Coherent Output Power with Groups of Microphones

By considering coherence between pressure sensors in an aircraft engine core duct and microphones in the far field, estimates can be made of the relative contribution of core noise sources to the noise in far-field. For that purpose, several versions of the Coherent Output Power (COP) method have been proposed in the past. Those methods are limited to one or two degrees of freedom of the acoustic field at the location of the in-duct sensors. Effectively, this means that the application is limited to plane wave sound propagation and that higher order acoustic duct modes are not allowed. This sets an upper limit to the applicable frequency. This maximum frequency decreases with duct diameter, which means that the applicability to future UHBR geometries will be limited further. To overcome this limitation, a new COP version is proposed. This version can be applied to arbitrary numbers of in-duct sensors, and thereby to arbitrary numbers of degrees of freedom. Denoising (removing incoherent noise from the in-duct sensors) can be done a straightforward manner. The merits of the new method are demonstrated with measurements on a DGEN 380 engine. Particularly, we consider a frequency range with three degrees of freedom, i.e., three propagating broadband noise duct modes. The far-field COP results are compared with the CLEAN-SC directivity reconstruction of the noise from the core exhaust. The agreement is very good: within 1 dB for most of the far-field microphones. The far-field COP levels were further decomposed into contributions from different in-duct modes. The results look plausible.