# Evaluation of a Microporous Acoustic Liner Using Advanced Noise Control Fan Engine +
## 7 Concluding Remarks  
Eight 45\({}^{\circ}\) segments of two-layer graded microporous (\(\mu\)PHB) acoustic liners (the innermost layer is made of large-size particles with an mean particle diameter of 625 \(\mu\)m and the outer layer is made of medium-size particles with an mean particle diameter of 160 \(\mu\)m), with each layer having a thickness of 25 mm and a total thickness of 50 mm, were fabricated to evaluate the microstructurally controlled graded material liner on a real ANCF engine. The liner was assembled in the aluminum rig and tested on an ANCF engine at a University of Notre Dame facility. The test conditions involved acquiring far-field acoustics data for baseline and liner configurations. Far-field acoustic measurements were acquired \begin{table}
\begin{tabular}{p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**Feature** & **Graded Microporous Liner** & **T-Liner Metamaterial Liner [33]** &
\begin{tabular}{} **Stepwise Gradient Metal** \\ **Foam Liner [34]** \\ \end{tabular} \\ \hline
**Noise Control** & Microporous controlled viscous and thermal losses & Resonance-based attenuation & Large, interconnected pores \\ \hline
**Tonal Noise Reduction** & Effective with 0.6 dB to 13 dB reduction across multiple locations (mic 19 to 23). Maximum = 13 dB & Up to 18.6 dB (mic 16) as reported by the authors Maximum = 18.6 dB & Not reported \\ \hline
**Broadband Noise Reduction** & 2 to 3 dB reduction over multiple locations (mic 19 to 23). Maximum = 3 dB & 3.7 dB at mic 16 and 2.9 dB at mic 13, no effect reported elsewhere & No or marginal reduction; enhanced noise levels with the liner at some locations, as reported by the authors. \\ \hline
**RPM Effectiveness (Tonal and Broadband)** & Effective SPL reductions at all tested RPMs (1500–2000) & Effective at 1800 RPM, as reported by the authors & Reported only 2000 RPM where SPL increased with the liner relative to baseline configuration. \\ \hline
**Performance Consistency** & Effective, reliable, and consistent noise reduction across multiple frequencies and locations & Highly location-dependent (effective at a few points) & Unstable and can enhance noise levels due to impedance mismatch \\ \hline
**Effect on Overall** & Attenuates noise effectively, aiming to improve the noise environment & Good but localized noise control and not widespread & No or marginal reduction along with enhanced SPLs with the liner at some locations \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Comparison of acoustic liners’ performance in ANCF tests. using 30 microphones placed at a 12-foot radius from the duct centerline and 10 feet high. Fifteen (1-15) of these were in an arc around the inlet exit plane (0-90\({}^{\circ}\) measured from the inlet axis, Fwd array) and fifteen (16-30) were in an arc around the exhaust exit plane (90-160\({}^{\circ}\) At array, with 180\({}^{\circ}\) being the exhaust axis). The tone and broadband SPL response data points were extracted separately by running MATLAB scripts. The evaluation of the impact of the proposed concept led to specific conclusions:  
* Fwd array microphones at the inlet exit plane recorded insignificant or no difference in SPLs between the baseline and liner, and this is expected as the microphone locations are at the inlet ahead of the liner setup. * At array microphones 19 to 23 placed at an angle of 105 to 125 deg at the exhaust recorded a significant reduction in tonal SPLs of 0.6 to 13 dB and broadband SPLs (dB loss) of about 2 to 3 dB for shaft speeds of 1500 to 2000 RPM. * At array microphones 16-18 and 24-30 recorded about 1 to 1.5 dB broadband dB loss for shaft speeds of 1500 to 2000 RPM. * Experimental ANCF testing was repeatable and consistent, with the standard deviations of the measurements being small and the margin of errors with the 95% confidence interval being quite reasonable and within industry standards. Note that the measurement also involved environmental factors like sustained wind and highway vehicle noise during the time of testing.