# Evaluation of Noise-Reduction Techniques for Gas-Turbine Test Stands: A Preliminary Analysis
## 6 Results  
### Noise-Attenuation Assessment--Without Flow  
The following section presents the acoustic results obtained from the test-bench measurements and the application of the two previously described methods. The following images illustrate the test results for various attenuator configurations. As shown in the graph in Figure 10, attenuation rapidly decreases at higher frequencies. This phenomenon, described by [PERSON] as the \"beaming effect\", occurs because high-frequency sound waves tend to travel directly through the gaps between the baffles. Consequently, they bypass the sound-absorbing material. [PERSON] suggests that high frequencies are not effectively absorbed due to this direct path. Instead of interacting with the absorptive material, the sound waves essentially travel the gaps with minimal influence, avoid absorption, and continue to propagate. This effect does not take place in a staggered configuration, where the insertion loss is higher at high frequencies. When analyzing Figure 10, we see that [PERSON]'s method generally aligns with the attenuation curves. However, a sharp decrease in predicted attenuation becomes evident at higher frequencies. The experimental results demonstrate that even for configurations with linearly arranged baffles, high-frequency acoustic waves (>6 kHz) undergo significant acoustic attenuation; finite-element numerical simulations capture acoustic attenuations at high frequencies very well. We can mention that while simulations and measurements show the maximum attenuation peak centered around 1.5 kHz, the Embleton curve has its central frequency around 2 kHz. This shift of approximately 500 Hz is mainly due to the characteristics of the mineral wool of which the baffles are made. As we have already mentioned, [PERSON]'s method ignores any changes in the density of the mineral wool. In the case of the attenuator configurations with linearly arranged baffles, the numerical simulations prove a high degree of fidelity to the measurements. However, [PERSON]'s method tends to overestimate attenuation as the length of the attenuator increases. For the staggered baffle configuration, the finite-element method predicts higher attenuation values than the measurements. This difference likely arises from two factors. First, the CAD model assumes perfect baffle positioning, whereas real-world installations may involve slight deviations due to manufacturing tolerances or assembly processes. These deviations can introduce imperfections that reduce the effectiveness of the baffles. Second, high attenuation in staggered baffle configurations can make measurements susceptible to noise from collateral sound paths. This additional noise can contribute to lower measured attenuation values. One last aspect to note is that numerical simulation achieves good correlation even at low frequencies, which is very important, especially in applications where such attenuators are used. Figure 11 presents a comparison graph of the average insertion loss for all configurations investigated in this study. Since the primary focus of this research was to identify the optimal configuration for a sound attenuator in a gas-turbine test stand, a three-baffle staggered arrangement emerges as the most suitable choice. Figure 10: Comparison of the insertion loss IL [dB] results between measurements and numerical modeling: (**a**) Conf. **I—**single baffled_linear; (**b**) Conf. **II—**double baffled_linear; (**c**) Conf. **III—**triple baffled_linear; (**d**) Conf. **IV—**double baffled_staggered; (**e**) Conf. **V—**triple baffled_staggered. Figure 12 presents a comparison of sound-pressure levels obtained using Actran software [32] for a linear three-baffle configuration and a staggered three-baffle configuration. The results demonstrate a more attenuated acoustic field within the staggered baffle variant compared to the linear one. Consequently, the staggered configuration appears to be a more suitable choice for the final test-stand application.