# Evaluation of Noise-Reduction Techniques for Gas-Turbine Test Stands: A Preliminary Analysis
## 4 Test-Bench Description  
A downscaled model was tested under anechoic conditions to evaluate the attenuator's performance. The testing stand, depicted in Figure 6, consists of a duct with a length of \(L_{duct}=5.7\) m and a rectangular cross-section with a width of \(L_{duct}=1\) m and a height of \(h_{duct}=0.5\) m. One end of the stand is closed, while the other remains open. At the closed end, inside the testing bench, an acoustic source (omnidirectional acoustic source Bruel & Kjaer 4292-L (Bruel & Kjaer, Naerum, Denmark)) was placed approximately 2 m away from the baffles, emitting pink noise in the frequency range of 50-16,000 Hz, with an overall acoustic pressure level of approximately 125 dB, thus creating a strong diffuse field inside. At the open end, approximately 2 m away from it, four microphones were placed, according to Figure 6, and the acoustic spectra were logarithmically averaged. The stand was constructed from 18-mm-thick chipboard panels with a density of 680-740 kg/m\({}^{3}\). To stiffen the test bench and reduce acoustic transmission through its walls, the chipboard panels were doubled. To minimize unwanted noise, the tests were conducted inside the anechoic chamber of INCDT COMOTI (Bucharest, Romania). The anechoic chamber (Figure 6a), designed and executed according to ISO 3745 [23] requirements, has a volume of 1200 m\({}^{3}\), with dimensions 15 \(\times\) 10 \(\times\) 8 m and with a wall absorption coefficient of 99%, in a frequency range of 150 Hz up to 20,000 Hz [24]. Figure 5: Comparison of acoustic fields in 2D vs. 3D simulations: (**a**) 130 Hz, (**b**) 1030 Hz, (**c**) 2130 Hz. Figure 6: Test bench inside the anechoic room and microphone locations: (**a**) Test bench in the anechoic room; (**b**) Perforated sandwich panel baffle inside the test bench. The baffled sandwich panels were made from basalt mineral wool, with a density of 80 kg/m\({}^{3}\), and a steel perforated sheet with a percentage of openings of \(POA=30\%\), hole diameters \(\varphi=4\) mm, and panel thickness \(b=0.5\) mm, baffle length \(L=0.6\) m, baffle height \(h=0.5\) m, and baffle width \(l=0.1\) m, as presented in Figure 6b. The literature in the field has shown that interleaving or staggering baffles leads to improved acoustic performance. Therefore, this study analyzed two attenuator models with staggered baffles and compared their acoustic attenuation results to linear baffle attenuators of the same total baffle length. Configurations II and IV, as well as configurations III and V, use the same total baffle length. However, in Configurations IV and V, the baffles are staggered. Figure 7 presents schematic representations of the tested attenuator configurations. Configuration I consisted of a single attenuator section with parallel baffles. The second configuration involved testing of two baffle sections placed end to end. Similarly, the third configuration consisted of three baffle sections placed end to end. In contrast, the fourth configuration employed two staggered sections, while configuration V utilized three staggered baffle sections. For all configurations, the distance between individual baffles and the staggered sections was maintained at \(l_{s}=0.1\) m. Additionally, the exposed edges of the attenuators (those facing the acoustic waves) were covered with perforated panels, as presented in Figure 8.  
In the present study, for the evaluation of Configurations IV and V with staggered topology, an adaptation of Embleton's method is proposed to account for the intercalated (staggered) arrangement of the baffles. Since Embleton did not present a calculation method for staggered baffle configurations, we used the calculation for the linear version and modified the formula to address the differences between the calculated and measured results. The attenuation correction for the staggered baffle configuration (Figure 9) includes three factors: \(l_{c}\)--distance addition correction, \(l_{s}\)--distance between two levels of baffles, and \(l_{y}\)--air channel width between baffles.