Extending Acoustic Liner Bandwidth with Simple Embedded Septa

An acoustic liner design incorporating single-hole perforate septa was evaluated by the NASA Langley Liner Physics Team. The design, termed the ‘Simplified Septa’ concept, incorporates embedded septa as found in traditional, multidegree-of-freedom liners. Generally, such septa are composed of multihole perforates or porous mesh, whereas this concept uses septa with one hole per cell. An optimization scheme was employed to determine septa placement and hole diameters based on a target impedance cost function. Versions of the concept are tested in the NASA Langley Normal Incidence Tube (NIT) to determine impedance and absorption spectra for swept tonal excitation at 120 and 140 dB. Experimental results are compared to predictions from a liner model based on the Zwikker-Kosten Transmission Line (ZKTL) code and show good agreement for no-flow conditions.