Terms & Definitions
Attenuation In an exhaust system, attenuation refers to the reduction in sound level measured between an engine without a muffler and the same engine with a muffler. Backpressure Backpressure is the force necessary to flow exhaust gas through the system. Backpressure reduces horsepower and fuel economy - for turbocharged diesel engines - about 0.5% decrease in horsepower and fuel economy per inch Hg backpressure. Typically measured in inches of mercury (Hg) or inches of water (H 2 O) - 1.0" (25mm) Hg - 13.6" (345mm) H 2 O CFM Cubic feet per minute of airflow. dBA The sound level measured in decibels using a sound level meter weighted for the “A” scale frequency response. It has over 82% correlation with loudness as determined by the typical human ear (dB without the “A” scale weighting overall has only 70% correlation to loudness). Decibel (dB) A unit used to express the relative difference in acoustic power. Whenever dB is used a reference level is implied. The reference value of 0.0002 dynes/cm2 = 0dB. DOC Muffler Diesel Oxidation Catalyst (DOC) Muffler. This muffler design contains a catalyst inside that reduces emissions while reducing exhaust noise. Also referred to as a catalytic converter muffler (CCM). DPF Muffler Diesel Particulate Filter (DPF) Muffler. This muffler design contains a particulate filter that requires Ultra Low Sulfur Diesel fuel. The filter eliminates a greater amount of particulate matter compared to a DOC Muffler, but has a higher activation temperature (260ºC). Like a DOC, it also reduces emissions while reducing exhaust noise.
Dynamometer Test A test that allows an engine to be run to full horsepower and rated RPM while remaining stationary. Exhaust Noise Level Sound level of the exhaust system, typically measured in dBA. N/A Naturally aspirated Resonator A device inserted in the exhaust system ahead of the muffler that provides additional exhaust noise silencing without adding significant backpressure. Sound Level Meter A sound level measuring device that includes a microphone, an amplifier, an output meter and sound frequency networks for measuring sound levels in decibels, Total Vehicle Noise The sum of exhaust noise level measurement plus fan, intake, mechanical and tire noise. Noise levels add logarithmically (equal noise levels combined add 3 dB; example 75 dB + 75 dB = 78 dB).
12 t Medium- and Heavy-Duty Exhaust Products t 78
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