Multiple drumlike silencer for low frequency duct noise reflection

Low frequency duct noise is difficult to tackle by passive means within a certain space limitation. Drumlike silencer offers broadband sound reflection in the low-frequency region, e.g. 0.1–0.3 times the first cut-on frequency of the central duct. Optimization was carried out for the logarithmic bandwidth in which the transmission loss (TL) is greater than a pre-determined criterion value. The prediction was validated in laboratory but it was anticipated that the required high level of tension is difficult to implement in practice. It is found that the optimal tension is approximately proportional to the 3rd power of membrane length, so a partitioned membrane is expected to reduce the required tension. In order to optimize the partitioned drumlike silencer, a new cost function of total transmission loss, or insertion loss, with a frequency weighting in favour of a certain wide band of low frequencies is introduced. The new cost function removes the undesirable discontinuous behaviour occurring in the previous optimization process. It is also found that the partitioned silencer can drastically improve the weighted total TL and the tension required is indeed much reduced. The main performance improvement derives from around the lower frequency limit of interest.