Perforated panel sound absorbing systems

This thesis documents the development of a perforated panel system impedance model, and the use of this model to improve and create higher performing perforated panels by Atkar, a company who specialises in MDF (Medium Density Fibreboard) perforated panels. There are several established models published for predicting the specific acoustic impedance of perforated panels systems, especially when backed with and without a bulk porous absorber in the subsequent air cavity behind the perforated panel, or a thin resistive textile glued to the panel. However; these models, based on the equivalent electrical circuit approach (EECA), tend to under-predict the random incidence sound absorption coefficient at high frequencies (3 kHz and above). Other issues include the lack of taking into the account the impedance of the panel material itself, as well as less modern theory for the addition of a thin resistive textile into the system. The proposed model in the thesis is to use the transfer matrix method to calculate the specific acoustic impedance of each layer, which is used to improve the high frequency random incidence absorption coefficient prediction, as well as include a measured panel material impedance, which is placed in parallel with the specific acoustic impedance of the perforations of the panel, and the measurement of the specific acoustic impedance of a thin resistive textile in front of an air cavity, where the air cavity impedance is theoretically removed from measurements, leaving the impedance of the textile. This model was then used to improve the current Atkar perforated panel systems, which include the use of a thin resistive textile behind a perforated panel. It was found the currently used resistive textile is too resistive when placed behind a perforated panel, because its impedance is divided by the open area perforation ratio of the perforated panel. Because of this high resistance, introduction of a bulk porous material into the air cavity improved the random incidence sound absorption coefficient minimally. The implementation of an appropriate flow resistance textile, matched to the desired open area perforation ratio of the perforated panel, improved the random incidence sound absorption in the low to mid frequencies, as well as further improving the system when a bulk porous absorber was placed into the air cavity, as the less resistive textile allows more sound energy to penetrate the bulk porous absorber. This model was then used to theoretically model layered perforated panel systems, which include the use of multiple open area perforation ratio panels together in the one system, as well as the introduction of multiple resistive textiles, bulk porous absorbers in between the panels, and the introduction of a micro-perforated panel facing, which is a first for Atkar. Some of these higher performing panel systems were then constructed and tested in a reverberation room, where very high values of NRC were achieved. Air cavity manipulation was also used to improve the low frequency random incidence sound absorption coefficient, by creating locally reacting cavities, as well as using impedance transformers such as horns. While in theory, these systems can be very beneficial in improving the low frequency sound absorption coefficient, especially with a combination of horns in different length air cavities, these systems are not yet cost effective to use on a large scale. The use of active noise control was also modelled in conjunction with a perforated panel and a resistive textile, which resulted in little or no improvement compared to the improved and current perforated panels systems.