Airborne noise reduction of agricultural tractor cabs
Graham, Neil Anthony
This thesis is concerned with the airborne noise reduction\ud of agricultural tractor cabs. The characteristics of the\ud noise radiated from a non-cab-equipped tractor have been\ud evaluated and used in conjunction with the classical\ud theories of airborne sound transmission loss to predict the\ud airborne noise reductions of tractor cabs. An experimental\ud cab has beep constructed and used to evaluate these\ud predictions.\ud The theoretical and experimental airborne noise reductions do\ud not show a good agreement. This has been shown to be due to\ud two ressons. Firstly, there are anomalies in the classical\ud airborne sound transmission loss theory whereby the theoretical\ud and experimental sound transmission loss spectra do not show\ud good agreement. Secondly, one-dimensional standing wave\ud resonances in the cab cavity have been shown to give rise to\ud an irregular plateau in the experimental airborne noise reduction\ud spectra.\ud Anomalies have been shown to exist between airborne sound\ud transmission loss theory and reverberant room transmission\ud loss measurements. These have arisen because the theory has\ud not hitherto considered panel flexure. Modifications have\ud been introduced which take account of panel flexure and these\ud show that the dependence of classical theory on the angle of\ud incident excitation is false and angular dependence occurs\ud on the transmitted side of the panel, giving rise to a\ud coincidence effect on the transmitted side which is independent\ud of the angle of incidence. These modifications give rise to\ud complete qualitative explanation of the shape of empirical\ud airborne sound transmission loss spectra. Quantitative\ud modifications have not been introduced because of lack of\ud information regarding the angular nature of the transmitted\ud waves.
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