Much more work needed on noise reduction, says RRL report
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byTony Wilding • As a review of the state of knowledge of traffic noise, and the current international situation in regard to research in the subject, the Road Research Laboratory Report RRLR357 (CM last week) is first-class. It also sets down in considerable detail knowledge from a wide variety of sources, including the results of work done on finding the effects of traffic noise on people, on sources of traffic noise and on methods of reduction.
The section dealing with the characteristics and levels of vehicle noise, its sources and methods of reduction are particularly valuable. But the report is open to possible criticism on the apparently glib and casual way in which it states that "in the long term it should be possible to reduce engine noise by about 10 dB(A) by radically redesigning the engine structure". A reduction of 10 dB(A) is truly considerable and the phrase "in the long term" is vague—it could mean five, 10 or even 20 years,
Costs and benefits
Highlighted by the report is the fact that in spite of the amount of knowledge of vehicle noise that has been built up (and detailed in the report), there are still many areas where much more work is needed. According to the RRL, basic things such as the effects of acceleration and payload on the sound level of goods vehicles "are not well understood", the mechanism of road noise generation is "not at present understood-, and so on. The report also includes in its recommendations that further research is needed into the current costs of traffic noise reduction measures and the methods of valuing the benefits from traffic noise reduction. Clearly, as in many other fields, there is going to be no improvement in the situation without costs. The working group who compiled the report do not even hazard a guess at the possible cost of measures that it suggests for cutting down traffic noise, neither are they able to put any value, either monetary or otherwise, at this stage on the benefits of having a quieter environment. Costs are obviously going to be very high. The report quotes up to £20 per foot run for noise-barrier construction for roads; while this will be a Government or local-authority charge, it will have to be passed on to the public or, more particularly, the road user.
Measures to reduce noise emanating from vehicles will also cost money and there is no possible alternative but that this must be included in the initial vehicle price.
But the report cannot be expected to cover points such as these for they are not included in its terms of reference. The preface points out that this is the first report of the working group, which consists largely of scientists and engineers from the Road Research Laboratory, central and local government, with one man from the Motor Industry Research Association representing the industry.
Other reports will presently come from the working group for as well as the first two parts of its terms of reference—reviewing the state of knowledge and current research and to identify fields were research into traffic noise needs to have increased effort—the group is committed to "co-ordinate traffic noise research within the UK and encourage the exchange of information with other countries and to advise on the application of traffic research findings".
The most interesting section of the report is that dealing with the sources, and reductions possible, of vehicle noise, It is said that extensive investigations have shown that the predominant noise from diesel engines is produced by the rapid rise in cylinder pressure following combustion. Secondary mechanical sources of noise such as pistons, injection systems and so on do little to change the overall level although in petrol engines these mechanical noises can make a significant contribution because the pressure rise is smoother. Turbo-charging a diesel engine smoothes out the abrupt rise in cylinder pressure and this is put down as the reason why these types of engine are quieter than the naturally aspirated type under most conditions.
Noise of an engine has been shown, says the report, to increase linearly with the logarithm of rotational speed. Relationships determined for naturally aspirated and turbocharged diesels and for petrol engines indicate that the advantages gained from the smoother pressure rise of the petrol and turbocharged diesel types are reduced as speed increases, says the report. An interesting fact also pointed out is that the effect of load on the cylinder-pressure diagram is very marked on a petrol engine but small on a diesel and therefore the change in noise level between the no-load and full-load conditions is rarely more than 3 dB(A) for a diesel engine but can be as high as 10 dB(A) for a petrol engine.
Recent work at the Institute of Sound and Vibration Research at the ,University of Southampton is quoted in the report as showing that the bore size is more important in determining the level of noise produced by an engine than is swept volume, as had previously been thought. A formula has been developed and it is shown that tests applied to calculate the sound level from 10 different engines have indicated that the difference between the predicted sound level and the
measured sound level is very small. '
There is also reference to work done by CAV engineers on the establishment of a radical change in engine structure design and, while the research team has established that with this change in structure there can be a 10 dB(A) reduction in sound level, a structure-research engine built by ISVR has given a 7 or 8 dBIA) reduction of level. It is said that although such an engine is quite feasible, none is yet in production.
Surveys of the noise emitted by current production vehicle diesel engines show that the noise measured on test beds at rated maximum speeds and loads lie between 99 and 110 dB(A). This is said to be a far greater variation than has been observed in the past and it is suggested that the reason is the introduction of the so-called compact engines which are specifically designed to occupy minimum overall space and which are generally "over-square" and so have high rated speeds. As a consequence, the noise from these compact engines is greater, because of the increase in bore size and also because of the higher operating speeds.
Enclosure of the engine and transmission has been found to give a useful reduction of noise, says the report, and for this reason it is recommended that the design of commercial vehicles should include engine enclosures. The report accepts the disadvantage that an enclosure interferes with access for maintenance and reduces cooling by air flow over the engine and through the radiator but, quoting some examples of what has been done, it says that a lined engine and gearbox enclosure will weigh about 100lb and give a total reduction of between 7 and 11 dB(A). More elaborate enclosuFes are said to have achieved reductions of between 15 and 20 dB(A) while an alternative method of enclosure is said to be to cover the radiating surfaces of the engine with an impervious material resiliently supported on the engine; this will be cheaper than full enclosure and reduce the cooling problems.
Manifold noise
When referring to inlet and exhaust noise from internal combustion engines, the report says that both are caused by gas columns which vibrate at high-pressure amplitudes and which communicate directly with the atmosphere. The common principle of controlling these sources of noise is to use silencers which reduce the induced pressure fluctuations. Inlet noise increases with increasing load by as much as 15 dB(A) between no-load and full-load in the case of a diesel engine—and exhaust noise can be influenced to a large extent by the characteristics of the exhaust valve and timing. The British Internal Combustion Engine Research Institute is said to have achieved between 10 and 15 dB(A) reduction of exhaust noise by attention to exhaust aspects of engine design.
In references to fan noise the report mentions that a special coupling to reduce fan speed at high engine speeds is one of the most effective ways of controlling noise from this source. It also says that aerodynamic noise is probably not significant outside the vehicle and refers to work done investigating the