Acoustic absorber design

The aim of the Acoustic Absorber Project was to investigate the performance of a range of materials as acoustic absorbers. A literature search on acoustic absorbers was carried out first and is presented with a summary of commercially available absorbers in the Absorber Survey. Modifications were made to the Reverberation Room in the Department of Mechanical Engineering, University of Canterbury. Tests showed that the Room modifications and diffuser installation improved its sound field diffusivity and uniformity, ensuring reliable absorption measurements. Apparatus was then built and used to measure the flow resistance of porous materials. This equipment was pivotal to the successful specification of materials used as acoustic absorbers. More than fifty different absorbers were tested in the refurbished Reverberation Room to determine their absorption coefficients. Subsequent analysis was carried out to compare the different materials, thicknesses and systems used as absorbers. Various models were used and developed to predict the measured results. The models produced similar trends to the measured data but with lower absorption coefficients. It was found that tuned absorbers could be produced from CMSG foam with impervious films, giving high absorption in selected frequencies. Wideband absorbers could be made at low cost from low density foam, polyester or fibreglass with fabric coverings, each optimised for flow resistance. Contoured foams were also found to be very effective wideband absorbers. Optimal acoustic absorbers can now be designed and produced to satisfy different absorption requirements