Design & Development of Helmholtz Resonator for Low Frequency Exhaust Noise Reduction in Commercial Vehicles

In recent times there has been rising demand for noise level reduction in commercial vehicles. Vehicle engine exhaust system is one of the key sources of noise at driver ear, especially in smaller wheel base vehicles, as well as critical for meeting pass by noise regulations. Several techniques are used to reduce the noise level of an exhaust system such as resonators, dissipative mufflers for low & high frequencies respectively. In this paper sound transmission loss (STL) measurement for a LMD bus exhaust system was carried out at rig level. It has been found from the measured data that noise attenuation of current exhaust system is poor in low frequency zone & therefore lower STL frequencies were identified. To attenuate the noises at identified frequencies Helmholtz resonator was introduced, which is particularly effective for low frequency noise attenuation. A design is conceptualized and developed based on Helmholtz resonator calculation for target frequencies and duty cycle gas temperatures. Further, effect of designed resonators on exhaust back pressure in CFD was analyzed. Prototypes were developed & NVH performance trials were carried out at vehicle level. It has been observed from the results that noise reduction can be achieved at passenger ear level (PEL) & in pass by noise (PBN) test. This paper covers complete NVH development cycle for a real-time noise scenario i.e. measurements & analysis, design, development & final verification on vehicle level. It explains possible measures for decreasing exhaust noise and can be used as guideline for related applications.