# Evaluation of a Microporous Acoustic Liner Using Advanced Noise Control Fan Engine +
## 6 Test Results
### Comparison of Test Results Between Baseline and Liner Configurations
#### 6.2.2 Broadband Sound Pressure Level Response  
In the case of broadband response, the data are in the form of signals, so these measurements need to be smoothened to eliminate the noise and analyze the data. The raw broadband response of SPL versus frequency for baseline and liner configurations for microphone 21 for 2000 RPM, as shown in Figure 21, requires smoothening. As mentioned, there is a need to smoothen the data to analyze and interpret the results. Hence, the measured data were smoothened using a robust locally weighted regression (rLowess) method [31] in MATLAB. This powerful method was applied to the fatigue model for data reduction [32], as adapted here. Figure 22 shows the smoothened broadband response of SPL versus frequency for baseline and liner configurations. The gray shading represents the standard deviation for each configuration, which is minimal in both cases. The overall margin of error in the measurements is small (<2%). Due to the large volume of the data set, individual values are not reported. From the results, we see that the liner configuration reduces the  
Figure 21: Raw broadband SPL versus frequency for baseline and liner configurations. Figure 20: Tonal blade passing frequency dB loss recorded by Aft array microphone 21 at different shaft speeds [29]. noise level beyond the frequency of 500 Hz, and the reduction in noise level increases with the frequency. The broadband dB loss (\(\text{SPL}_{\text{baseline}}-\text{SPL}_{\text{liner}}\)) is noticeable beyond 500 Hz, increases with the frequency, and varies between 2 to 3 dB beyond 1300 Hz. The clear distinction between the baseline and liner test results shows the effectiveness of our microporous liner technology. 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. A dB loss for a broadband response versus frequency between the baseline and the liner; the configurations are as shown in Figure 23. A typical region of 105 to 125 deg (microphones 19, 20, 21, 22, and 23 are plotted, respectively) was analyzed. From the result, we see that for a shaft order of 20 (667 Hz), the dB loss is \(>\)1 dB and increases with a further increase in the frequency with a maximum dB loss of 2.70 dB. Also, a significant dB loss was recorded in the region of microphones 19 to 23 (highlighted in Figure 14). Based on this test result, we can design (control microstructure) the acoustic liner through appropriate selection of the particle size, distribution through the thickness, layer thickness, and total thickness to improve the broadband dB loss. Figure 23: Broadband dB loss versus frequency of Aft microphones 19 to 23. Figure 22: Smoothened broadband SPL versus frequency for baseline and liner configurations. The broadband dB loss for microphone 21 at different shaft speeds of 1500 to 2000 RPM is shown in Figure 24. From the results, we see that for the shaft order 40 and beyond, for all of the shaft speed cases, the dB loss is at least 1.5 dB and increases with an increase in the shaft order varying from 2 to 3 dB. From this, we can conclude that the liner shows similar dB loss as recorded by microphone 21 at all of the tested shaft speeds. We noticed that the liner was effective with similar recorded noise levels by microphones 19 to 23 (the highlighted region in Figure 14) at different shaft speeds of 1500 to 2000 RPM. The present results are for a two-layer graded \(\upmu\)PHB composite liner of 50 mm thickness. This paper demonstrates the idea that a passive system of 3D microstructure controlled graded porous media (bulk material system) can be effective in mitigating noise. While effective, further optimization can enhance noise attenuation. Future research can focus on the following:  
1. Layer thickness ratio--Exploration by varying the layer thickness ratios to maximize sound absorption and transmission loss. 2.