The shell noise radiated from the exhaust system silencer is contributing more to the global radiation from the car and it will influence the pass-by and interior noise of the car. The origin of the shell noise is mainly from the internal pressure pulsation and the mechanical vibration directly related to the engine where the first predominated as exciting force at least in the rear part of the exhaust system. A critical exciting force is built up from the formation of shock waves in the frontal parts of the exhaust system. They appear at a cold start when the speed of sound is changing due to the temperature gradient. The shock waves become critical with modern engines where the valve opening is steeper and earlier. In the critical the valve opening is steeper and earlier. In the critical silencer the first change of section should not be too abrupt, which will decrease the strength of the shock wave. Another important design parameter is the connection between the baffle and the shell. Experiments have shown that higher tension forces between the baffle and the shell will improve the shell noise. This is why hot silencers radiate less noise than cold ones. Different experimental methodologies have been tried out and the microphone measurements have been found to be the most convenient and practical to use although it is measuring the near field. Impedance measurements can be used to detect resonances in cold conditions but only relative comparisons can be made between two silencers under certain conditions. (A)
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