Program for the computation of absorptive silencers in straight ducts

Abstract

The design of silencers is frequently based on results from standardised measurements which are assembled in the form of a data-base. Some silencer manufacturers offer their clients programs on diskette (free of charge) which employ such data-bases, together with interpolation between available data points, to aid silencer selection for specified applications. However, such programs cannot be applied if the required operating parameters are outside the data-base range. Similar comments apply when using the so-called trapezoid diagrams from the relevant literature. These present in simplified form the results of analytical procedures, but in a manner simple enough for calculations using personal computers. In this paper a computer program is presented which allows the calculation of the insertion loss of splitter silencers in such a way that the results are comparable to results measured according to ISO 7235. Different methods were used for the evaluation of the components that contribute to the insertion loss. The propagation loss in the air passage due to absorption by the splitter material is obtained using analytical calculation procedures. The amount of reflected sound at the splitters is determined from a large pool of insertion loss data previously measured according to the standards at the Institut für Bauphysik (IBP). The data pool is analysed using regression methods in order to obtain statistically the components of propagation loss and reflection loss. From this analysis, approximate formulae are derived for the reflection loss and a correction is obtained which accounts for the bypass transmission along the splitter frame. This correction is used to modify the propagation loss for the calculation of the insertion loss. The calculated insertion loss is then obtained by the sum of the propagation loss, the reflection loss and the bypass correction. Comparison of calculated and measured insertion loss is presented, showing good agreement. A user programme for the calculation of silencers on personal computers was developed from these procedures and has been applied already in a bigger project ( Eckoldt, D. et al., IBP-Bericht B-TA 3/1993, 1993).