This SAE Recommended Practice establishes the instrumentation, test site, and test procedure for determining the maximum exterior sound level for snowmobiles.
Sound propagation is directly related to the ground cover and provides the largest variation to the measured result. A correction factor is introduced to improve year round test repeatability of the results on grass surfaces by correcting their spectra to be similar to snow covered ground spectrums.
Measured sound pressure levels are also highly dependent on the degree of track slip present when performing the vehicle acceleration. Operators should attempt to limit track slip as much as possible while maintaining the requirements described in 5.1.1.
Sound pressure measurements on the test surfaces described in 4.4 have been known to vary as much as 6 to 10 dBA due to atmospheric, surface condition and vehicle fluctuation. The most recent update is focused on accounting for the the variability in the surface related variation. Over 210 sets of data from 2 winters and summers were used to understand this variation in snowmobile sound testing. The correction factor for the grass surface is introduced to accommodate the surface differences from different grass test sites as well as the difference between summer and winter sound propagation. The correction factor presented is the result of using an average winter speaker response as a reference to determine the adjustment to the response measured on grass surfaces. It should be noted that the average snow surface response used may not represent all snow surface test conditions allowed within this standard. The correction factor may be updated to represent a wider set of snow conditions as more data become available. With the correction factor in place, the pass/fail criterion is much more robust from test site to test site and season to season.