NASA Langley Research Center has developed a new approach for designing aircraft liner cores for noise reduction. The cores are comprised of multiple honeycomb-shaped chambers, each of which may contain one or more embedded porous septa. By varying the resistance and insertion depth of the septa between adjacent honeycomb core chambers, the number of tuning frequencies is increased for the full liner. High-resistance septa can also be used to change the effective chamber depth. Together, these design features cause the acoustic liner to absorb sound over a broad frequency range.
Conventional broadband acoustic liners are generally limited to two-layer configurations, with the internal septa placed at a constant depth for ease of manufacturing. This constant-depth constraint limits the ability of the liner to achieve broadband noise absorption needed for today's aircraft.
In this design, the number of internal septa along with their respective insertion depths and DC flow resistance are carefully selected to achieve optimum broadband sound absorption over a targeted frequency range.