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Fig. 4. — Absorption coefficients of two 1/2,1 layers of felt with interven.ng air space.
1. For what may be considered as continuous masonry including walls of gypsum tile, clay tile, solid plaster, and brick the reduction of
sound in transmission increases according to a fairly definite law with the increasing weight of such constructions.
2. Felts and other fibrous porous materials, used alone, are not effective sound insulators as compared with stiff, heavy, and impervious materials. As generally employed for sound insulating purposes, either as an inner-lining for double constructions, or as a fill between wood members in single construction, they do not produce any very marked effect in increasing the degree of sound insulation.
3. Double wall constructions furnish a higher degree of sound insulation than do single walls of equal weight. The greater the structural and spatial separation of the two component structures, the greater the degree of sound insulation afforded.
Fig. 5 presents the results of some of the tests in the transmission of sound by more usual types of wall construction. The vertical scale gives the average logarithm of the reduction of sound intensity for 17 different tones ranging from 128 to 4096 vibs. per sec. The horizontal scale gives the logarithm of the weight per square foot of the partition structure. The points lying along the straight line are masonry partitions including walls of clay and gypsum tile, both hollow and solid, and of different thicknesses, as well as solid plaster on metal lath, from 1 V2 " to 4 % )} The double partitions were of gypsum tile, with complete structural separation between the single units. The effects of the various types of fill as well as the effect of bridging the intervening air space are shown. Unless otherwise indicated, the separation between the double walls was 2 inches.
It appears then that when insulation from external noise is necessary the double wall construction is the most effiective means of securing it with wall construction of moderate weight. Equally important is the matter of preventing the entrance of sound by means of openings, including doors and windows, and ducts for heating or ventilation. The ventilating system of a talking moving picture stage requires especial care in the planning. Quiet operation should be a first consideration in specifying the fans, pumps and other machinery employed in air conditioning. Flexible canvas coupling between the fan and the ducts should be specified to prevent the conduction of vibrations along duct walls. The cross section of ducts should be figured large enough to allow lining with sound absorbent
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