Acceleration Noise Metric for Vehicles with CVT Transmission 2011-01-0150

In recent years, Continuously Variable Transmissions (CVTs) have made significant inroads into passenger cars because of advantages over traditional Automatic Transmissions (ATs) such as improved efficiency, reduced weight and smoother operation. However, from an acceleration sound quality perspective, drivers generally seem to prefer the AT sound over the CVT sound, especially in sub-compact/compact vehicle segment with small displacement engines and cost-conscious sound packaging.

Vehicles equipped with ATs maintain a linear relationship between vehicle speed and engine RPM during wide-open throttle (WOT) acceleration that is dictated by fixed gear ratios. Vehicles with CVTs typically rise rapidly to a high engine RPM (near peak engine torque) and then dwell at a constant engine RPM as the vehicle speed continues to increase by varying CVT pulley ratios. This nonlinear relationship between engine speed and vehicle speed is often perceived negatively by occupants of vehicles with CVTs, particularly those mated to small displacement engines. From an acceleration sound standpoint, occupants experience an extended period of ‘droning’ or ‘humming’ sound; the powertrain sound level and pitch remains relatively constant as the vehicle speed increases during WOT acceleration.

This paper aims to provide a method of quantifying the effects on passenger perception of acceleration noise produced by the modified rpm trajectory of the CVT thereby providing a method of more accurately comparing occupant perception of acceleration noise produced by a CVT equipped vehicle with that of traditional ATs. Subjective ratings of the acceleration noise produced my multiple AT and CVT vehicles were attained and correlated with various metrics. The subjective ratings of acceleration noise showed a strong inverse correlation with the time duration of high loudness level. Application of a sound pressure level / duration tradeoff identified in the literature was examined as a method of improving setting of noise targets for CVTs.