Journal of the Society of Motion Picture Engineers (1930-1949)

phase on, has a half wavelength, X/2, equal to the distance between the ears (Fig. 1). Therefore, if: 11 SO X = 6.78 X 2/12; then/ = — = A 1130 7S212 That is, the maximum possible frequency for binaural phase detection by this theoretical observer is in the order of 1000 cycles/sec or less. Most speech sound sources possess frequencies both above and below the crossover frequency range of from 800 to 1000 cycles. Not only does this enable the observer to compare angular location by both phase and amplitude methods (and to derive a more accurate location), but since phase shift of a given frequency is a function of both angular position as well as distance, it provides a measure of the distance to the sound source. In addition to the localization system defined by the base distance between the ears and the mental computation of angles, the mind has an additional distance-computing ability based upon the ratio of direct sound to reverberant sound impinging upon the eardrum. Microphone Placement In view of the above, it becomes immediately apparent that in order to record binaurally for later binaural reproduction some care should be exercised in microphone placement. The first basic principle underlying microphone placement is that the perpendicular bisector of the line joining the pickup microphones represents the center line of a fictional listener's position. During reproduction, the loudspeaker placement should be such that the perpendicular bisector of the line joining the loudspeakers coincides with the real listener's center line. This arrangement results in both depth and lateral stereophonic "image" location, dependent upon both phase and the intensity ratio of the direct sound picked up by the two microphones. The second principle underlying microphone placement affects the apparent position of the sound behind the immediate foreground. The distance of the source from a single microphone is also determined mentally by a comparison of the reverberant sound to the direct sound. The most accurate mental calculation is made when this ratio is not in the extremes. Therefore, both exceedingly close and overly distant microphone placements are to be avoided. Under a strict binaural microphone arrangement the two microphones should be relatively close together and have individual pickup patterns approximating those of the human ear; the placing of an acoustic septum between the microphones would be desirable. Under an expanded arrangement, wherein a simple stereophonic system is obtained, the microphones are spaced quite widely apart and a third microphone with isolation amplifiers and attenuators is added midway between these two primary microphones. The object of the center microphone is to feed a small amount of sound energy to both recording channels and thereby to correct for the spatial distortion occasioned by moving the primary microphones apart.4 Unless this correction is made, some depth location error occurs, especially in the area midway between the primary microphones. If some depth location error may be permitted, which it may be if the sound is not associated with a concurrent motion picture, then it is readily possible to omit the center microphone. Binaural Presence — Listening The physiological sense satisfaction that yields the psychological impression of being present in a nonexistent room is the startling factor in listening to a binaural recording for the first time. The sense of "presence" obtained is Otto C. Bixler: Binaural Recorder 111