## 4 Experimental Verification
### Construction of the Testbed  
The airflow-free acoustic performance test system for the muffler is shown in Figure 11. This testbed is mainly composed of the noise generator and testing system, which includes the sound source, microphones, acoustic acquisition system, muffler test specimens, and terminal muffler. It is designed to evaluate the acoustic attenuation performance of the split-stream rushing muffler. Specifically, the microphone used was an MPA201 (1/2 inch, BSWA Technology Co., Ltd., Beijing, China) sensor, the impedance tube was a CIT30 model, and the acoustic data acquisition system employed a BOACH-DRH-DR08 data acquisition card. In this test system, the sound source is provided by a loudspeaker. The sound source signals are generated by the signal generation module of the multi-channel data acquisition and analysis device and are amplified by the power amplifier. At the outlet section, different resistive loads are applied for several measurements, as the transmission loss of the split-stream rushing muffler is tested using the two-load method. The first loading condition involves no additional loads, while the second loading condition includes additional loads of cross-sectional pipes with sudden resistance expansion to achieve varying test loads. The positions of the four microphones on the pipe are determined according to the standards ASTM E1050-98 and GB/Z 27764-2011 [26; 27]. Meanwhile, the sensing films of the microphones are kept aligned with the pipe wall of the testbed. The distance between each pair of microphones and the highest frequency in the test must satisfy the following condition:  
\[l\leq\frac{c_{0}}{2f_{m}} \tag{46}\]  
Figure 11: Assembly of the transmission loss testbed and devices. where \(c_{0}\) is the sound velocity, \(f_{m}\) is the highest test frequency, and \(l\) is the distance between two microphones. In general, \(l\) should be small for testing high-frequency signals but large for testing low-frequency signals. According to earlier studies on the split-stream rushing muffler, this research focuses on low-frequency signals. Therefore, \(l\) is set to 30 mm and \(f_{m}\) is approximately 5667 Hz. Microphones are installed at the same positions on both the upstream and downstream sections, with the shortest distance between the two pairs of microphones and the muffler being 130 mm. As depicted in Figure 12, the sound source is placed outside of the anechoic chamber during the construction of the testbed, while the remaining components are positioned inside to reduce disturbances to the acoustic test. Before testing, verifying whether the background noise in the pipes met the test conditions was necessary. During the airflow-free transmission loss test, the loudspeaker is first turned off, and the background noise level in the pipes is about 48 dB. Subsequently, the loudspeaker is turned on, and the measured sound pressure level of the noise at the same pipe position is about 70 dB. The sound pressure level difference between the sound source signals and the background noise exceeds 10 dB, which satisfies the test requirements of GB/T 4759-2009 [28]. During the measurement, the temperature in the pipes is 30 \({}^{\circ}\)C, while \(c_{0}\) = 343 m/s and \(\rho_{0}\) = 1.182 kg/m\({}^{3}\). During the transmission loss test, calibrating the microphones is essential to ensure the accuracy and repeatability of the test results. First, it must be confirmed that the microphones and calibration equipment (e.g., acoustic calibrators or standard microphones) are in proper working condition and within their calibration period. Following this, the microphones are calibrated according to the steps outlined below. (1) Connect the microphones to the test system (e.g., frequency analyzer or sound level meter). (2) Insert the microphones into the acoustic calibrator and ensure a proper seal to prevent sound leakage. (3) Turn on the acoustic calibrator and apply signals with known sound pressure levels (e.g., 90 dB SPL).