The acoustic performance of a splitter duct silencer of the sound stream type with gourd-shaped absorbers was studied. Silencers of this type are applied to obstruct the transmission waves that pass directly through the straight airways when silencers of the parallel baffle type are used. These splitter duct attenuators are usually installed in a HVAC duct or an air passage of large sectional dimensions, and the lowest order several modes in it come into the important audible range. When the number of the far-field modes possible in all of the connecting straight ducts is M, the number of the transmission factors between every mode incidence and every mode transmission is amount to M squared. To determine these transmission factors, the incoming and outgoing wave pressures of N + 1 modes at each interface must be decomposed generally from the sound pressures at 2(N + 1) positions around each interface for M independent sound field conditions. A significant preciseness is required in the sound pressure observation to achieve this. Last year [1], we presented transmission factors obtained from sound pressures by a measurement and a BEM numerical simulation at frequencies below second mode cutoff. The agreement of these was satisfactory. However, as the number of the possible far-field modes increases, difficulty increases in meeting the preciseness in the pressure measurement. In this study, we carried out the numerical simulations rigorously to obtain precise sound pressures in the duct. Consequently, for 2D attenuators of a cound-stream type and a parallel baffle type as shown in Figure 1, we attained the purpose to determine and to compare the transmission factors between the incidence and transmission modes possible at frequencies below eighth mode cutof1.
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