Perception of Diesel Engine Gear Rattle Noise

Component sound quality is an important factor in the design of competitive diesel engines. One component noise that causes complaints is the gear rattle that originates in the front-of-engine gear train which drives the fuel pump and other accessories. The rattle is caused by repeated tooth impacts resulting from fluctuations in differential torsional acceleration of the driving gears. These impacts generate a broadband, impulsive noise that is often perceived as annoying. In most previous work, the overall sound quality of diesel engines has been considered without specifically focusing on predicting the perception of gear rattle. Gear rattle level has been quantified based on angular acceleration measurements, but those measurements can be difficult to perform. Here, the emphasis was on developing a metric based on subjective testing of the perception of gear rattle. In the first part of the present work, a method to simulate gear rattle noise and incorporate it into a no-gear-rattle (baseline) recording was developed. That procedure enabled controlled variation of rattle within the total engine noise signal. The simulations were then used in a psychoacoustic test that was designed to quantify detectable levels, perception of growth, and increase in annoyance due to the presence of gear rattle noise. Forty subjects participated in the threshold detection tests and a paired comparison annoyance test. The responses of people who reported having experience with diesel engines were compared to those of a more general population. The subjects with diesel engine experience were found to be better at detecting gear rattle noise and found rattle more annoying than the other subjects, particularly at high rattle levels. Current work is focused on development of metrics that accurately reflect human responses to gear rattle.