Measurement and simulation study of an exhaust system noise complaint

A noise complaint was issued during a 1000 hour endurance test for a Wärtsilä W32E engine with cylinder pressure 235 bar. Sound measurements were taken, and the cause of the complaint was determined to be the first stage exhaust silencer, which radiates noise in the 100 Hz third-octave band (i.e. engine order 7.5). Actran finite element software was used to create a vibro- Acoustic model of the exhaust silencer in order to investigate the noise problem identified as occurring at 94 Hz. The simulations indicated that a combination of factors resulted in the noise problem; the engine load, which determines the temperature in the ductwork and silencer, the engine speed, which determines the engine order frequencies that are excited, and the length of the duct leading to the silencer, having a resonance at 94 Hz if engine load is at 100%. Several potential silencer modifications were simulated, and an extension of the silencer inlet was proposed. The silencer was thus modified, and the sound measurements were repeated. The result was a 10 dB decrease in total sound level measured around the silencer exterior.