PAPERS PRESENTED AT GENERAL SESSIONS 37 



four panel door of birch veneer. Curve 2 is for a solid 

 oak door, I34 inches thick. Curve 3 shows the reduction 

 produced by a double walled "ice box" door, made of 

 hea^-y pine, the intervening space being filled with heat 

 insulating material. Interestingly enough, it was found 

 that a solid steel door, I4 of an inch in thickness, was 

 much more effective in reducing the amount of the trans- 

 mitted sound than were any of the other door construc- 

 tions tested. Curve 4 indicates that, in general, only 

 about 1/1000 of the sound was transmitted by this door. 

 The net results of these investigations point to the con- 

 clusion that the highest degree of sound insulation is in 

 the use of materials having the greatest mass and stitf- 

 ness, rather than in those in which the natural damping 

 is greatest. 



It has been generally supposed that soft flexible mater- 

 ials possess great virtues in preventing the passage of 

 sound. This opinion, no doubt, arises from their known 

 value as insulators for heat, as well as to their property 

 of being highly absorbent of sound. Our experiences in 

 the use of such materials in securing the necessary sound 

 insulation in the laboratory led to serious Cjuestion as 

 to the validity of this belief. Accordingly a study of a 

 number of materials of this general character has been 

 made. Due to its wide spread use as a sound absorbent 

 for the reduction of reverberation, uncovered hair felt 

 was the first material tested. In Figiire 8, the logarithm- 

 ic reductions of intensity of transmitted sound produced 

 by different thicknesses of this material are shown. The 

 point of most practical significance is the relatively small 

 reduction of intensity of the transmitted sound as com- 

 pared with material such as glass and steel. For all but 

 the highest tones, four inches of this material is less ef- 

 fective than one-fourth of an inch of plate glass. It will 

 be further noted that the sound insulating efficiency rises 

 rapidly with rising pitch. If the logarithmic reduction 

 be plotted against the thickness of the material as in 

 Figure 9, the relation is seen to be linear, that is, each 

 succeeding layer reduces the intensity of the transmitted 

 sound by a constant ratio. This leads to the conclusion 

 that the reduction of intensitv bv such a material is a 



