44 SECTIONAL ADDRESSES 



the initiative and efforts of the industry itself which, as a whole, merits 

 high praise for its substantial contributions to the silence of motor 

 vehicles, for example the flexible mounting of engines, the counter- 

 balanced crank shaft, silent gearing, the straight-through type of exhaust 

 silencer, and the development of car bodies which are sound insulated 

 and sound absorbent. In the matter of both exhaust and general silencing, 

 certain motor car manufacturers have achieved remarkable perfection. 

 It is a notable comment on the success of the general quietening of cars 

 that tyre noise (about 75 phons) has become conspicuous at moderate 

 speeds, especially on particular road surfaces on wet days. Even in the 

 matter of certain motor cycles and sports cars, it must be conceded that 

 their acoustic blemishes have been largely stimulated by a section of the 

 public which happily now finds itself in a small and dwindling minority. 



The Ministry of Transport tests were carried out under conditions of 

 both use and abuse, with the object of arriving at a measure of the noise 

 potentialities of a vehicle in the hands of a driver who is prepared, on 

 occasion, to misuse it and so give rise to an objectionably large output 

 of noise, for example, by racing a stationary engine at high speed, or 

 travelling at high acceleration in a low gear. 



The Committee, while appreciating that the effect of noise on the 

 human being is partly psychological, satisfied itself that the problem 

 before it was one mainly of loudness, and that the N.P.L. objective noise 

 meter, which was used throughout the tests, provided a trustworthy 

 index of the average noise levels experienced by the observers present. 



Measurements were first conducted on stationary vehicles with racing 

 engines. These were followed by tests on running vehicles in various 

 gears at a variety of steady speeds and also when accelerating under full 

 throttle, both on the level and when climbing hills. Each vehicle was 

 driven either by the firm's driver or the owner during the tests, and the 

 goods- and passenger-carrying vehicles were tested fully laden. Some 

 of the tests were carried out on Brooklands Track, others in various parts 

 of Middlesex and Surrey under a wide diversity of favourable and un- 

 favourable weather conditions. 



In general, the measurements showed that, omnibus paribus, a vehicle 

 emits most noise at its highest speed, so that the gain in quietness from 

 the imposition of the 30 m.p.h. speed restriction in built-up areas would 

 seem to be worth while. In the case of the tests on cars and motor 

 cycles when running on the level in top gear, the average noise emission 

 at steady road speed was found to increase by about 4 phons for each 

 10 m.p.h. increment of speed, while at full throttle the corresponding 

 figure was of the order of 2 or 3 phons. In the case of cars or motor 

 cycles, the noise at 30 m.p.h. steady speed was on the average about 

 5 phons less than when accelerated at full throttle at that speed. It 

 was also established that when a vehicle was travelling at full throttle at a 

 specified speed in a given gear the noise was substantially the same whether 

 the vehicle was accelerating on the level through the instantaneous speed 

 in question, or climbing a hill at the same steady speed. 



A few tests were conducted with cars and lorries free-wheeling, i.e. 

 with the engine stopped and the gear in neutral. The loudnesses measured, 



