Beamforming Method for Extracting the Broadband Noise Sources of Counter-Rotating Open Rotors

Counter-rotating open rotors (CRORs) are known to have advantageous propulsive efficiency properties but, at the same time, many noise emission issues related to this technology still need to be investigated. The noise they generate consists of tonal and broadband components, of which the tonal components often appear in narrow frequency bands with large amplitudes. During beamforming investigations, this often makes sorting them out from among other noise sources in the same frequency bin rather straightforward. On the other hand, the broadband noise sources can be characterized as having small amplitudes: in general, they are not the dominant noise sources in many frequency bins; but, overall, a significant noise component that needs to be investigated. The literature has provided a single microphone signal preprocessing method for removing the tonal components, hence isolating the broadband noise of a CROR: the signal of which has been shown to be appropriate for the investigation of broadband spectra. In this paper, this preprocessing method is further developed into a beamforming method that can be used in order to localize the broadband noise sources of CRORs. The resulting beamforming maps have been compared to the results of earlier broadband noise CROR studies, showing the validity of the new method and the advantages of using it as compared to methods used in other investigations.