高速走行する列車から発生する明かり区間圧力変動の発生源

抄録

It is known that pressure fluctuations are generated from intermediate vehicles of Shinkansen trains. Those pressure fluctuations involve the infra-sound phenomenon with a wavelength longer than several meters. Past studies showed that the infra-sound consists of the aerodynamic and structure-borne components, and the aerodynamic component is originated in the high-speed airflow around vehicles. The generating mechanism of the aerodynamic component, however, had been remained unclear. In this paper, a new methodology applicable to railway field tests is proposed to evaluate low-frequency aerodynamic sound less than 100Hz. A field test was conducted at a flat land without noise barriers, in which fifteen ultra-low frequency microphones constituting a linear microphone array were arranged in line along rails. In order to determine the frequency spectrum and distance decay rate, another five ultra-low frequency microphones were deployed at measurement points 8.7m to 50m apart from the nearest track center. The reason why the flat measurement site was selected was to suppress the bridge noise and focus on the aerodynamic noise during a train passage. On processing the microphone array signal to identify sound sources in field tests, much attention should be paid for the number of averaging; the number of averaging were limited so that sound source identification was difficult in practice. In this analysis, therefore, more than 100 trains were measured and ensemble-averaged to clarify the relationship between the number of averaging and precision of determining sound sources. Our field test campaign showed that low-frequency aerodynamic sound was locally distributed around all sections between two neighbouring bogies and pantograph sections.