Assessment of Acoustic Liner Hole Clustering in a Normal-Incidence Impedance Tube

Characterization of nonuniform, tightly-packed, facesheet hole layouts (defined here as “hole clustering”) is shown in a normal-incidence environment, both experimentally and analytically. Nineteen additively-manufactured acoustic liner samples are tested in a normal-incidence impedance tube at various frequencies, sound pressure levels (SPLs), and source types (tonal and broadband). These samples vary in geometry, including the use of both uniformly-distributed and hole-clustered facesheets. Resonant frequency differences between these two facesheet types are documented and shown for all cases. An impedance prediction model is developed, relying on the theory of radiating pistons to properly capture impedance changes associated with hole-clustering. This includes samples with both constant- and variable-depth chambers backing the facesheet. It is shown experimentally that a nonuniformly-distributed, hole-clustered facesheet can provide improved lower-frequency absorption compared to a standard uniformly-distributed facesheet. This behavior can be accurately predicted at frequencies up to 3000 Hz and at low to mid SPLs using the developed impedance model, at least for the acoustic liner geometries considered in this study.