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412 CINEMATOGRAPHIC ANNUAL
80 loudness units above minimum audibility. The top and bottom lines are identical with the top and bottom lines respectively, of the curves in Fig. 2. This curve indicates that in going from loudness 20 to 40, it is necessary to increase the gain of the amplifying system 20 db for the frequencies lying in the middle region. On the other hand, in order to go from a loudness 20 to loudness 40, at the low frequencies, say around 60, it is only necessary to go 6 to 8 db. However, loudness is mainly interpreted by the middle region. If, therefore, a sound which was originally made at loudness 20 were reproduced by increasing the intensity by all its components by 20 db, it is obvious that the loudness in the bass would lie slightly above the curve representing a loudness of 60, i.e. more than 20 loudness units too high. Such reproduction would sound over-bassed or "heavy." This is one of the reasons why the human voice sounds heavy when reproduced at a level considerably higher than that at which the person actually spoke.
This effect is inherent in the ear, and as the recording becomes more and more perfect, the loudness level, at which music or speech is reproduced, becomes more and more important. This ends the preliminary review.
The technic of acoustic control is based on letting the camera be the eye and the microphone be the ear of an imaginary person viewing the scene. It might be interesting, therefore, to consider briefly how a person observes, that is, how he sees and hears what is taking place around him.
When a person is viewing a real scene in real life, he is viewing it with lenses — that is, the eyes, and pickup devices — that is, the ears, which are in a fixed relationship, one to the other. This observer is equipped with two eyes and two ears. The two eyes enable him to appreciate distance or depth with much more facility than would be possible with one eye, while the two ears enable him to appreciate direction and perhaps, to a slight extent, depth where sound is concerned. The point of importance, however, is the fact that the eyes and ears maintain a fixed relationship to one another.
The method by which direction is determined with either one or two eyes is obvious and need not be discussed. The factors which enter into the appreciation of depths or perspective of sound are the ones of interest here.