It is well known that an array can be designed to reject noise arriving from one (or several discrete) fixed directions, by adjusting the nulls of its pattern to coincide with the unwanted directions. It is not possible to design an array to accept only discrete directions; nor is it possible to design it to reject an entire solid angle of noise. In these properties the array (or space-frequency filter) is entirely analogous to the ordinary time-frequency filter. We show here that a similar property exists for the radial coordinate. A source of noise in the vicinity of a receiving station may have a small power output and yet greatly interfere with the reception of distant signals. It is possible to eliminate entirely the noise due to a point source by suitably combining the outputs of two transducers located at the receiving point. Such a filter operates by virtue of the sphericity of the noise wave front, and is effective over a broad band of frequencies. The technique can be immediately extended to the case of several discrete noise sources. In the general case of a continuum of noise sources, it is no longer possible to remove all the noise, but an “optimum radial filter,” in either the rms or the absolute-value sense, must be designed on the basis of the “radial spectrum” of the noise. (This work was supported by the Office of Naval Research.)