# Evaluation of a Microporous Acoustic Liner Using Advanced Noise Control Fan Engine +
###### Abstract  
A novel microstructurally controlled graded micro-porous material was developed and experimentally validated for noise reduction through a normal incidence impedance test. Extensive parametric studies were conducted to understand the influence of test specimen size, particle size, porosity, pore size, and its distribution on acoustic absorption and transmission loss. Based on previous research, this study evaluates the application of graded microporous material as an acoustic liner technology for aircraft turbomachine engines. The liner was fabricated in eight 45\({}^{\circ}\) segments, assembled in an aluminum test rig, and tested on NASA Glenn Research Center's Advanced Noise Control Fan (ANCF) low-speed test bed for tonal and broadband noise. The study demonstrates that microstructurally controlled graded microporous material is very effective in dissipating sound energy with reductions in tonal sound pressure level (SPL) of 2 to 13 dB at blade passing frequencies and reductions in broadband SPL of about 2 to 3 dB for the shaft order greater than 40. While the proposed two-layer graded liner model successfully validated the concept, additional design optimization is needed to enhance performance further. This work highlights the potential of graded microporous material as next-generation acoustic liners, offering lightweight, efficient, and scalable aircraft engine noise reduction solutions. A 2025 15 4734  
The approaches to reducing aircraft noise can be categorized into two kinds: 1. Air-frame noise and 2. Engine noise. Among the two, engine noise reduction is the primary focus. The introduction of turbofans resulted in a significant reduction in conventional jet engine noise (see Figure 1a,b) [6] at that time. However, turbofans introduced a new source of noise from the compressor fan (see Figure 1b) [6]. Although the overall noise profile has been noticeably reduced, the fan noise is predominant in the broadband, low-to-mid-frequency range and produces tonal noise, which is annoying at the same frequency [7]. The current technology used in aircraft turbomachine engines is honeycomb liners with frontal perforated face sheets (see Figure 2a); another type is with wire mesh (see Figure 2b) [8]. The particular design is lightweight and strong due to the honeycomb core. This single degree of freedom (SDOF) design is a quarter-wavelength resonator, a Helmholtz resonator, or a combination and has limited ability to attenuate the broad range of noise generated by the engine. The absorption characteristic of the liner depends on the resonator depth (in this case, the liner depth) [9]. The double degree of freedom (DDOF) system (see Figure 3) [8] further increases the liner depth to achieve a second peak of sound absorption to enhance the liner's overall performance. These conventional liner technologies exhibit spatially concentrated absorption, block lateral dissipation of sound waves, limited bandwidth absorption, and strong resonance and work best only for tones [10]. Furthermore, to achieve low- and mid-frequency attenuation, the liner depth must be increased, but with the increase in liner depth, the liner's performance diminishes. Moreover, there are constraints on the size of the liners,  
Figure 1: (**a**) Conventional jet engine noise sources [6] and (**b**) turbofan engine noise sources [6]. Figure 2: (**a**) Conventional SDOF liner [8] and (**b**) wire mesh SDOF liner [8]. and an alternative approach to liner development must be explored that has the ability to increase sound absorption over a broadband frequency range in addition to tones. The extended reacting bulk liner is an alternative strategy to overcome the limitations of conventional liners with a zero-resistance face sheet, unblocked lateral wave, continuous distribution of resistance, attenuated internal wave propagation, reduced internal sound speed, subdued depth-related resonance, and improved absorption bandwidth [10]. Porous sound absorption materials are promising for the upcoming generation of turbine engine acoustic liners.