A.— MATHEMATICAL AND PHYSICAL SCIENCES 45 



which naturally set a limit to the quietness attainable, ranged between 

 70 and 80 phons at 30 m.p.h. 



The noise levels recorded during the various running and racing engine 

 tests extended over a wide range — in round figures, 40 phons, i.e. from 

 75 to 115 B.S. phons, at a distance of 18 feet sideways or 25 feet to the 

 front or the rear. A consensus of opinion among the observers present 

 at the tests suggested that a transition point between tolerably noisy 

 vehicles and unduly noisy ones was in the region of 90 B.S. phons. 



In general, the following conclusions emerged from the tests on new 

 vehicles : — • 



(a) Confirming the common impression, the average present-day 

 ' ordinary ' motor car is not offensively noisy, except at high engine 

 speeds. 



(b) Certain ' sports ' cars (though not all) are too noisy but can clearly 

 be improved without much difficulty. 



(c) Except at moderate steady speeds, many motor cycles make far too 

 much noise. 



(d) Certain commercial vehicles are somewhat noisy but could probably 

 be substantially improved without a great deal of trouble. 



The Committee were therefore led to make the following recommenda- 

 tions for new vehicles which, while designed to ameliorate traffic noise in 

 all localities, take particular cognisance of ' built-up ' areas, in which regions 

 the effects of excessive noise are undoubtedly the most disturbing : — 



(i) ' No new motor cycle, motor car, heavy motor car or motor tractor, 

 as defined in Section 2 of the Road Traffic Act, 1930, shall be offered 

 for sale or for use on the public highway unless it complies with the 

 requirements stated below, the specified tests of which must be con- 

 ducted in an open space free from the proximity of buildings, etc. 



(a) ' Normal running test. — When the vehicle is running at a road 

 speed of 30 m.p.h. (or at such lower maximum speed as is 

 attainable by the vehicle or is legally prescribed for it) with the 

 engine at full throttle and using the gear preferred by the driver, 

 then the loudness at a point distant laterally 18 feet from the 

 midpoint of the vehicle shall not exceed 90 phons (B.S.). 

 Measurements shall be made on each side of the vehicle and the 

 mean taken. The running conditions specified may be realised 

 either on the level or on an incline, using appropriate accelera- 

 tion, braking, loading, etc. 



(b) ' Racing engine test. — When the vehicle is stationary and the 

 engine is running at the speed at which it would give its 

 maximum power, the loudness at a point distant 25 feet behind 

 the open end of the exhaust pipe shall not exceed 90 phons 

 (B.S.). 



(2) ' A latitude of 5 phons on the above noise limits shall be allowed 

 for a period of two years on the following classes of vehicles : — 



