A commercial muffler has to be designed to meet the various requirements such as acoustical, geometrical, aerodynamic, mechanical and economical criteria which become more severe as the time goes on.
The 3-pipe through-flow system was designed owing to the needs of the development of a muffler with a proper geometry meeting the minimum given space, a good acoustical performance satisfying the required minimum noise level and a minimum loss of the mechanical performance due to the back pressure of the exhaust or intake system. To investigate the more precise nature of the designed model than before(4) including the mean flow effect, a simple bench test facility was made and the experiment associated with the above-mentioned concepts was performed.
As an experimental results, in the case of the acoustically long 3-pipe resonator, when the resonator reaches a high porosity combination that is about 36.9% or more in the inner tube and is about 16.5% or more in the middle tube, the stop band is broadened to the cutoff frequency of the middle inserted tube while maintaining the maximum insertion loss(IL) the same as the simple expansion chamber or the 2-pipe resonator with high porosity. In the case of the acoustically short 3-pipe resonator, the secondary peak frequency in IL spectrum is shifted to the high frequency region under the condition that the porosity of the inner tube is fixed while that of the middle tube is varied. With mean flow the 3-pipe resonator is still more effective in the acoustical performance than the simple expansion chamber or the 2-pipe resonator in high frequency region (above 5KHz) in spite that the acoustical performance (IL) of the resonator is considerably reduced as the flow velocity is increased.
As a mechanical performance indicator, the pressure drop and the back pressure due to 3-pipe resonator is considerably small compared with the conventional 2-pipe resonator and the simple expansion chamber with the same dimensions, so it would be possible to improve the existing source-exhaust system performance. Also, it is observed that the "singing" phenomena occurred at the 2-pipe resonators with high porosities greatly reduced or removed in the 3-pipe resonator.
Therefore, the designed 3-pipe resonator model proved to be a good candidate muffler element for improving the acoustical performance in high frequency region and the source system mechanical performance without enlarging the muffler volume.
