The potential of a circular source, having radially symmetrical but otherwise arbitrary distributionv(r0) of velocity over its face, is expanded asψ = z −1e ikR(a0+a1z−1 + ⋯)[cylindrical coordinates (r,θ,z); center of source atr − z = 0]. We finda0to depend only on the total volume velocity at the source. Ifv(r0) is known, allancan be calculated; if it is unknown, limits onancan be set.