Sound wave propagation in the boundary layer around moving source

Sound wave propagation from a moving sound source, such as the Shinkansen, is affected not only by the source movement but also by the airflow around the source. Previous theoretical studies showed that variations in sound wave propagation depend on the airflow distribution. However, few experimental studies have been conducted, because the experimental study of the sound propagation from a moving sound source requires the installation of microphones, which inevitably affect the airflow. Thus, the actual effect of airflow remains unclear. In this study, we experimentally investigate the effect of airflow around a moving sound source using a parallel phase-shifting interferometer, which is a noncontact optical sound measurement method. We conducted wind tunnel tests with a scaled train model, which includes a sound source to visualize sound wave affected by a boundary layer. Additionally, we conducted moving-model tests to investigate the sound propagation around the moving scaled train model. The results show that the sound wave propagation characteristics change, and several phenomena, such as wavefront distortion, sound trapped in the boundary layer, and shadow zone formation, occur. Additionally, sound wavelength modulation and sound frequency modulation were observed in the wind tunnel and moving-model tests, respectively.