SP
Technical Research Institute of Sweden
Box
857
SE-501
15 Borεs
Four different snowmobiles have been
tested using different methods. The operating conditions have been
accelerating, cruising and stationary and two different measurement distances
and two different microphone heights have been used. The maximum pass-by sound
pressure levels were measured in both 24 km/h and 50 km/h using both
time-weighting F and S. In addition the acoustic impedance of the snow and
grass covered ground was determined. The measurements were also carried out in
frequency bands and these results were used to put up an acoustic source model
of the snowmobile, which was then used to calculate the acoustic ground effect
for propagation over different distances and ground surfaces.
The most important conclusions of the
measurements and theoretical calculations are:
All
snowmobiles tested failed to meet the American requirements for maximum pass-by
sound pressure level during acceleration;
time
weighting S used by SAE-standards yields around 2 dB lower measurement values
than time-weighting F normally used for road vehicle testing;
type approval measurements should be carried out on snow.
The snow depth is not very critical as long as it exceeds 7,5
cm;
selection of measurement distance and time-weighting is not
very critical as long as the limit values are adjusted accordingly;
grassland may influence the operating conditions and should
not be used for type approval measurements;
operating
conditions should be both cruising and accelerating. The reference speed for acceleration is not critical. However, when
cruising, 50 km/h makes much more noise than 24 km/h;
subtraction of two dB from the measured value as in SAE J192 is not compatible with the European Directive on noise emission from equipment for use outdoors.