Noise Path Analysis Process Evaluation of Automotive Shock Absorber Transient Noise

Shock absorber transient noise, often referred to as “chuckle” or “loose lumber”, has been a vehicle level noise and vibration concern for many years. The noise often occurs with lightly damped shock tuning under small road inputs at low speed. This transient type noise is of particular concern to the operator because it can sound like mechanical looseness in the chassis.

This noise concern is generally addressed late in the design cycle and the options of a fix are limited to a change in damper tuning or added mass. A need for a wider design envelope exists to address this concern which must include noise paths into the structure and body sensitivity.

The study documented in this paper walks through the process of acquiring this noise on the road and reproducing it in the lab on a 4-post hydraulic test rig. The noise path analysis process is then evaluated in detail for this transient type of noise to compare the results of acquiring the NPA data as a fully disconnected suspension, fully connected suspension and a reduced 3-point shock attachment. The study then evaluates the fully disconnected suspension method for a square matrix compared to an over-defined matrix. Finally a comparison of frequency versus time domain noise path analysis is performed.