Electronic Correlator for Diffusion Studies

We have built, from off-the-shelf arithmetic modules, an electronic cross-correlator which measures R = p1p2¯/(p12p22¯)12 as 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, and 13-octave bands 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 which R = 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.