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ACOUSTICS 95
distribution. In ventilation, concentration of forced air must be avoided because of noise and draughts. Therefore, diffusion must take place, and to this end the auditorium walls and ceiling are used to spray new air, equally distributed over the audience. The process, if accomplished scientifically, is beneficial; it neither distracts attention nor causes discomfort. Similarly the mathematical determination of the effects of porosity and flexibility in all the materials to be used in the interior finish and furnishings of a theatre is necessary in deciding just what treatment the acoustical engineers are to give the walls and ceiling to insure good effects.
When a sound wave travelling in the air reaches a wall it communicates itself to the wall and is converted therein into waves of exactly the same shape and of the same number per second but of rather smaller size; for a little of the power is lost in transferring the waves from one medium to another. The extent of this weakening depends upon the thickness and on the material and structure of the wall. However, because sound is produced by waves, it can be focussed, refracted, and reflected, just like light. When reverberation is present in excessive amount it may cause an overlapping and blurring, especially annoying in the case of speech. It must likewise be borne in mind that when reverberation is reduced, loudness diminishes in almost the same ratio; therefore, to allow for both conditions, the proper point must be determined.
Since the action of sound may be calculated in a precise manner the proper corrective measures may be prescribed to reduce reverberation to the amount best suited for the auditorium under consideration. Reverberation increases, for instance, with the size and height of the room. It may also be reduced in auditoriums that have balconies which contain heavily upholstered seats or that have other recesses in the construction. Excessive absorbent materials