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SOUND PROJECTION AND THE AUDIENCE 487
in arranging drapes and hangings on the walls, it is not necessary to cover a wall area directly opposite drapes used over windows. In general, parallel surfaces are best controlled by covering only one of the pair of surfaces. In the case of parallel side walls, for example, one wall might alternate with a standard width of wallboard running from floor to ceiling, and no covering for the same width. The opposite wall would likewise alternate except that a covered portion of the first wall would be directly opposite an uncovered portion of the second wall, and vice versa. The rear wall should be almost completely covered if possible.
It would seem apparent that the best control of sound quality is achieved when the sound heard by each listener is that from the loudspeaker alone, unaffected by any blurring caused by the multiple reflections from the highly sound-reflective walls of a reverberant room. Although this theoretical ideal does not obtain in practice, it is possible to select the loudspeaker type so that sound radiated by the loudspeaker is directed only to the audience and not "sprayed" all over the walls, ceiling, and other surfaces where it ' ' bounces, ' ' arriving at the listener 's ears appreciably later than the sound that issued directly from the mouth of the loudspeaker. To accomplish the desired objective in the most satisfactory manner calls for a horn-type loudspeaker. One several feet long is preferable to the very short horns used in such loudspeakers such as the Altec-Lansing 604. An excellent horn is the Jensen K4-244, but, unfortunately, like all good horns of this kind it is unwieldly in comparison with the relatively small box-enclosed dynamic cone loudspeaker customarily furnished as part of a sound projector.
If the sound radiation of a loudspeaker is non-directional, as is the case with a dynamic cone loudspeaker operating at low frequency, it may be thought of as being located at the center of a sphere of radiation into which sound energy is being transmitted. Remembering that the location of the loudspeaker is also the location of the screen, it is apparent that most of the sound energy is not directed to the audience at all but is radiated to the walls, ceiling, floor, etc., where it is either absorbed and wasted, or partially reflected to the listeners' ears where it "blurs" the direct sound that was heard a moment earlier. A rough calculation will show the high order of magnitude of this waste of energy; it is merely necessary to take the ratio of the volume into which sound is radiated and compare it with the volume in which sound is desired. The former is represented by a sphere of radius equal to the distance from the loudspeaker to the farthest listener in the room, the latter is represented by that sector of the sphere within which the ears of the audience are located. For a loudspeaker mounted at the top center of the screen, the solid