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With the growing recognition that pavement selection can be an effective traffic noise abatement tool, there has been increased need for developing methods to characterize tire/road noise generation for existing and experimental highway surfaces. To address this need, sound intensity measured on-board a test vehicle has been developed as an alternative technique to wayside, passby, or trailer methods. As part of this development, the relationship between sound intensity measured close to a moving tire contact patch and coastby sound pressure data measured at stationary point 7.5 meters away has been demonstrated for different tires and road surfaces. A protocol for sound intensity measurement on existing highways in traffic has also been developed. Using these, a library of the tire/road noise levels has been assembled for California State Highways and experimental highway test sections.

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.