We have built, from off‐the‐shelf arithmetic modules, an electronic cross‐correlator which measuresR = p1p2¯/(p12p22¯)12as a function of the distance from point 1 to point 2. It is compact, accurate, and stable. To illustrate its use, we have made surveys along a horizontal diagonal of the NBS reverberation room. Pure tones, FM signals, and13‐octavebands of noise were used at frequencies (or center frequencies) in the range 125–2000 Hz. In some cases microphone No. 1 was fixed and No. 2 was moved, but in most cases the microphone separation was fixed at λ/2, for whichR= 0 in the case of a three‐dimensional diffuse field, and the array translated slowly along the diagonal. The second method appears to be the better.