NOISE CONTROL HELP BY D.S.I.R.

Taupo Times, Volume XIV, Issue 46, 15 June 1965, Page 10

NOISE CONTROL HELP BY D.S.I.R.

We live in a world that science and industry have mechanised to aid mankind, and in many cases the law of diminishing returns is coming into effect as the noise of mechanisation takes its toll on man's nerves and hearing.

In this respect advice and help have frequently been sought from those sections of the Department of Scientific and Industrial Kesearch that work in this field, especially the Auckland industrial development dlvision. I Most noises can be reduced, j to the benefit of health and ; well-being. This is important in industry where, to some extent, a manufacturer's product (whether refrigerator, motor mower or scooter) reflects good or bad public relations for both the industry concerned and science generally. The ear is extremely complex; its response varies with the frequency of a noise in a way that depends upon its loudness. Some measure of agreement has been reached internationally as to what can be taken as reasonable approximations to the ear's response. Three charaeteristics are used for soft, medium and loud noises. Because of the wide range of audible sounds, from those quieter than a ticking watch to the roar of a jet plane (noises much louder than this cause actual physical pain), sound measurements are normally made on a logarithmic scale, the unit being called a decibel. An increase U sound levels of one decibel means pressure variations"! The zero of this scale is set

at an internationally-used | pressure variation, so that a I noise level of no decibels does ! not imply silence; in fact it ! is almost within the audible j range. Background noises are important. For instance, a dripping tap can sound very loud in some circumstances. A good first test is the subjective one: What does it sound like to a critical observer ? For loud noises, the hear-ing-loss problem must be considered. The New Zealand Health Department is investigathig a scheme to protect hearing in industry wherever working conditions have noise levels about 85 decibels.

The D.S.I.R. physics and engineering laboratory at Gracefield has already measured and analysed sound in several Government industries at Wellington and New Plymouth. Local private industry has also shown much interest in this scheme. To reduce a noise, the first place to study is its source. A slight redesign may eonsiderably lower the noise level. For instance, fan-in-duced noises can be cut by 24 decibels by halving fan speeds. Other factors affecting fan noises include gaps between the stator and rotor, fan and motor bearing noises, and projections into the air flow. Each type of mechanised equipment has its own source of noise, but science has devised effective silencing methods. An automobile muffler is one example where noise is absorbed. Another method is to isolate the offendlng equipment by removing it to another room or to place baffles at strategic positions around the noise source. This is frequently possible in industry. However, not all work done for clients by the division is routine; much of it is of considerable interest. Most industrial problems involve i noisy machines in reverberant rooms or workshope, The division' s generally - adopted treatment is to make a tape [recording of the noise on a

high-quality portable machine and later analyse the tape at the laboratory. Some excessively noisy industrial factories have had the internal walls treated with a highly acousticabsorbent material, and the effect of the resulting very-much-shorter reverberation time has been remarkable. Not only are noise levels reduced, but their quality and character are changed, creating in the ear and brain a feeling of less annoyance. Another aspect of the division's work is measuring reverberation times in public rooms and churches. When a church or hall is built, it is

frequently found that the acoustic properties need improving. The division helps architects correct such deficiencies by determining reverberation times at various selected frequencies. In churches, a compromise has to be made between the best reverberation times for speech and organ music. For clear and intedligible speech, a maximum of about one second is best, but organs and choral music require two or three seconds.