National institute of agricultural engineering report no. 1 : Tractor and cab noise 1971 (January)

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although in some respects superficial. The discussion of noise background and its causes is good, including such sources as cavitation and “pseudo-sound”. The fifth essay, of 69 pages, is by R. W. G. Haslett of Kelvin Hughes Ltd. It is a thorough treatment based mainly on the author’s already-published and deservedly well-known work. It is perhaps useful to get it all together, but there is nothing new here. Target shape and material are taken into account, with both theoretical and experimental results; fish target strengths are dealt with from model experiments, correlated with available measurements on real fish and some theory. The short essay by V. G. Welsby of the University of Birmingham on the non-linear interaction of acoustic waves gives a valuable introduction of this subject, so rapidly growing in importance and interest. It is a mathematical treatment, but with much physical argument and a consideration of possible applications in sonar systems. The last two essays, of 23 and 14 pages, by D. M. J. P. Manley and B. Ray respectively, cover a miscellany of topics. Manley treats briefly what he calls “underwater instrumentation” -sources of underwater sound (explosive, spark, boomer); electro-acoustic transducers; telemetering systems; current, wave and tide measurement; underwater navigation systems; and side-scan sonar and mud probes. It is really too miscellaneous a collection to be very useful, but does help to complete the picture. Ray discusses audio communication between free divers, and gives a good summary of the many problems, particularly those of forming the speech itself within the diver’s environment. As can be seen, the contents of the book are patchy and unbalanced, although of good quality. It is notable, for example, that surface scattering is treated in depth, but scattering from the sea-bed only in passing. Transducers receive only brief treatment, and arrays none at all. The book is, as we said at the beginning, a collection of essays, by authors who happened to be available to give them as lectures. It must be judged as such, and not as a general textbook; and as such it can be recommended to all those concerned with underwater acoustics at a fairly advanced level. D. G. TUCKER NATIONAL INSTITUTEOF AGRICULTURAL ENGINEERING REPORTNO. 1: TRACTORANDCABNOISE 1971 (January). Wrest Park, Silsoe, Bedford: National Institute of Agricultural Engineering. Price E2. Noise in tractor cabs is becoming recognized as a significant problem in tractor design, both from the point of view of the hearing damage risk to which the driver is exposed, and the work output of the tractor-driver unit. In recent years there has been a rapid increase in the number of tractors fitted with cabs, and this has also been accompanied by a trend towards increased engine size. Tests under working conditions described in this report indicate that the noise level at the driver’s ear position on tractors without cabs is in the range 90 to 103 dBA, and that the fitting of cabs raises the levels to 95 to 110 dBA, an average increase of about 5 dBA. In some cases the increase is as much as 10 dBA. The report contains eight papers, ranging in content from an explanation of terms and units used in acoustics, through the topics of noise measurements, legal requirements, the derivation of a desirable noise limit from the hearing damage risk aspect, and discussions of the sources and means of control of the noise level. The second half of the report is devoted to display material giving a considerable number of results of tests carried out at Silsoe on noise reduction measures and the influence of cab structures. The paper, which sets out to derive a noise limit, concludes that 90 dBA is a realistic test level. This is derived both from hearing damage risk considerations and also the observed fact that drivers will not tolerate the high noise levels produced when the tractor is working

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at its maximum duty, and on average adjust their speed to give a noise level some 9 dBA below the test level. This indicates that the high noise level imposes an economic penalty on the output of the man-machine unit. The main sources of cab noise are shown to be airborne noise transmitted through the front bulkhead, noise radiated by the transmission housing and noise radiated by vibration of the cab structure, coupled with amplification by multiple reflections within the cab. Vibration isolation of the cab structure from the tractor itself can produce significant noise reductions, particularly at low frequencies. It is also desirable to incorporate a bulkhead between the cab and the engine to reduce noise radiated directly from the engine casing, and to fit a cab floor to reduce noise radiated from the transmission housing. Care must be exercised here to avoid overheating of the transmission. The basic mechanisms of engine noise production and radiation are discussed, and it is concluded that a radically redesigned engine is necessary to achieve worthwhile noise reductions. This collection of papers and data forms a useful resume and progress report on work on tractor noise up to its date of publication, and clearly demonstrates the need for reduced noise levels; at the same time it points out the difficulties, technical, political and economic, encountered in producing acceptably quiet tractors. A.H. MIDDLETON