There is an industrial requirement to develop a method whereby acoustic measurements taken inside a model fan rig can be processed to produce estimates of the far-field noise level and directivity. The primary application is to fan rigs tested at Anecom in Germany.
The current method, developed at the ISVR, is based on a single axial array of microphones and the so-called ‘beamformer’ algorithm to produce an in-duct directivity of the noise field as a function of frequency and the in-duct propagation angle. A computed transfer function is then used to project the in-duct noise level at each angle to the far-field. The projection relies on the application of Snell’s law to determine the mapping between the in-duct and far-field angle which is one to one in the absence of mean flow and duct area change. However the mapping of the in-duct to far-field angles, cannot, as yet, be extended into the cone-of-silence or into the forward arc. Various solutions have been considered and the most promising involves the introduction of an additional fixed azimuthal array. Measurements from the combined or ‘cross-array’’ are then processed to determine the radial and azimuthal mode statistics. A conventional radiation computation for each mode in turn will then yield the far-field at all radiation angles. Application of the beam-former method has been applied to one set of fan rig test data and the improved method is being applied in future tests at Anecom.
This work is funded by European projects VITAL (now completed) and OPENAIR.