Curves have been calculated for the total radiation resistance and reactance of a single circular piston as a function of frequency when another circular piston is vibrating in the same infinite plane, the distance of separation of the two pistons being a parameter. The curves shown are for equal velocity amplitudes and for phase differences of 0° and 180°. These results enable one to calculate the impedance of a piston when there are present an arbitrary number of pistons in arbitrary phases.