This paper reports the results of an exterior tire noise investigation whose purpose was the determination of present passenger car tire noise levels and the development of a passenger car tire noise test. Several operational and measurement parameters including road surface texture and microphone distance and height were studied. The investigation resulted in a passenger car tire noise test procedure taking into account the importance of conducting tire noise tests on a standardized road surface texture.
Fourteen sets of tires, consisting of thirteen 1975 experimental and production constructions and one ASTM skid test tire were used. Testing consisted of coasting at various speeds on three different road surfaces past microphones which were located at 25 and 50 feet from the vehicle centerline at ground level and 4 feet height. The major results were:
Road Surface Macrotexture change (Portland cement, epoxy paint, and asphalt) exhibited greater noise variation than the difference between the 11 tire constructions, which included one snow tire, tested on the same surface.
Load Variation from light (driver only) to maximum design load showed only small changes of up to 1 dBA for a particular car.
Tire Wear may be a factor for the first 200 miles. After this break-in period, variation up to the tested 1000 miles is minor.
Vehicle Speed has a major effect on tire noise; a 2 mph variation can result in up to 1 dBA change at medium speed.
Tire Noise contribution can be significant in 1979 when the vehicle acceptance limit is 75 dBA in Florida and Oregon.
In summary, surface and speed effect is significant and has to be controlled precisely for repeatable results. In spite of the wide variety of tires which included the almost treadless skid tire and a snow tire, the tire noise variance averaged only 2.5 dBA. This indicates that noise reductions beyond 2 dBA for passenger car tires will be very difficult to achieve with changes in the tread design.