Measurement of Flow-Generated Noise inside Mufflers

Flow-generated noise has recently received a lot of attention within the process of designing exhaust and intake systems. Flow-generated noise can limit the amount of sound reduction a muffler can introduce inside ducts. This is more important in the modern system design where mufflers are compact and the flow speeds become higher in different sections inside the muffler. In this paper, three measurement techniques are used to measure the flow-generated noise from a duct element. The first is based on calculating the sound power levels inside a reverberation room according to ISO 3741. The radiated noise is measured from the muffler body as a source of noise, then from the tail pipe as an active one-port source. The second is based on sound power measurements inside the ducts using the active two-port theory. The third is measuring the sound pressure radiation inside an anechoic room. There has been a lot of work done to calculate the flow generated noise from simple duct elements but little has been published on full mufflers. In this paper, a compact muffler was studied. A 1D model based on the two-port theory was built for this muffler and whereas the flow-generation mechanisms were included in the active two-port formulation, and calculated using different scaling laws from literature for different duct elements such as orifices, pipes and open ends. Comparisons between simulations and measurements were performed to prove the reliability of the flow noise simulation technique.