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NOISE is virtually a continuous experience to those of normal hearing. Even a person living alone on top of a mountain and waking up in the middle of the night will hear a noise of some kind, It may be the wind, or a bird, or his own breathing. To the resident of a house on a busy main road, the night-long roar of traffic normally drowns such noises and they will go unnoticed. But to a hypochondriac living in a quiet road (or on top of a mountain) they could on occasion be sleep-preventing phenomena. Noise is. therefore, unwanted sound rather than something that can be given an exact value. It is a subjective experience.
Loudness in decibels (dB) can be measured with a meter; to measure a noise it is necessary to regulate the meter reading to comply with normal public reaction to noise as decided by a group of subjects representing a typical cross-section of the public.
Most people are less responsive to noises of low frequency and high frequency than to those of medium frequency; for example, a noise of given loudness that is produced by a jet plane is less offensive to the average human than a noise of equal loudness produced by a vehicle at a medium frequency. Only a specialist with a scientific knowledge of the subject is capable of understanding the highly complex factors involved and such specialists have, on their own admission, a lot to learn about the exact nature of the many types of noise produced by a diesel or petrol engine.
A paper on "The interpretation of noise measurements" presented during a conference on noise at Southampton University in March by Mr. D. C. Lavender of the Motor Industry Research Association (CM March 28 1969) would probably have been the least interesting to the average vehicle operator of any of a number of papers that dealt with motor vehicles. But the paper was important in a practical way, because it explained the necessity of "weighting" a noise meter to adjust its response to comply with the normal human response. An understanding of this necessity is basic to an appreciation of the noise problem arising from the use of motor vehicles or any other type of machine.
There are three internationally standardized weighting networks that can be applied to a meter in (according to Mr. Lavender) an endeavour to match the response of the instrument to the response of the human ear. These comprise "A", "B" and "C" weightings, the -A" in dBA indicating that the meter is conditioned by an A weighting network to give an appropriate response relative to human response. An A weighting reduces (or attenuates) the noise reading recorded when the noise has a low frequency compared with the dB reading and slightly increases the reading at the medium frequencies to which humans are more keenly responsive.
Experiments
Mr. Lavender described a number of experiments involving commercial vehicles and claimed that a simple sound-level meter with an A weighting network gave satisfactory results with regard to the "approximate public reaction to noise". The meter gives a correct rank order for noises having similar spectra.
This supports the general contention that the A weighting is more realistic than the B or C weightings and it is likely, therefore, that the dBA noise rating will be employed for many years to come. It should be appreciated, however, that subjective response is a variable that can influence the individual's opinion of the acceptability of a noise and it is pertinent that the public is becoming more noiseconscious. It is possible that in years to come the public will demand a lower level of noise.
The prospect of strict enforcement of the noise regulations has already had an effect on the design of diesel engines, notably in the case of the Ford Turbo 360 diesel. As pointed out by Mr. F. Spellacy of the Ford Company, who also presented a paper at the conference, this turbocharged unit operates at 400 rpm less than its naturally-aspirated equivalent. It develops 17 per cent more power.
It would seem highly probable that Ford would have uprated the 360 by increasing the .rpm in preference to turbochargeing the unit if this had not been unacceptable on account of the noise problem. Radiated mechanical noise increases with speed and is more difficult to control than any other type of noise.
Looking into the more distant future Dr. T. Priede of the Institute of Sound and Vibration Research mentioned the possibility of engine outputs being increased by turbocharging up to four times the existing average as a means of reducing the noise produced by engines of a given output. This is an exciting prospect, and it is to be hoped that if engines of this calibre are eventually produced the public will approve. In due course it may be necessary to cater for a different range of public susceptibilities and to apply a different kind of weighting network to noise meters.