An introduction to NASA’s broadband acoustic absorbers that resemble natural reeds

Thin, lightweight, and durable broadband acoustic absorbers capable of absorbing sounds over a wide frequency range, especially below 1000 Hz, while also surviving harsh operational conditions such as exposure to sprays of liquid and solid debris and high temperatures are desired for many noise control applications. While today’s commercially available broadband acoustic liners are impressive, such as melamine foam and perforate-over-honeycomb structures, each style has its limitations. Motivated by the need to reduce aircraft engine noise pollution NASA has recently patented a broadband acoustic absorber that claims some benefit over existing acoustic liners. Inspired by nature, these structures resemble the geometry and acoustic absorption of bundles of natural reeds, slender grasses that grow in wetlands across the world. Proof-of-concept experiments have begun at NASA. This report summarizes the design, fabrication, and normal incidence impedance tube tests performed for assemblies of natural reeds and additively-manufactured plastic prototypes that resemble the irregular geometry of bundles of natural reeds. Some synthetic prototypes were tested with and without perforated face sheets. Results indicate that there are a number of synthetic designs that exhibit substantial acoustic absorption in the frequency range of 500 Hz to 3000 Hz, and especially below 1000 Hz, as compared to baseline acoustic absorbers of similar thicknesses and weights. Many of these prototypes have an average acoustic absorption coefficient greater than 0.6. Additionally, an annular prototype was designed and printed but not yet subjected to tests. This annular prototype of a multifunctional structure designed to transfer heat and absorb sound was developed to fit inside the NASA Glenn Research Center’s DGEN Aeropropulsion Research Turbofan engine testbed. This invention can be considered and developed for a variety of aerospace, automotive, industrial, and architectural noise control applications.