A Glossary of Air and Emissions Technologies’ Terms

Absorption: The mechanism by which acoustic energy is converted into heat by friction in absorptive media. See Pack. Airborne: A noise that is transmitted by the atmosphere and can be heard outside the car. Air Cleaner Housing: The housing that retains the air filter and forces the route of combustion air through the filter medium. Air Filter: The medium that eliminates the major particulate from the air used in the engine combustion process. Purolator is an air filter supplier. Also called an " air cleaner ". Air Gap: Air gap pipes, including manifolds, comprise a tube within a tube and a gap of generally at least 4 mm in between. Used to reduce thermal inertia for faster catalyst warm up and/or to improve heat and noise insulation. Air Meter: A sensor that measures the amount (volume) of air that passes through the intake. Often mounted in the air intake duct. Air2Air: A customized term referring to the design of the whole air management system, including the induction system, the cold-end exhaust and hot-end exhaust. The concept is that, if the air management system is designed as a whole, the outcome will be a more efficient (performance, complexity and cost) air management solution. ArvinMeritor Gas Noise Simulation (AGNES): A method of predicting exhaust gas flow noise, developed internally by ArvinMeritor. Attenuate: To lessen or decrease the amount of noise. Primarily used in reference to low-end periods. Attenuation: A measure of the noise reduction achieved by a given exhaust design. Backpressure: The total resistance to flow in the exhaust system. Typically, the lower the backpressure, the better the engine performance. Barrelly: An exhaust system noise that sounds as if the system was dumping into a tank or barrel. Bean Can: A typical bean can consists of a perforated tube, with or without insulation, enclosed by a concentric outer shell. Used for muffler tuning. Bellows: A thin metal cylinder with convolutions. It is very flexible and is used as part of flexible tube components.