A.— MATHEMATICAL AND PHYSICAL SCIENCES 55 



that the sound also falls upon flanking walls (or floors) and is so conducted 

 to other rooms. The effect, which may be appreciated by putting the 

 ear against the flanking surfaces, is likely to be of only minor importance 

 unless the walls or floors have insulating values appreciably greater than 

 that of a 9-inch brick wall. 



(ii) The insulation of floors.~The problem is more difficult in the 

 case of floors, for which the important aspect of acoustical insulation is 

 that of reducing the transmission of impact sounds such as footsteps. 

 To measure the insulation of a test floor, it is subjected to blows from a 

 set of mechanically driven hammers designed to simulate heavy footsteps. 

 The noise heard below the floor is measured subjectively by a team of 

 observers. Since no satisfactory method of determining absolute values 

 for insulation against impact sounds has yet been developed, the results 

 are necessarily comparative and show the amount by which the insulation 

 of the test floor exceeds that of some floor of ordinary construction which 

 is accepted as a standard. 



It appears that for floors, as for walls, a composite structure is a 

 necessary concomitant of good insulation. Three general methods of 

 providing such a structure have been investigated, viz. : — 



(a) To lay on the floor a soft material such as carpet on underfelt, or 

 linoleum with a sponge rubber or similar backing ; 



(b) To lay a ' floating floor,' i.e. a supplementary floor supported on 

 insulating material on the structural floor ; 



(c) To mount an insulated false ceiling below the floor, for example, 

 on insulating hooks. 



The first method provides better insulation for sharp blows than for 

 dull blows (such as heavy footsteps), but may be unacceptable on the 

 ground of expense and, in the case of working class dwellings, on account 

 of the unsuitability of the material. The second method is capable of 

 providing good insulation together with a hard upper surface at not 

 too great a cost. So far, two classes of floating floor have been examined, 

 viz. a concrete floor standing upon a number of suitably proportioned 

 rubber blocks ; and a wooden " raft " floor resting upon a continuous 

 layer of soft cushioning material such as eel-grass or glass-silk blanket. 

 Both are examples of successful floor treatment, though the underlying 

 factors are not as yet completely investigated. Experiments with the ' 

 concrete and rubber-block construction have shown that such leakage of 

 sound as occurs from the floating to the structural floor is partly through 

 the rubber supports and partly through the air interspace. A suspended 

 ceiling alone is not usually as effective as the floor treatments, but may be 

 used in combination with them to obtain a greater insulation when 

 necessary, the effects being additive. Neither does a suspended ceiling 

 isolate an impact sound and so prevent transmission to other parts of a 

 building as does a floating floor. 



The standard of acceptable insulation for a floor is commonly taken 

 as at least 15 to 20 phons better than that of a bare solid or hollow-tile 

 concrete floor for a test impact which simulates heavy footsteps. It 



