Managing the acoustic signature of military vehicles can play a critical role in the safety of soldiers. Low-frequency sounds propagate through the atmosphere, resulting in unacceptable acoustic vehicle detection ranges, requiring relatively large silencer structures to mitigate. Currently, these requirements are met by using a custom muffler that is hand-assembled using low-volume prototyping manufacturing techniques. This method results in significant engineering and manufacturing time.

A parametric model of the muffler concept.

For the purposes of analysis, muffler systems may be broken up into acoustic elements that can be represented in a one-dimensional acoustic circuit analog. This circuit analog may then be solved to predict the performance characteristics of the muffler. In the case of exhaust systems, the performance characteristics are usually composed of two separate fields. The first is transfer loss, or insertion loss, which characterizes the acoustic performance of the muffler. The second is back pressure, which characterizes the restriction applied to the engine.

Once the models for the various elements in a muffler system are defined, the performance of the system can be found using the transfer matrix method that involves representing each passive element in the muffler system as a two-port element, and then finding a 2x2 matrix, known as a transfer matrix, which describes the interaction between elements. The transfer matrices describe the relationship between pressure and mass velocity through each of the ports.

In a muffler system, the circuit diagram is usually composed of a source and source impedance, an n-element chain of elements, and radiation impedance. In general, there are three different types of two-port elements. Distributed elements are long compared to the wavelengths being analyzed. This type of element is usually used to represent the piping in the muffler system. Shunt elements are elements in which the pressure field is uniform across them, but which allow mass flow to be diverted. Series elements have a constant mass flow across the ports, but cause a pressure drop.

Once each transfer matrix is found, they can be used to calculate the pressure and mass flow. Similar to calculating pressure drop across a muffler system, the back pressure created by an element may be estimated using the transfer matrix method. Once the transfer matrices have been calculated, the same method used to estimate the acoustic performance of a muffler system may be used to calculate the flow resistance.

To be an effective product, the modular software needs to severely reduce the engineering time spent designing a muffler solution. The goal of the system is to automate the procedure enough to allow an acoustics layperson to develop a full muffler solution without the aid of a NVH engineer.

A point of consideration in developing the acoustic model is that eventually a manufacturing model will need to be developed from the solution. This means that care needs to be taken so that the algorithm does not find unmanufacturable solutions. Currently, it is anticipated that simple, conservative, geometric rules will need to be programmed into the system that take into account the entered space claim. However, because it will be difficult to prove that the constraints hold in all possible cases, a collision detection algorithm should be run when the manufacturing model is generated. If interference is detected, another solution will need to be found.

Existing technology will be adapted to meet modular exhaust design needs. Parametrically limited muffler designs that are established in this effort will be used to develop meaningful lists of muffler components for specific or broad applications. These component lists provide data necessary to optimize the shop layout plan, schedule, selection, and use of machinery/tooling, and the handling of inventory and materials. Mufflers will be sized parametrically so that the manufacturability of the muffler is controlled.

Costs and lead times for modular mufflers will be significantly reduced due to several important factors. The complicated shapes, manufacturing challenges, and large amount of engineering required of custom exhaust systems has historically driven the high costs and lead time.

This work was done by Alan Hufnagel of the Army TARDEC; and Kevin Nelson, Greg Kangas, and Steve Mattson of Great Lakes Sound & Vibration. ARL-0175