Measured amplitude modulation of radiated sound from vibrational and aerodynamic sound sources moving at low Mach number

Both amplitude modulation and frequency modulation occur in a sound field radiated from moving sound sources. However, the amplitude modulation for different source types has not been clarified experimentally. In this study, the directivities of sound radiated from an ultrasonic transducer attached to a moving scaled train model and used as a simplified vibrational sound source were measured. The surface of the vibrational source was parallel to the movement direction of the source. The directivities of an aeolian tone sound generated using a cylinder attached to the train model were also measured; the cylinder was used as an aerodynamic sound source for a simplified dipole component of aerodynamic sound, which is dominant at low Mach numbers, generated from pressure fluctuation. The experimental results show that the acoustic intensity of sound radiated from a vibrational source increases roughly in proportion to the fourth power of the Doppler factor, whereas that of the dipole component of aerodynamic sound increases roughly in proportion to the eighth power of the Doppler factor. Sound radiated from an aerodynamic source moving at low Mach numbers leads to a greater amplitude modulation due to source movement compared with sound radiated from a vibrational source.