Muffler operation

Model Airplane News, Jun 2000 by Gierke, Dave

A successful muffler must reduce pressure fluctuations in the engine's exhaust gas-acoustically. Discounting sound-canceling systems, two primary muffler types accomplish this: the expansion chamber and the resonator.

EXPANSION CHAMBERS

The simplest expansion chamber is the Helmholz chamber (Figure 1) that, if properly designed, works remarkably well despite its obvious simplicity. A sophisticated variant is shown in Figure 2 and has proven to be a very effective solution. Single chambers, double chambers, double chambers with external connecting tubes (Figure 3), and double chambers with internal connecting baffles (Figure 4) were all designed in accordance with the "plane-wave theory." This theory assumes that sound is transmitted through a tube in the form of a one-- dimensional plane wave. Wherever the tubes encounter sectional area changes, part of the sound is transmitted along the tube, and part is reflected toward its source. The reflected component represents a reduction in sound. Expansion-- chamber mufflers reduce noise by taking advantage of these reflections. The theory also says that below a certain frequency (known as the "cut-off frequency") expansion-chamber mufflers are relatively ineffective. To be effective, these chambers must be designed for an rpm band (range) that extends to the engine's operational upper limit.

RESONATORS

Mufflers of the single-chamberresonator type (Figure 5) consist of a resonant chamber that is connected in parallel with the exhaust pipe by one or more tubes or orifices. Resonance refers to a reinforcement of vibrating sound in a tube caused by waves from another chamber vibrating at nearly the same rate. In certain frequency ranges, as determined by the engine's operating speed, the impedance (for our purpose, resistance) at the connector is much lower than the tailpipe's impedance. The resonant chamber then acts as a short circuit that reflects most of the incident sound toward the source (as with the expansion chamber). Therefore, the amount of sound energy that is permitted to go beyond the muffler into the tailpipe is reduced. In multi-chamber resonators (Figure 6), engineers use equations to determine orifice impedances while simultaneously incorporating the plane-wave theory to determine the action of the central tube. Multiple resonators, like multiple expansion chambers, have a cutoff frequency.

Of course, there are combinations of expansion and resonant chambers that offer the potential for improved muffler performance by combining the attributes of both types (Figure 7).

Muffler design is a complicated, timeconsuming process and requires years of specialized training in many fields. But theory rarely translates into instantaneously successful sound-attenuating hardware. This is where the experimenter/entrepreneur fits into the picture. Exercising empirical methods, small companies have dominated the muffler aftermarket. This isn't all bad, as some remarkable designs have been produced, especially over the past two decades. The truth is, the muffler isn't the problem any longer; the propeller is the limiting factor for further reductions in noise!

For further reading: Sound and Model Aeronautics (1991) from The Academy Of Model Aeronautics; "Hey! Keep the Noise Down!," D. Gierke, Model Airplane News, December 1995; "Sound Advice From Europe," Paulson, Model Airplane News, November and December 1995.