DISPERSION OF SOUND 301 



efficient per centimeter for 1500, 3000, 6000 and 10,000 cycles as a function 

 of the humidity is shown in Fig. 12.13. 



M. Sound Transmission through Partitions '^'^•'^^''^^''^^''^'^''^'^^. — The problem 

 of sound transmission through partitions and walls is complicated because 

 of the many factors involved. The problem of the mass controlled single 

 wall partition is very simple. The sound insulation of this type of parti- 

 tion is proportional to the mass and frequency. For the usual building 

 materials and walls of ordinary dimensions supported at the edges, the 

 problem is that of the clamped rectangular plate with distributed resistance 

 throughout the plate and lumped damping at the edges. Obviously, the 

 performance of this system depends upon the size, the ratio of the two 

 linear dimensions, the weight of the material, the damping in the material 

 and the edge supports. This type of problem is not amenable to an ana- 

 lytical solution. 



The transmittivity of a partition is defined as the ratio of the intensity 

 in the sound transmitted by the partition to the intensity in the sound in- 

 cident upon the partition. The transmission loss, in decibels, introduced 

 by the partition is given by 



T.L. = 10 logio T = 10 logio - 12.11 



It T 



where /{ = intensity of the incident sound, 



It = intensity of the transmitted sound, and 

 T = transmittivity or transmission coefficient. ■ 

 The coefficient of transmission t is a quantity which pertains alone to the 

 partition and is independent of the acoustic properties of the rooms which 

 it separates. 



The reduction factor is the ratio of the sound energy density in the room 

 containing the sound source to the sound energy in the adjoining receiving 

 room. The reduction factor, in decibels, is given by 



R.F. = T.L. + 10 logio - 12.12 



o 



where ^ = total absorption in the receiving room, and 

 S = area of the test pattern. 



2" Raylelgh, " Theory of Sound," Macmillan Co., London. 

 2^ Eckhardt and Chrisler, Bureau of Standards, Paper No. 526. 

 ^2 Knudsen, " Architectural Acoustics," John Wiley and Sons, New York, 1932. 

 2^ Sabine, " Acoustics and Architecture," McGraw Hill Book Co., New York, 

 1932. 



^''Watson, " Acoustics of Buildings," John Wiley and Sons, New York, 1923. 

 2"^ Morrical, K. C, Jour. Acous. Soc. Amer., Vol. XI, No. 2, p. 211, 1939. 



