**5. Conclusions**

*Noise and Vibration Control - From Theory to Practice*

*Peak frequency in each case of the aperture ratio [29].*

corresponds to that of the general acrylic surface. To clarify the acoustic characteristics of the tube bank duct, the acoustic frequency responses have been calculated using the monopole point sources (without the flow) that are supposed to be vortices generated on the downstream side behind the tube as shown in **Figure 31(d)** and **(e)**. The magnitude of the point sources is 1 Pa at all frequencies. The point source is located at (0.25*H, −*3.86*H,* 0) downstream of the tube bank, and *H* is 200 mm in the height of the tube bank duct. In order to excite the resonance mode in the duct width direction,

**Figure 32** represents the acoustic frequency responses that have been calculated using the monopole point source (without the flow). The monitor point is located

the sound source was arranged asymmetrically with respect to the YZ plane.

*Acoustic modes at each peak frequency [29]. (a) ϕ = 0%. (b) ϕ = 1%. (c) ϕ = 2%. (d) ϕ = 4%.*

**Table 2.**

**76**

**Figure 33.**


*Noise and Vibration Control - From Theory to Practice*
