One Type of Acoustic Distortion in Sound Picture Sets

Abstract

The addition of sound to motion pictures has introduced many new problems in acoustics which have heretofore been subjected to but meager quantitative analysis. Judgment of “good” or “bad” acoustics has been based on the results of direct listening rather than on those of sound recording. A room or sound picture set may be acoustically good for binaural listening at a given place but may not be good for recording with a microphone placed at that particular position. One reason is that when one listens to a sound he uses two pick-up devices, the two ears. Any distortion of the sound at one ear may be balanced by that which the second receives. In the case of recording we use a single pick-up device, a microphone, which sends on to the sound record any distortion which may be present at the point where it is placed. Due to the fact that the ears add sound effects without reference to phase while multiple microphones combine amplitudes, the use of two microphones does not accomplish the balancing process obtained by the ears. Distortions of the sound field at the position of the microphone should, therefore, be avoided. If a sound record is to give a satisfactory reproduction of the original sound, the microphone must be placed at a point where the intensities of all the frequencies bear the same relation to each other that they did near the source. <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">**</sup> Any departure from this relation results in distortion. One of the main causes of this type of distortion is the interference at the microphone of two or more sound waves coming from the same source by different paths. While this phenomenon is not novel, the character and extent of the distortion which it introduces in sound picture recording has not been fully appreciated. Recent experiments carried on in the Bell Telephone Laboratories have shown that this distortion is responsible for a hollow unnatural quality when it occurs in sound picture records.