The importance of intelligible communication in noise has focused attention on noise-canceling microphones, and one of the attendant problems is that of assigning valid figures of merit to such microphones for the purpose of comparing their noise-canceling properties. Differential microphones have responses which are a function of both the direction from which the sound comes and the distance between the microphone and source. It is here proposed that a simple extension of the concept of directivity factor and directivity index would be useful as a valid figure of merit for actual microphones. Up to the present time the ability to discriminate against noise has been evaluated by taking the ratio of the on-axis response of the microphone at minimum talking distance to the on-axis response for plane waves and multiplying this by the directivity factor (modern version). This is usually a theoretical calculation; and, since real microphones do not follow accurately the theoretical decrease in response with distance, actual measurements of the response, as a function of orientation and distance for a given frequency, could be substituted in the following formula to give a noise discrimination factor, NDF: NDF = ∫ 04π ∫ ab drdΩ ∫ 04π ∫ ab f2(r, Ω)drdΩ, from which the noise discrimination index would be NDI = 10log10 NDF, where f(r, Ω) is the ratio of the response of the given microphone at orientation angle Ω and distance r to its response at Ω = 0 and r = a, the minimum talking distance.