Sound Metric Balance of Engine Cooling Fan Noise to enable Delivery of Good Exterior Sound Quality 2011-01-1530

The drive for lower CO₂ emissions places ever greater demand on cooling dissipation for a multi-cylinder internal combustion engine. This challenge has increased the requirements of the engine cooling system, particularly in countries where high ambient temperatures prevail and HVAC usage is high. Environmental necessity coupled with market demands have resulted in cars which emit a higher level of cooling fan noise which is intrusive in an urban environment and objectionable to customers.

Conventional quantification of noise using traditional units and metrics was found to be insufficient for effective Sound Quality analysis. To assist Bentley Motors, a high performance luxury vehicle manufacturer, with its brand cachet and its commitment to the environment and customer, a new sound metric analysis has been devised to help the business deliver an ever-quieter exterior power unit cooling system.

A rig simulating a vehicle cooling fan installation has been constructed to compare Bentley fans with competitor fans. Ranking the fans in terms of overall sound pressure level for a given fan speed was found to correlate poorly with the subjectively perceived "best" fan. Sound metrics were used to characterize the fans, with "sharpness" displaying good correlation with subjective perception of tonality and "loudness" correlating well with flow noise. Furthermore, it was found that a linear balance between sharpness and loudness with increased speed led to improved perception throughout the speed range. Areas of poor balance were subjectively found to be speeds where either tonality or flow noise dominates human perception. Acoustic targets in terms of balance between sharpness and loudness were established for Bentley fans.