What's the Purpose of Exhaust Resonators?

Exhaust resonators are based on the concept of Helmholtz resonance, a phenomenon that explains resonance in a cavity.

Helmholtz resonance is best explained by an example of air being blown across the opening of an empty water bottle. When air is blown across the opening, the external force of it causes the air inside the bottle to be compressed, thereby creating an increase in pressure. This increase leads to air being expelled out of the bottle.

Now, the bottle has a rarefied lower pressure which sucks air back into the bottle. This increase and decrease in pressure in the bottle due to the displacement of air causes a continuing oscillation like that of a spring.

This principle is employed in the manufacture of exhaust resonators, in order to reduce the noise produced by the engine. Resonators are often confused with mufflers, since they have similar functions however, they are two distinct devices.

A muffler functions to simply reduce the volume of the exhaust sound, but a resonator can selectively cancel or amplify certain frequencies of sound, so as to produce a custom sound.

The cylinder has two openings on one end and is closed at the other end. It has a built in partition, towards the closed end, that has a hole in the dividing wall. The latter closed part of the cylinder is the resonance chamber and its length decides the type of sound that is produced.

Waves enter via one of the openings of the tube and exit via the other. In between entering and exiting, the wave passes through the length of the tube, encounters the aperture, enters the resonance chamber, bounces back, exits the chamber, and finally exits the tube.

● When a sound wave from the engine exhaust passes through the aperture into the resonator chamber, it is partially reflected and a part of it enters the chamber. Since, audible sound travels in the form of alternate low and high pressure waves, the waves entering and leaving the chamber are also of alternate and opposite pressure values.

The specific length of the chamber causes the exit of a wave to occur just after the entry of the succeeding wave. Theoretically, this causes the high pressure wave emerging from the chamber to coincide with the reflected part of the succeeding low pressure wave and vice versa, thereby allowing both waves to be canceled out.

● In actuality, the sound originating from the working of the machine is composed of a variety of different sound frequencies, and these frequencies change according to the change in engine speed, the theoretical function of the resonator is not achieved.

To overcome this limitation, resonators are designed in such a way that they display their functions over the frequency range across which the engine makes the most noise.

This allows the body to absorb some of the pressure pulses. In addition, the inlet and outlet to the chamber are perforated, so as to achieve numerous pressure pulses, that bounce about in the main tube. Most of these tiny pulses cancel each other or are absorbed by the body of the resonator.

The remaining waves that reach the chamber are canceled out to a certain extent via the normal functioning of the chamber. Hence, reducing a considerable amount of the generated noise.

In certain "tuned" exhaust systems, the resonance produced is used to suck out the combustion products from the combustion chamber at a specified range of speeds.

These exhaust resonators can be removed or augmented by the vehicle owners to suit their needs. They can be modified in certain ways so as to produce the sound quality that the owner desires.