S6 SECTIONAL ADDRESSES 



may be noted that the noise of such impacts heard below a concrete floor 

 is practically as loud as that heard above. 



(iii) Sound - absorbent treatment of rooms. — As already mentioned, it 

 is often expedient to subject the surfaces of noisy rooms to treatment 

 with sound-absorbent. This serves a double purpose. It is firstly a 

 necessary adjunct of a sound-insulating wall if it is to operate to advantage, 

 and secondly it serves to reduce the amount of noise built up by repeated 

 reflections at the room surfaces. The extent of the quietening value is, 

 however, limited, as that part of the sound heard directly is not ordinarily 

 affected. 



There is a great variety of sound-absorbent materials on the market, 

 ranging from stone and tiles with structural properties to soft flexible 

 materials. Some of the latter are of a fluffy porous nature, so that covers 

 of open texture, or perforated, or even pin-pricked are commonly pro- 

 vided. Some acoustic absorbents have coefficients as high as 90 per 

 cent, and most of them absorb high notes better than low. Meyer has 

 recently developed a non-perforated cover, which may have hygienic 

 advantages, e.g. for hospital purposes. This consists, for example, of 

 thin metal sheets mounted on a wooden framework so that they are spaced 

 an inch or two from a. wall to which is attached absorbent material. 



Finally, there are two or three other points which may require attention 

 when plans are being made for sound-insulating a building. The first 

 is machinery noise which is liable to be conducted through the structure 

 of a building unless the machinery is properly insulated. For the pur- 

 pose, it should be mounted upon an undamped elastic support so weighted 

 and proportioned that the frequency of vibration of the machinery on 

 this support is low compared'with the frequency of the noise generated. 



The second point concerns noise conducted through metal pipes, for 

 example, water pipes. While some of the noise originating in a tap or 

 a circulating pump travels through the water, much of it is directly con- 

 ducted by the piping itself. It has been found at the National Physical 

 Laboratory that a beneficial reduction (10-15 ^b) in the noise transmitted 

 along a water pipe can be achieved by replacing a few feet of the pipe 

 with rubber hose. 



A third point is the noise of ventilating fans forming part of a system 

 of artificial ventilation which is likely to be an essential accompaniment 

 of a sound-insulated building. Care should be taken that the tip speed 

 of ventilating fans does not greatly exceed 50 feet per second. Ventilating 

 ducts should be lined with sound-absorbent, and a length of canvas hose 

 may advantageously be inserted at some point in a duct. ' 



The foregoing will, I hope, afford a notion of some of the organised 

 steps which are being taken in this country to combat the evil of un- 

 necessary noise. To find practicable solutions to the many ramifications 

 of the problem is, I submit, of material significance to every section of 

 the community. 



