**3. Countermeasure for self-sustained tone using baffle plate**

#### **3.1 Setup of experiment**

In this section, we describe the experiments performed by Ishihara et al. [12, 26]. They have investigated the appropriate insertion position of the baffle plate for the suppression of the self-sustained tone and the mechanism of suppressing the self-sustained tone by inserting the baffle plate. The setup of the experiment is shown in **Figure 3(a)** and the duct used in this experiment is shown in **Figure 13**. The tube bank consists of an array of bronze tubes whose diameter is *D* = 6 mm. The array geometry is represented in **Figure 3(b)**, where the spacings *T*/*D* and *L*/*D* are 2.0. In the tube bank, there are 5 rows of tubes in the flow direction and 18 tubes in the width direction, which is perpendicular to the flow, and the length in the flow direction is 60 mm. The sound pressure signal is measured using the microphone set near the duct outlet as shown in **Figure 3(a)**, and converted to frequency domain with FFT analyzer. One tube bank is installed in the duct in the present experiment as shown in **Figure 13**. The baffle plate with a length of 60 mm, thickness of 3 mm, and height of 200 mm is inserted in the center of the tube bank

**Figure 13.** *Setup of experiment ([12]).*

as shown in **Figure 13**. The position of this baffle plate is varied in the flow direction in increments of a row pitch (12 mm) in the tube array. The patterns of the baffle plate position are shown in **Figure 14**. An effective position of the baffle plate for suppressing the self-sustained tone is derived from this experiment.
