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86 J. P. MAXFIELD [j. s. M. p. E.
The method by which we determine direction with either one or two eyes is obvious and need not be discussed. The factors which enter into our appreciation of depth or perspective in sound are the ones of interest to this paper. It is probable that the most important factor, particularly where monaural hearing is concerned, is that which deals with the relative change in loudness of the direct and reflected sound. Since the intensity of the reflected sound varies relatively little from place to place in a room, while the direct sound from the source to the pickup device varies quite rapidly with its distance, the ratio of the intensity of the direct to the reflected sound also varies considerably. Hence, as a source of sound such as a person speaking recedes from the microphone, the loudness of the voice appears to decrease slightly while the reverberation appears to increase materially. With binaural listening, this is unconsciously interpreted as distance. It has been found that this effect, when properly controlled, can also be interpreted as distance with monaural listening.
In the case of a talking motion picture, the camera has only one lens and the recording system only one ear, so that those effects which were brought about by the binocular seeing and binaural hearing cannot be made use of. Long experience with the photography has enabled the cameramen to create a part of the depth illusion by the proper choice of the focal length of the lens used and by the proper type of lighting. Fortunately, for the acoustic engineer, the impression of depth depends upon factors which are almost as effective with monaural as with binaural listening ; namely, the change in the ratio of the intensity of the direct sound to the reverberation present.
The loss of direction brought about by the use of one ear only, causes some rather unexpected results. When two ears are used, a person has the ability to consciously pay attention to sounds coming from a given direction to the partial exclusion of sounds coming from other directions. With the loss of the sense of direction which accompanies the use of monaural hearing, this conscious discrimination becomes impossible and the incidental noises occurring in a scene, as well as any reverberation which may be present, are apparently increased to such an extent that they unduly intrude themselves on the hearer's notice. It is, therefore, necessary to hold these noises, including the reverberation, down to a lower loudness than normal if a scene recorded monaurally