Method for Measuring the Sound Intensity of Rotating Broadband Aerodynamic Noise Sources

A method was investigated for the measurement of the near field sound intensity of rotating broadband noise sources. The method used two sound intensity probes held stationary in the laboratory reference frame, which were conditionally sampled to track the motion of pre-defined rotating source regions along their circumferential path. The cross-correlation coefficient of the sound intensity for a single location in the rotating frame of reference was then calculated from the two stationary probe signals. Multiple probe spacings provided the information required for an estimate of the cross-correlation function, which was then Fourier transformed to yield the sound intensity spectrum. The method was validated using known stationary random signals, and the numerical simulation of a rotating source. The source spectrum in each case was successfully estimated despite leakage, a high variance, and a bias error in the intensity. The method was then applied to a mixed flow automotive cooling fan and the results were compared to flow measurements.