An investigation of the adjustable element concept for design of automotive exhaust mufflers

The concept of the adjustable muffler is felt to be a step in the direction of easing the present tedium of constructing and testing prototype mufflers. The evaluation of such a muffler also furthers the knowledge of the interaction of acoustic elements. A review of past and present efforts in the design and evaluation of acoustic filters and filter elements is herein presented. Some consideration is given to phenomena, such as tube attenuation and transverse modes of vibration, which are encountered in muffler evaluation techniques. The characteristics of wave guides are discussed at length. The solution of the plane wave equation is presented and its applicability to standing wave tube measurements is scrutinized. The derivation of reflection factor and transmission factor equations is reviewed, and these equations are related to standing wave tube measurements. Furthermore, the standing wave tube which was used in this investigation is described. Methods of sound filtration are presented, and a theoretical foundation is constructed with a discussion of acoustic elements and their analogous mechanical and electrical components. A lumped parameter means of predicting muffler performance is also delineated, and the filtering characteristics of simple elements such as orifices and chambers are discussed. Also described are the design and evaluation of an adjustable muffler. The experimental means of evaluating the reflection and transmission characteristics are presented, and tables and graphs are incorporated to portray the effect of acoustic element adjustment on muffler performance. Furthermore, the effect of adjustment on automotive exhaust spectrums is presented and comparison of pure tone and spectral analysis is given. Inclusive in the conclusions of this investigation is the conviction that construction and evaluation of adjustable element mufflers lead to a greater understanding of the action and interaction of acoustic elements, and that this knowledge, in turn, will greatly simplify the design of silencers for specific applications --Abstract, pages ii-iii