# Evaluation of a Microporous Acoustic Liner Using Advanced Noise Control Fan Engine +
## 6 Test Results
### Comparison of Test Results Between Baseline and Liner Configurations
#### 6.2.1 Tonal Sound Pressure Level Response  
Figure 17 shows the tonal SPL (dB) versus frequency for both baseline and liner configurations for microphone 21 for 2000 RPM. There are 8 \(\times\) Blade Passing Frequencies (BPFs) in addition to tonal noise at other frequencies. From the results, we did notice a reduction in sound pressure level between the baseline and liner beyond 500 Hz. Tonal noise constitutes the major part of the overall fan noise, especially the blade passing frequency (BPF), which is generally the dominating component. Therefore, a separate tonal BPF response analysis was performed to understand and interpret the effectiveness  
Figure 17: Tonal SPL versus frequency for the baseline and liner configurations. of the liner. Figure 18. shows the mean values of the tonal BPF sound pressure level of the baseline and liner configurations, along with their respective standard deviations. The standard deviation is minimal at all BPFs, indicating consistent material performance. Additionally, with a confidence interval of 95%, the margin of error was calculated and tabulated for all eight BPFs, as shown in Table 1. From Table 1, the margin of error range is quite reasonable and within industry standards; also, it involves environmental factors like sustained wind and highway vehicle noise during the time of testing. Furthermore, from Figure 18, the liner effectively reduces the SPL (\(\text{SPL}_{\text{baseline}}-\text{SPL}_{\text{liner}}\)) at all eight BPFs. A minimum of 0.6 dB at 533 Hz (1 st BPF) to a maximum of up to 8 dB at 2133 Hz (4 th BPF) as recorded by microphone 21 can be noticed. The standard deviations and margins of error at a 95% confidence interval for microphones 19 to 23 were similar to mic 21 at 2000 RPM and other RPMs. To calculate the dB loss (\(\text{SPL}_{\text{baseline}}-\text{SPL}_{\text{liner}}\)), the mean values of the respective sound pressure levels of baseline and liner configurations were used. For the five Aft array microphones 19 to 23, wherein the reduction in SPL was significant, the dB loss (\(\text{SPL}_{\text{baseline}}-\text{SPL}_{\text{liner}}\)) at the 8\(\times\) BPFs is shown in Figure 19. We see that all five microphones, 19 to 23, record significant dB loss with the liner relative to the baseline configuration at all 8\(\times\) BPFs. The tonal BPF dB loss and maximum dB loss for respective frequencies, as recorded by corresponding Aft microphones (between 19 to 23), \begin{table}
\begin{tabular}{c c c} \hline
**Blade Passing** & \multicolumn{2}{c}{**Margin of Error—95\% Confidence Interval**} \\ \cline{2-3}
**Frequency, Hz** & **Baseline Configuration** & **Linear Configuration** \\ \hline
533 (1 st BPF) & 87.09 \(\pm\) 0.15 (\(\pm\)0.18\%) & 86.54 \(\pm\) 1.26 (\(\pm\)1.46\%) \\
1067 (2 nd BPF) & 83.44 \(\pm\) 1.78 (\(\pm\)2.13\%) & 78.82 \(\pm\) 0.49 (\(\pm\)0.62\%) \\
1600 (3 rd BPF) & 77.29 \(\pm\) 1.66 (\(\pm\)2.14\%) & 74.73 \(\pm\) 0.52 (\(\pm\)0.69\%) \\
2133 (4 th BPF) & 80.62 \(\pm\) 1.02 (\(\pm\)1.26\%) & 72.93 \(\pm\) 1.88 (\(\pm\)2.58\%) \\
2667 (5 th BPF) & 78.31 \(\pm\) 0.69 (\(\pm\)0.88\%) & 71.09 \(\pm\) 0.45 (\(\pm\)0.63\%) \\
3200 (6 th BPF) & 68.78 \(\pm\) 0.98 (\(\pm\)1.43\%) & 63.87 \(\pm\) 0.95 (\(\pm\)1.48\%) \\
3733 (7 th BPF) & 65.97 \(\pm\) 2.93 (\(\pm\)4.44\%) & 63.21 \(\pm\) 2.24 (\(\pm\)3.55\%) \\
4267 (8 th BPF) & 67.25 \(\pm\) 2.10 (\(\pm\)3.13\%) & 60.31 \(\pm\) 0.41 (\(\pm\)0.67\%) \\ \hline \end{tabular}
\end{table} 
Table 1: Margin of error of microphone 21 (2000 RPM) data for baseline and liner configurations. Figure 18: Tonal blade passing frequency SPL for baseline and liner configurations. are listed in Table 2. A maximum dB loss of 13 dB was recorded by microphone 19 at the 4 th BPF (see Table 2). As mentioned, tonal noise constitutes the major part of the overall fan noise, especially the BPF noise, which is generally the most dominant component. Figure 20. shows the tonal BPF dB loss versus shaft order of microphone 21 for 8\(\times\) BPFs at turbomachine shaft speeds of 1500 to 2000 RPM with increments of 100 RPM.