Ride the Wave
Muffler design for noise attenuation
Sound is nothing more than energy traveling in a waveform. As with most forms of energy, it's possible to either redirect it or change it into another form. Sound waves from a car's exhaust are no different. A sound wave is generated every time an exhaust valve opens, releasing high-pressure gas into the exhaust port. Then the sound propagates down the exhaust pipe. If you've ever heard an engine with short open pipes, you know how loud an engine is with no sound control apparatus.
There are essentially two ways to decrease (or attenuate) exhaust noise. The first is a dissipative method. Mufflers are filled with mechanical packing: usually steel, glass or synthetic fiber. The theory is that sound travels through the fibers and transforms from wave energy to heat through friction between those fibers. It's an effective method, but sometimes requires more space than is available to achieve the desired level of attenuation. Because of their design, dissipative mufflers generally give the highest flow rates with the least backpressure. Exhaust gases move through a perforated tube (that's first wrapped in a screen to keep the packing in place). The muffler's internal volume and the size of the perforations determine frequency attenuation. Both factors are employed to tune the muffler to a specific frequency range. These mufflers are often referred to as 'turbo' or 'straight-through' and are common in aftermarket applications.
The second method is reactive. This form of attenuation reflects sound back toward the source instead of allowing it to escape from the tailpipe. There are a couple of reactive muffler forms. The first is a Helmholtz resonator, a cavity of a tuned size used to catch and reverse a specific frequency. This concept is similar to making sound by blowing over the top of a bottle. Energy is absorbed into the chamber in the first half of the cycle and then sent back during the second. Returning sound waves cancel out some new ones, decreasing total energy. It's highly effective, but only within a chosen range.
Another form of reactive noise attenuation is through a low-pass filter, often referred to as an expansion chamber. Whenever there's a change of area in an exhaust system's cross-section, some wave energy is reflected back toward its origin. A low-pass filter is not as efficient as a Helmholtz resonator, but it absorbs a greater range of frequencies. Also, a low-pass filter will still let some sound through, even from the frequency range it is designed to work in.
Many muffler manufacturers employ a combination of both methods. Helmholtz resonators and low-pass filters can be filled with packing material to give a decent amount of dissipative attenuation. OEM exhausts are designed using mostly reactive attenuation, since the mufflers are only being designed for one particular application. Aftermarket manufacturers, however, quite often have universal mufflers for use on several applications, so a design cannot be so highly specialized. Tip choice can also affect exhaust tone and volume, via changes of the cross-sectional area at the junction with the tip and also where flow goes into the atmosphere. Enthusiasts looking for small changes in sound can do some actual tuning through this method.
When choosing an aftermarket exhaust, enthusiasts can select from several different designs and prices. The muffler type chosen by a manufacturer will give an idea of intended performance. Many of the more expensive systems will offer a higher level of engineering and a more refined sound at closer-to-factory volume levels. More economical systems and those more focused on weight reduction will likely use dissipative mufflers that will be louder and may drone at certain rev levels. If you are concerned about noise and still expect maximum performance gains, it will be well worth the extra investment in a higher-end system.
-Michael Febbo