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Errors in sound intensity measurements are examined for situations in which a high reactive component is present. This occurs when the indicated sound intensity level is much smaller than the sound pressure level. In these situations, significant errors in the sound intensity measurement can occur for even very small phase mismatch between microphone channels. High reactive components may be encountered in the near field of an extended source such as a body panel, when the intensity vector does not coincide with the sensitive axis of the probe, or when measurements are made in the presence of high background noise, reverberant sound fields, air flow, or standing waves. Expressions for calculating sound intensity error due to phase mismatch with high reactive components present are developed and examples of calculated error for common instrumention are provided. Cases of sound intensity measurements with known amounts of phase mismatch are also examined.

In the late 1970’s and early 1980’s, Jing-Yau Chung along with Joseph Pope published several external General Motors reports on the then novel measurement of sound intensity (SI) using the two-microphone, cross-spectral method. Application of this measurement method was then extended to sound intensity measurements in flow. Through component wind tunnel measurements, it was determined that the intensity of noise sources could be accurately measured up to a level of 15 dB below the sound pressure level generated by flow noise on microphones. An initial application of this method was to the identification of noise sources alongside rolling truck tires. It was then extended to the measurement of the aerodynamic noise generated by protrusions added to automotive vehicle designs. These included items such as outside rearview mirrors, windshield wipers, A-pillar offsets, grille whistles, roof racks, underbodies, and fixed-mast radio antennas.