When a sound source moves, not only frequency modulation (the Doppler effect) but also amplitude modulation occurs in the radiated sound. At high velocities, these modulations can significantly impact the amplitude and A-weighted sound pressure level. However, the influence of such modulations of aerodynamic sound sources remains unquantified in the evaluation of noise metrics for high-speed railway noise. In this study, we modeled aerodynamic sound sources of high-speed railways as a sound source group of dipole point sources to represent a dipole component of aerodynamic sound, which is dominant at low Mach numbers, generated from pressure fluctuation. The other sources of railway noise (e.g. rolling noise, traction noise, impact noise, squeal noise) are ignored to focus on the effects of aerodynamic source movement. Then the noise metrics were numerically computed when the source group is moving at a constant velocity on a straight trajectory. The computed metrics were compared between cases with and without the consideration of the modulations caused by source movement. Results show that the level differences of the metrics between the cases with and without their consideration increased as the source velocity increases, reaching approximately 3 dB at Mach number of 0.3.
Makino et al. (Wed,) studied this question.