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.