MEASUKEMENT OF NOISE KAYE 



189 



In the case of the single rigid type of wall, the weight is the 

 primary factor, the insulation value (in decibels) being proportional 

 to the logarithm of the mass of the wall per square foot of area. 

 Prisoners in the castle dungeons of old could not have been greatly 

 troubled by air-borne sounds ! Figure 10 illustrates the measure- 

 ments of Davis and Littler at the National Physical Laboratory. 

 Their results, together with those of Knudsen, the Bureau of 

 Standards, and others on single partitions, are summarized in 



200 500 1000 2000 3000 



" Frecjuencij o^ Tesf Nofe (circles per second .) 



FiGCEB 10. — Insulating values of various materials for notes of different 

 frequencies. (Davis and Littler) 



Table 2 for a medium frequency (512 cycles per second). The insu- 

 lation values are in general rather less for low frequencies and rather 

 more for high frequencies. As panels transmit sound mainly by 

 diaphragm action resonance effects may come into operation at low 

 frequencies, but under normal conditions are probably of secondary 

 importance. 



As a rough working rule, doubling the mass increases the insula- 

 tion value by about 5 decibels, though resonance effects may spoil 

 the relation. 



In the case of porous flexible materials Knudsen states that the 

 insulating value is proportional to the mass of the wall per square 



