Seatbelt retractors as important part of modern safety systems are mounted in any automotive vehicle. Their internal locking mechanism is based on mechanically sensing elements. When the vehicle is run over rough road tracks, the retractor oscillates by spatial mode shapes and its interior components are subjected to vibrations in all 6 degrees of freedoms (DOF). Functional backlash of sensing elements cause impacts with neighbouring parts and leads to weak, but persistent rattle sound, being often rated acoustically annoying in the vehicle.
Current acoustic retractor bench tests use exclusively uni-directional excitations. Therefore, a silent 2 DOF test bench is developed to investigate the effect of multi-dimensional excitation on retractor acoustics, combining two slip-tables, each driven independently by a shaker. Tests on this prototype test bench show, that cross coupling between the two perpendicular directions is less than 1%, allowing to control both directions independently. The setup generates low idle noise and is free of resonances up to 150 Hz.
N10 loudness readings from combined horizontal - vertical parameter scans with varying ratios and levels reveal the horizontal direction being loudness determining - the vertical direction has minor influence on loudness. Expert sound evaluators however judge the retractor sound characteristics from 2DOF-bench testing loading closer to in-vehicle observations. Therefore, an attempt was made to correlate in-vehicle ratings of four retractors in three different vehicle types with the sound perception on a bench. Ratings suggest, that the perceived annoyance of in-vehicle retractor rattle depends on both retractor and vehicle. As retractors with significant different acoustics were selected, a correlation could be identified. There is room for improvement as multi-sensoric effects might influence the acoustic perception as well, being yet not thoroughly understood.