| Pass-by noise for snowmobiles as well as other vehicles is an increasing concern. Although not the only source, a good portion of the noise emitted by snowmobiles originates from the exhaust system. Space and weight restrictions on a snowmobile can make it difficult to attenuate exhaust noise to a desirable level only using passive mufflers. The use of a semi-active exhaust system, either stand alone or in conjunction with a traditional passive muffler, offers the opportunity to achieve good exhaust noise attenuation while keeping size and weight to a minimum.;The semi-active muffler approach achieves this by making the muffler element more efficient. This is accomplished by actively changing the physical parameters of the muffler. This changes its attenuation characteristics to be most beneficial for attenuating the exhaust noise at any engine speed or operating condition. Knowledge of the engine exhaust noise characteristics along with measurements of engine rpm and other parameters will allow this to be done in real time.;In this dissertation, a thorough review of muffler acoustics is presented. The 4-pole parameter representation of mufflers is reviewed. Methods are employed for determining the 4-pole parameters of mufflers using experimental and boundary element modeling techniques. Muffler performance is quantified by Transmission Loss (TL) or Insertion Loss (IL) defined in terms of the 4-pole parameters. TL results determined by models and experiments are corroborated.;A semi-active muffler design is introduced, developed, and analyzed including refinement of the design after initial engine testing. The semi-active muffler is tested for functionality in the laboratory as well on a snowmobile engine. It is shown to function as designed, demonstrating the feasibility of the semi-active muffler concept. However, the added complexities of a semi-active muffler evidenced by this study suggest that only substantial size and weight savings coupled with improved attenuation performance would make the use of a semi-active muffler worthwhile. The findings tend to indicate that this may be very difficult to achieve. Nonetheless, the information provided in this dissertation will prove useful to future design of mufflers both passive and semi-active alike. |
