Interior and exterior noise reduction measures were tested in
heavy vehicles. Major noise sources were identified as the engine,
transmission, body, components, rolling of tires, and aerodynamic
sources. Improvements were made on the intake and exhaust systems,
the engine compartment, and the driver's compartment by
incorporation of mineral wool. Engine performance was not
sacrificed and engine covers were not used. However, significant
results were achieved by emphasizing modifications of the engine
and transmission and without exhausting other noise-reduction
possibilities.
Tests on various, heavy vehicles indicated different relative
influence of noise sources and correspondingly different levels of
noise-reduction were achieved. Sometimes small modifications gave
significant results, sometimes major alterations were required.
Accordingly, no general noise-reduction levels can be cited. Even
adaptation of vehicles to electrical drive systems did not show
significant improvement without other simultaneous modifications.
The costs of incorporating the tested improvements into series
production were judged to be 2 to 4% of vehicle selling price.
Areas of further potential improvement include the engine fan and
the gas pedal linkage