54 SECTIONAL ADDRESSES 



relation is, of course, impaired by the presence of cracks or badly fitting 

 joints ; a surprisingly large amount of sound can be so trarismitted. 



Measurements show that to increase appreciably the insulating value 

 of a single wall, involves a prohibitive addition to the weight ; for 

 example, doubling the weight only adds rather less than 5 decibels to 

 the insulation. The minimum standard of acceptable insulation for a 

 party wall against air-borne sounds is commonly adopted as being that 

 of a plastered 9-inch brick wall (about 55 db). The main escape from 

 the weight relation lies in the use of double or composite partitions free 

 from rigid couplings. Double air-spaced partitions may be divided 

 conveniently into two classes : — 



(a) Those using light flexible materials in which the mechanical linkage 

 via the edges is small ; and 



(b) Those using heavy rigid components, in which case linkage via 

 the edges is likely to be substantial. 



The first class has been shown by Constable to exhibit resonance eflFects 

 due to coupling by the air in the interspace, so that the insulating value 

 depends on the spacing and incidentally displays a minimum. The 

 insulation is improved by introducing sound absorbent material in the 

 interspace in such a manner that it does not act as a link between 

 the components. As a practical illustration of these observations, it is 

 recommended for good insulation that double windows should have a 

 sound-absorbent lining at the boundary of the interspace, and that the 

 spacing should exceed a certain minimum, e.g. 4 inches for 21 oz. glass, 

 a properly constructed double window being as sound proof as a 9-inch 

 brick wall. For thicker glass the separation can be reduced. Double 

 windows conforming to these requirements are now commercially avail- 

 able, some of which permit the windows to be opened and still afford an 

 acceptable degree of insulation. 



In the case of double partitions constructed from heavy rigid materials, 

 coupling via the edges dominates that due to the air, so that the insulating 

 value can be increased appreciably by framing the components with 

 insulation, e.g. cork strip, round their margins. For example, a cavity 

 partition consisting of two 2-inch clinker slabs separated by a 2-inch air 

 gap and marginally insulated by cork is acoustically as effective as a solid 

 9-inch brick wall and costs appreciably less, though if the insulation is 

 omitted, the partition is little better than a single wall of the same total 

 weight. 



Recent tests show that the insulating value of a single solid wall benefits 

 by the application to both sides of plastered building-board fixed to 

 battens secured by insulating clips. It is of considerable interest to note 

 that the traditional partition of lath and plaster on each side of wooden 

 studding (or a similar partition with fibre board) is definitely superior to 

 a single partition of the same weight. A fire-resisting version consisting 

 of plastered expanded metal on concrete studding affords insulation equal 

 to that of a plastered 9-inch brick wall (of three times the weight). 



It should be noted that the insulating value of a wall (or floor) against 

 air-borne sounds originating in a room is often set a limit by the fact 



