42 SECTIONAL ADDRESSES 



I20 feet lengths are now being employed on the L.N.E.R., while it is 

 stated that lengths as long as 2,700 feet have been welded together on 

 German main lines. Rail monocars with pneumatic tyres, now used 

 for certain local services in chis country, extinguish rail noises to such a 

 degree as to be almost uncanny, particularly from outside. Express-train 

 travel in this country may be experienced in its most comfortable form 

 on such trains as the ' Silver Jubilee,' with its generous use of absorbent 

 in the coaches, permanently-closed double windows and air-conditioned 

 ventilation, conversation being readily possible at all speeds. 



Whatever the acoustic difficulties may be in trains above ground, they 

 are multipled many times in an underground railway system. The 

 London Passenger Transport Board have achieved considerable success 

 in their experiments on the acuie problem of noise which is presented by 

 the tube railways. Rail-joint noise is lessened by the use of 90 feet instead 

 of 42 feet rails, which are then welded together in position. Further 

 improvement comes from the employment of a rail-grinding car, which 

 removes dents and imperfections from the rails and leaves them smooth. 

 In addition, the experiment is being tried of continuous shields, made of 

 asbestos and other materials, which project from the sides of the tunnel 

 almost to the footplates of the train, so trapping and absorbing the noise 

 coming from underneath. 



It is stated that these several measures result in an 80 per cent, diminu- 

 tion of the noise, which presumably corresponds to a reduction of about 

 7 phons. Incidental noises such as wheel, gear and motor noise are 

 reduced by the use of asbestos pads. Asbestos brake-blocks advantage- 

 ously replace cast iron, while thicker glass windows help to reduce rattle. 

 Seats of absorbent rubber or similar material, instead of cane, assist in 

 lowering the noise level particularly in relatively empty coaches. A 

 persistent type of noise in tube railways is the grating of wheel flanges 

 in passing round sharp curves. 



Noise in the Air. 



The degree of silencing which has been achieved in aircraft serves as 

 an outstanding illustration of what can be done in the case of a gross noise 

 problem under conditions so discouraging that only a few years ago it 

 was commonly regarded as unlikely to find a satisfactory solution. It 

 was important that ways and means should be found, for the noise of the 

 engine had become the most serious deterrent to areoplane travel, 

 conversation in the cabin being quite impossible. 



Since 1929 an extensive programme has been undertaken by the 

 National Physical Laboratory for the Aeronautical Research Committee 

 of the Air Ministry. There are three main sources of aeroplane noise : 

 propeller noise, exhaust noise and engine clatter. The attack on the 

 problem came in a variety of ways, for example, by using propellers with 

 lower tip speeds, by better positioning of the engine exhausts, by more 

 effective exhaust silencing, by enclosing the engines so as to reduce engine 

 clatter, by increasing the relative isolation of the engine and cabin, by a 

 better placing or shielding of the cabin with reference to the propellers, 



