Analyses of Low-Frequency Motorcycle Noise Under Both Steady-State and Transient Operating Conditions

This paper presents experimental investigations of diagnosing and analyzing the low-frequency, low- SNR (Signal to Noise Ratio) noise sources of three motorcycles using a hybrid technology that consists of a passive SODAR (Sonic Detection And Ranging) and modified HELS (Helmholtz Equation Least Squares) methods. The former enables one to determine the precise locations of multiple sound sources in 3D space simultaneously over the entire frequency range that is consistent with a measurement microphone in non-ideal environment, where there are random background noise and unknown interfering signals. The latter enables one to reconstruct all acoustic quantities such as the acoustic pressure, acoustic intensity, time-averaged acoustic power, radiation patterns, and sound transmission paths through arbitrarily shaped vibrating structures. By combining a passive SODAR and modified HELS methods, engineers are able to reveal the low-frequency noise sources that cannot be accomplished by using other technologies. Test results demonstrate that the low-frequency motorcycle noises (< 150 Hz) are primarily emitted from the radiator area under the idle speed, and are from the clutch area during transient, for example, sudden accelerating operation condition. Comparisons of the low-frequency motorcycle noise from three different models are examined and their performances compared.