SAE J2521 has been the standard way of testing for NVH problems in Passenger Car brakes for over 30 years. It is the most appropriate way of evaluating noise events from Passenger Vehicles because it identifies short duration events which other analysis systems leave out. Although these short events are not considered under normal noise evaluations to be deleterious to health and safety, they are aesthetically undesirable and damage the vehicle brand.
From a manufacturers point of view, short events are very important hence the widespread adoption of the SAE standard.
When these short events were analysed 30 years ago, the only analysis system available was spectrum analysis by FFT. This means there was a price to pay in terms of measurement uncertainty for a reduced sample size. Typically, the FFT is taken from a sampled sound bite recorded at 44.1KHz. The FFT provides a measurement accuracy of ±12 Hz frequency and ±2.4 dBs. This paper proposes a method of significant improvement in accuracy and proposes changes to the SAE J2521 specification to improve its accuracy.
Slevin Edward James M.Sc. B.Tech., Microface Limited, Ireland