This Small Business Innovation Research Phase I project is aimed at developing micro-slit panel (MSP) sound absorbers that are effective over broad frequency ranges. MSP absorbers are reclaimable, non-combustible, and environmentally friendly. Traditional absorbers like foams and fibers deteriorate over time and trap oil and other contaminants. On the other hand, MSP absorbers are made from aluminum and plastic, with sound being attenuated due to viscous friction in the sub-millimeter size pores. The panels are separated from a hard surface and are most effective when the acoustic particle velocity is high in the slits. Previous work has demonstrated the viability of the materials in particular frequency bands. However, MSP absorbers will need to be effective over a broader frequency range in order for them to be used in a wider range of applications. The goal of the proposed work is to conduct measurement and numerical simulation studies aimed at improving the MSP performance by varying the slit size and porosity across a panel, and by designing a substrate that can be placed behind the panel. It is anticipated that the developed MSP absorbers will replace foams and fibers in many commercial applications.
The broader impact/commercial potential of this project will be the development micro-slit panel (MSP) absorbers that are superior to traditional fiberglass and foam absorbers. Conservative estimates for the potential volume of the product, if it were to be accepted by the architectural community, would be 20 million square feet per year. Areas of application would include ceiling deck panels in airports, train stations, bus terminals, and acoustical ducting in commercial buildings. These products would be especially advantageous in the health and food processing industries because they can be sterilized and cleaned. The materials would also benefit the automotive industry due to their light weight and reclaimability. Typical automobiles have over 50 kg of sound or vibration absorbing material. Using MSP absorbers would reduce this weight, improving fuel economy.
The NSF SBIR grant funded the research efforts of Ward Process dba American Acoustical Products (AAP) to develop micro-perforated panel (MPP) sound absorbers that will perform comparably to fibers and foams. For ordinary perforated panels, pore diameters are on the order of millimeters or even centimeters with little acoustic resistance. MPP absorbers have pore diameters sub-millimeter in size. Due to the small holes, MPP absorbers provide acoustic resistance, which enhances the sound attenuation. Unlike traditional sound absorbing materials like foam and fiber, MPP absorbers are unique because they are sustainable and reclaimable, non-combustible, rugged, fiber free, light weight, and aesthetically pleasing. However, there are two primary obstacles to the widespread acceptance of MPP absorbers. First, many believe that MPP absorbers are too costly for commercial use. This was certainly the case in the past, when holes were circular in shape and were cut using a laser. However, AAP produces lower cost but similar micro-slit panel (MSP) absorbers. Slits are non-circular and are cut or pressed into metal or plastic. The second and perhaps more important obstacle is that MPP absorbers are tuned and not broadband absorbers. The objective of the funded research was to develop MSP absorbers that will perform comparably to foams and fibers while retaining their inherent advantages. It was proposed that performance could be improved by: Spatially varying the porosity and slit size across the panel. Designing backing substrates to improve the absorption characteristics. Accordingly, the focus of the funded work was to examine each of these possibilities. It is anticipated that the absorbers will be especially advantageous for use in heating, ventilation, and air conditioning systems in schools, cruise ships, hospitals, clinics, and food processing facilities. The major findings of the funded research are as follows. Varying the effective porosity impacts the sound absorption and should be optimized per the backing substrate. However, varying the effective porosity across the panel will be insufficient in and of itself to produce a broadband sound absorber. Broadband absorption is best achieved by designing a backing substrate. The AAP team developed two suitable configurations that provided broadband absorption above 500 Hz that was a significant improvement over that for an MSP with an empty cavity. Based on these findings, the directions for a follow-on SBIR Phase II are clear. The likely objectives will be as follows Though MSP and backing substrates have been developed that provide broadband absorption, the final design for the backing substrate will need to be optimized based on both performance and cost. For the Phase I research, functionality was emphasized over cost. The Phase II proposal will place a greater emphasis on reducing cost. The MSP absorbers and backing substrates can likely be utilized most effectively if they are integrated into the system design. AAP aims to develop a suitable real world demonstration that will validate the advantages of using the MSP absorbers.
