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TECHNIQUE OF SOUND RECORDING 317
the sound is randomly reflected from the boundaries or walls of the pickup space or set, and from objects within the space. To reduce the ratio of reflected sound to direct sound still further, sets are made less reverberant than normal by the use of considerable quantities of strategically placed sound-absorbing material on the walls and other flat surfaces. Unfortunately, 16-mm projectors are often run in highly reverberant rooms; recorded reverberation must be kept to a minimum on the film if the overall result is to be intelligible under such unsatisfactory reproducing conditions, as the reverberation (echo) effect of projection is added to that of recording.
Another important factor is the degree of mobility of the sound source with respect to the microphone. Since pressure-type microphones are nondirectional at low frequencies and quite directional at high frequencies, the "wiggling about" of a speaker's head translates itself into objectionable changes in voice quality due to the changes in the amount of high frequencies arriving at the microphone. If more uniformly directional microphones — such as the cardioid or the velocity types — are used, the effect of "head wiggling" is less noticeable. Such microphone types do not discriminate as seriously wTith respect to frequency ; accordingly, the primary effect of "head wiggling" is to produce small changes in volume rather than serious changes in quality.
The problem of quality variation due to source mobility is far more serious when it is necessary for actors to move about a set to provide the desired pictorial action. Although sound pickup is not simple under these conditions, the final result does not suffer if adequate equipment and personnel are used. Most 16-mm films cannot afford the high costs that such production entails; to accomplish such recording requires the duplication of the complicated and expensive equipment and numerous personnel used for a Hollywood production.
Since many 16-mm films are recorded with a single off-stage voice, the problem of mobility of the sound source does not arise. It is usually sufficient to record within a relatively small space (the space may be as small as 600 cubic feet), making certain by attentive listening to the monitor speaker that satisfactory sound pickup is being obtained. It is especially important in the case of a small space that the length-to-widthto-height relationships be of the order of 1 : 1.25 : 1.5 or multiple? thereof ; this dimensional relationship among the three axes of the room is necessary to provide "randomness" to sound reflected from the room surfaces. Further data in condensed form may be obtained from the excel