Exploration of Bio-Inspired Acoustic Metasurface-Based Absorbers

Regulators around the world, including the United States Federal Aviation Administration, have adopted aircraft noise standards identified by the International Civil Aviation Organizations (ICAO) Committee on Aviation Environmental Protection (CAEP), driving aircraft manufacturers to produce quieter aircraft. Noise is a particular problem in aircraft, where the move toward lighter structures in both fixed and rotary wing aircraft has resulted in an increase in structural vibration and interior noise, as well as noise that can propagate to the ground and adversely affect communities surrounding airports. While vibration in the structure can be a concern at a wide range of frequencies, a relatively small amount of noise in the range 512-2048 HZ is critical to intelligibility of human speech. In order to increase the ability of those in an aircraft cabin to understand each other, methods of decreasing the noise in the cabin must be found, particularly in the range below 1000 Hz, where current acoustic liners do not perform well. Preliminary work found in the literature has indicated that it is possible to achieve complete acoustic absorption at a low frequency (125 Hz) with a thin resonant absorber. The proposed work seeks to develop thin, bio-\xad-inspired resonant acoustic absorbers with outstanding performance through developing a suite of design rules necessary to apply this technology over a broad range of frequencies. This work will investigate 3D printable curved structures because of the potential for reflections at the square corners of the design reported in the literature. Inspiration for the design to be investigated is taken from the Nautilus, a marine mollusk with a shell that is a natural example of a logarithmic spiral. The ability to selectively and effectively remove low frequency sound with structures designed for maximum effectiveness at the frequencies causing the most concern will allow a thin structure to exhibit substantially improved absorption of sound compared to conventional bulk absorbers. The goal is to develop thin, bio-inspired resonant acoustic absorbers with outstanding performance through developing a suite of design rules necessary to apply this technology over a broad range of frequencies.