals is called the multiplicity of the pointS. The multiplicity equals one if the velocityUremains subsonic and can be greater whenUbecomes supersonic. For an unsteady uniform flowU(t)î, rules are formulated for defining the smallest number ofIsubdomainsViofDwith the union ofViequal toD. Each subdomain has multiplicity 1 and a formula for the corresponding retarded time. The number of subdomainsViwith nonempty intersection is the multiplicitymof the intersection. The multiplicity is at mostI. Examples demonstrating these rules are presented for media at accelerating and/or decelerating supersonic speed.