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As new regulations constantly decrease the levels for acceptable noise, it has become increasingly important to minimize the exterior noise level of cars and trucks. As the levels approach what is technologically possible with available techniques, it becomes more important to have as much information about the noise radiating mechanism and the relationship to other mechanical processes in the vehicle. A typical passby measurement will therefore not only be a simple measurement of the maximum sound pressure level, but will include a detailed frequency analysis and order analysis, both as a function of the vehicle speed and position relative to the microphone. Simultaneously the throttle position, engine rpm and vehicle speed is measured and stored with the noise data. The result is a complete measurement record as a function of the vehicle position, with all relevant information.

The use of Kalman filter methods for high-performance order tracking of noise and vibration signals was introduced in 1993. Based on experience with that original formulation, further work has produced significant enhancements which greatly extend the ability of these methods to deal with several practical issues of concern in vehicle testing. This paper reports on advances in the areas of: RPM estimation accuracy, even for fast-changing events such as gear shifts; Higher order Kalman filters, with improved shapes for extracting modulated orders; Decoupling of close and even crossing orders by use of multiple RPM references; Significant speed improvement over the original algorithm. Besides obtaining the magnitude and phase of selected orders as a function of time or RPM, the harmonic content may be extracted as time-histories, with no phase or leakage distortion.