Measuring noise and setting standards

Press, Volume CXII, Issue 33095, 9 December 1972, Page 12

Measuring noise and setting standards

(By

ANGELA CROOME)

Noise pollution had low priority at the United Nations Environment Conference at Stockholm. In world terms, there were more urgent considerations to be tackled by the international delegates. Nonetheless, noise nuisance is an increasing problem with a number of international aspects.

Work on the complex character of noise has made Dr D. W. Robinson at the National Physical Laboratory, near London Airport, a world authority. The international value of noise research under Dr Robinson at the N.P.L. has come into particular prominence lately. The endeavours to achieve an acceptable noise level for cars and lorries and to introduce standards for busy airports is being tackled on a worldwide basis. Airport noise One branch of NJ*.L. noise research relates to the measurement of airport noise so that meaningful standards may be produced for the allowable din that aircraft engines may make on takeoff and landing. This is when they cause the maximum disturbance to the community. Whether it is a fine or a wet day makes more difference to the propagation of noise and therefore to people’s perception than a few decibels more or less in the absolute noise level of a jet engine. The same sort of factors apply to the propagation of motorway noise to nearby houses. Distance away and noise level are not of governing importance; a cornfield in front of the house (which tends to break up and absorb the frequencies) and garages in the gaps between houses (which reflect the sound back) have a decisive influence. Reliable prediction of what the noise burden ort people will be is the over-riding requirement. Index produced N.P.L. work has now produced a noise pollution level index (the initials quaintly come out N.P.L.I. which relates the physical description of different noise environments (comparatively easy to establish) to the subjective reaction of people (exceedingly tricky). It makes possible the prediction of noise nuisance and the reaction to it without, for in-

stance, having to build an airport handling a jumbo jet a minute to find out what the neighbours will think. Certain traffic situations are, however, too complex to permit direct extrapolation of the likely noise environment in the neighbourhood and here a mathematician at the NJP.L. unit has devised an acoustic modelling technique which accurately represents a range of conditions and the effects. The roads and buildings are scaled down 30 times and the noise source simulating the roar of moving traffic is correspondingly 30 times higher in frequency. Dr Michael Delany is seen in the accompanying photograph adjusting his model in an echo-free room before taking measurements.

Recently Britain set an example to other industrialised countries by introducing a code of practice for noisy factories which would guarantee that workers would not be deafened — as they are now — even if they worked there all their lives. Sawmills and metal-working shops are among the

noisiest industrial environments.

The code calls for an upper limit of 90 decibels in the average factory environment, but until now there has been no means of measuring the average noise level; indeed it is hard to design a noise meter of the simplest type because, for instance, 70 db. is 10,000,000 times louder than 1 db. Either you have a scale so large it is unmanageable or else small fluctuations fail to register. Meter developed A prototype noise average meter has now been developed to N.P.L. specifications which will enable factory inspectors and conscientious companies to establish whether conditions meet the code of practice. It reads from 70 db. (the level where if kept up continuously, impairment to hearing will result) to 140 db. which actually hurts. But at the same time it digests all the noises it has heard since “switch-on” and registers a running average. It is this accumulated average that matters and does the damage.

It is sometimes impossible to limit the noisiness of a factory environment. Here the only solution is for workmen to wear protective earmuffs. The effectiveness of those commercially available is difficult to measure and the photograph shows a rig where the design and reliability of ear-muffs is tested. Even when N.P.L. has perfected an ear-muff design one may suppose that it will have to launch a fashion campaign to persuade workers to wear them — “You too can be a Jet Age Pilot!” for instance. The teddy-bear look is surely a small price to pay to avoid premature loss of hearing. — From "Spectrum.”