Identification of a rotating sound source in a duct with high spatial resolution

Abstract

To identify the major sources of noise radiated from in-duct fluid machines, one needs to observe the source parameters such as pressure and velocity fields at the source plane. For this purpose, inverse estimation methods are generally used starting from measured pressures and a sound propagation model in the duct. In this paper, a technique is suggested for identifying the rotating noise source in a wide duct. For the detailed observation of sources with high spatial resolution, the evanescent wave is considered in the modeling based on the modal summation method. Also, the Doppler effect caused by rotation of the noise source is considered in the modeling. The validation experiment is conducted on the duct system excited by a rotating loudspeaker radiating a tonal sound in the absence of flow. The measured near-field pressure precisely shows spectral peaks at shifted frequencies due to the Doppler effect. The modal amplitude set related with the rotation of the loudspeaker is estimated to investigate the source parameters. The pressure in the near-fields very close to the source is regenerated by using the estimated modal amplitudes, and the maximum error is found to be less than -10 dB. The pressure and velocity fields at the source plane are restored by the estimated modal amplitudes, and the result clearly indicate the rotating loudspeaker as the main noise radiator within the rotating reference frame.