## 3 Experiment
### Cabin Noise Test Results  
#### 3.3.1 Time Domain Analysis  
The measured sound pressure values of microphones 1-4, situated at their typical positions in the 0-2000 Hz range in the cabin for 0-15 s, for the three separately installed microperforated plate composite acoustic structures are displayed in Figures 12-14. As shown, the sound pressure values at the typical positions in the cabin for all three structures show significant attenuation, and the attenuation effects of microphones 1 and 2 are generally better than those of microphones 3 and 4, which may be due to the closer proximity of microphones 3 and 4 to the noise source. To reflect the attenuation effect more intuitively, the sound pressure value was converted to SPL using the following equation:  
\[SPL=20\log(\frac{P_{e}}{P_{0}})dB \tag{28}\]  
where \(P_{e}\) and \(P_{0}\) are the RMS and standard values of the sound pressure, respectively, and \(P_{0}\) = 2 \(\times\) 10\({}^{-5}\) Pa. In the range of 0-2000 Hz, the SPL at the typical position in the cabin where the Combined MPP was installed was attenuated by 2-4 dB. Considering the design frequency range of the Combined MPP, the SPL at the typical position in the range of 500-2000 Hz was obtained using band-pass filtering. In this frequency range, the SPL at each microphone position was attenuated by 9.2 dB on average, which is equivalent to a noise reduction effect of approximately 65%. In the range of 0-2000 Hz, the SPL at the cabin location where the MPP+Porous structure was installed was attenuated by 2-3 dB. Considering the design frequency range of the MPP+Porous structure, the sound pressure signal in the range of 0-2000 Hz was band-pass-filtered, and the SPL at the typical location in the range of 500-2000 Hz was obtained. In this frequency range, the SPL at each microphone position was attenuated by 8.1 dB on average, equivalent to a noise reduction effect of approximately 61%. Figure 12: Sound pressure values at typical locations in the cabin (Combined MPP). **(a)** Microphone 1. **(b)** Microphone 2. **(c)** Microphone 3. **(d)** Microphone 4. In the range of 0-2000 Hz, the SPL at the cabin location where the MPP+PAM was installed was attenuated by 4.7 dB on average (a noise reduction effect of approximately 42%), because the noise at the engagement frequency of the low-frequency band was effectively controlled. In summary, all the microperforated plate composite acoustic structures exhibited satisfactory noise suppression effects in the target frequency range. Figure 14: Sound pressure values at typical locations in the cabin (MPP+PAM). (**a**) Microphone 1. (**b**) Microphone 2. (**c**) Microphone 3. (**d**) Microphone 4. Figure 13: Sound pressure values at typical locations in the cabin (MPP+Porous). (**a**) Microphone 1. (**b**) Microphone 2. (**c**) Microphone 3. (**d**) Microphone 4. Observing the frequency response curve in each figure, the following can be observed:  
1. Compared with the original wall panel, the three structures evidently attenuate the cabin SPL in the range of the resonance peak and its nearby frequency band, with a maximum attenuation greater than 20 dB. 2. Owing to the influence of the diffusion sound field, a new sound absorption band is formed, and the SPL with the three structures is attenuated by 10 dB, on average, over a wide frequency range of 1300-2000 Hz. The overall cabin noise level is effectively controlled. 3. The MPP+PAM attains a significant cabin SPL attenuation in the range of 300-450 Hz, with an average noise reduction effect of 45%. It also achieves a good low-frequency sound absorption effect and maintains satisfactory broadband noise reduction effects in the middle- and high-frequency bands. 4. Compared with the sound absorption performance of the ordinary MPP, those of the three structures are optimized to a certain extent. The Combined MPP substantially improves the sound absorption performance in the 600-1100 Hz and 1400-1800 Hz ranges. The MPP+Porous structure enhances the sound absorption performance in the range of 600-1200 Hz.