Experimental study of rasping noise in internal combustion engine exhaust systems

Abstract

The abnormal noise, known as rasping noise by several exhaust muffler designers, is a metallic intermittent noise which irritates the ear and has an adverse effect on vehicle marketability. It occurs in small cars having no more than two boxes in their exhaust systems. This phenomenon is highly noticed during a rapid acceleration or deceleration in cold conditions. However, it is sharply attenuated when the exhaust system is warmed up. An experimental study has been undertaken. Several exhaust systems, ranging from a simple pipe to a complete exhaust line, are tested. Pressure measurements in the line are performed by piezoelectric transducers. The tail pipe noise measurements are performed by a microphone. It has been observed that, unlike the usual exhaust noise that contains essentially second and fourth orders, the perception of the so-called rasping noise is associated with high-frequency components in the signal and needs higher-order analysis to be fully characterized. The experiments also show the steepening of the waves when rasping noise is listened. The high axial temperature gradient causes an increasing of that effect and probably a generation of shock waves which excite the main muffler and cause the metallic noise.