Low Profile, Low Frequency, Adaptively-Tuned Acoustic Liner

Conventional approaches to aircraft engine noise reduction via passive acoustic liners are limited in performance, particularly at lower frequencies, where improvements are gained through increased liner depth. Typical engine nacelle installation clearances, however, limit liner depth and prevent further improvements in low frequency noise reduction using these conventional approaches. The proposed innovation addresses these limitations via a low-profile, tunable acoustic liner for modern aircraft engines capable of significant noise attenuation at lower frequencies than currently achievable. The innovative approach lowers the resonant frequency and enables significant reductions in cavity size and volume. Significant net weight savings is achieved due to the large reductions in cavity volume (via corresponding decreases in cavity wall surface area). The end result is lower frequency noise attenuation with simultaneous reductions in liner depth and weight. The proposed innovations provide the following benefits for acoustic noise reduction: ? Optimum absorption of sound at frequencies half of those achievable with currently available technologies ? Decreased liner depth ? Decreased liner weight ? In-situ, automatic tunability for optimum absorption under different engines and engine conditions. ? Broadband operation through MDOF performance and individual impedance tuning

In February of 2010, the Aeronautics Science and Technology Subcommittee of the National Science and Technology Council (NSTC) released their biennial update on the National Aeronautics Research and Development Plan. In this report, the Energy and Environment R&D Goal #3 was stated to "Advance development of technologies and operational procedures to decrease the significant environmental impacts of the aviation system". A key element of reaching this goal was identified as the reduction of aircraft noise. Specifically, the report identifies a 32 dB target cumulative reduction in aircraft noise for the next generation aircraft (N+1), a 43 dB reduction for N+2 aircraft and a 25 year goal of 62 dB cumulative reduction for N+3 aircraft. In the NASA SBIR 2016 Phase 1 Solicitation ? Subtopic A1.02 Quiet Performance - Propulsion Noise Reduction Technology, "To reduce noise emissions from aircraft, tools and technologies are needed..." including "low-frequency liners (i.e., liners with optimum absorption frequencies half of the current ones but without increasing the liner depth)" and ?adaptive flow and noise-control technologies including smart structures for inlets, nozzles, and low-drag liners.? The proposed innovations directly address this stated need by providing the ability to absorb sound at the targeted lower frequencies, while maintaining or even decreasing the liner depth. They furthermore enable adaptive noise-control through a tunable acoustic liner impedance.

The commercial aircraft engine market is a prime target for the proposed technology. Due to the adaptive nature of the innovation, it can be applied to a wide variety of aircraft, requiring only minimal redesign, and will adapt in-situ for optimal noise suppression. With continually more stringent regulations on noise emissions from aircraft, engine and aircraft manufacturers are constantly seeking ways to reduce noise emissions while maintaining flight and fuel efficiency performance. The proposed technology provides a relatively low-cost, adaptive approach to achieving those noise reduction goals without negatively impacting weight, fuel efficiency or engine performance.

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