ERRORS IN STANDARD CONFIDENCE INTERVAL ESTIMATES AND OTHER STATISTICS, IN RELATION TO SOUND EMITTED FROM ROTATING MACHINERY, WITH APPLICATION TO A TRACTOR ENGINE COOLING FAN

摘要

Engine cooling system is the main noise source in a tractor. In an engine cooling system, where a fan operates together with a radiator/shroud in a compact engine compartment, the radiated noise is complex containing both harmonics of the blade passing frequency (BPF) and broadband noise. Existing experimental methods used to estimate the overall sound pressure level (SPL) are insufficient to characterize the complex noise structure. The goal of this work was to characterize the stochastic structure of the noise from a tractor engine cooling system using advanced statistical signal processing tools and to decide whether a given change in the engine shape has a significant effect on the noise components by hypothesis test. A standard octave band analysis indicated the most important bands. Families of common spectral estimators were used to detect tones and estimate the tonal power at multiples of the BPF of the measured noise process. A comparison is made between the confidence interval (C.I.) of the spectral estimator from our C.I. formula and from the standard formula that is utilized in commonly used spectral analysis software, such as Matlab. Our formula considers the mixed spectral nature of the process, while the standard formula assumes purely continuous spectrum. The comparison showed that the standard formula overestimates the C.I. at the tone's frequency by a factor proportional to the window size. This overestimation may fail to identify an actual significant effect of a specific design on the noise components. The C.I. of the 1/3 octave band level (OBL) was estimated with our formula and used to decide whether the presence of engine block significantly influences the noise OBLs.