Sound Quality Metric Development and Application for Impulsive Engine Noise

Many engine tick and knock issues are clearly audible, yet cannot be characterized by common sound quality metrics such as time-varying loudness, sharpness, fluctuation strength, or roughness. This paper summarizes the recent development and application of an objective metric that agrees with subjective impressions of impulsive engine noise. The metric is based on a general impulsive noise model [1], consisting of a psychoacoustic processing stage followed by a transient detection stage. The psychoacoustic stage is extracted from portions of a time-varying loudness model. The primary output of the impulsive engine noise model is a time series that indicates the location and “intensity” of impulsive engine noise events.

The information in this time series is reduced either to a single number metric, or to a frequency-based vector of numbers that indicates the amount of impulsiveness in the recorded sound. The frequency-based vector is a distribution of the impulsive engine noise metric as a function of frequency. This is a new development of the impulsive noise model and has proved useful in many applications to indicate which frequency bands contribute most to the overall metric value.

An overview of the model and application to various vehicle-level powertrain sounds are presented in this report. Results show distribution of the impulsive engine noise metric as a function of frequency and its agreement with subjective assessments is discussed.