The task of reducing the noise emitted by commercial vehicles is complicated; not only are the mechano-acoustic aspects complex but there are serious commercial limitations arising from the cost of noise reduction in a very competitive industry.
Noise radiated by the engine is the principal source on present-day commercial vehicles. The principles of design of enclosures to attenuate the noise are presented.
The work of vehicle noise reduction is illustrated by an example in which the effects of exhaust silencing and various designs of engine enclosure are illustrated.
It is concluded that it is possible, but not necessarily practicable, to get down to an I.S.O. test level of about 80 dBA with the appropriate amount of engine enclosure at a weight penalty of 9 ib/dBA. However, space limitations on vehicles fitted with larger engines than those used in this work may make the fitting of an enclosure extremely difficult, and may necessitate considerable redesign of the vehicle. 80 dBA is probably the lower limit of commercial vehicle noise and is set by tyre rolling noise.
The vehicle manufacturer can make use of the techniques described above to make vehicles considerably quieter, but any acoustical treatment involves penalties in weight, cost, space, etc. For this reason it is cheaper and more effective to plan noise control treatments at the design stage than it is to modify existing vehicles.