3) Model C: The acoustic ratio of model C is similar to that of model B. In addition, the dips are smaller than those of model B. As a result, this mixing method was adopted.
3.1.2 Optimization of Length of pipes
By using model C, the length of pipes as shown in Fig.5 was changed to improve the acoustic ratio. As a result, it was clear that the acoustic ratio could be a flat shape, as shown in Fig.7, when the length of pipe L1 equal to L2. The reason of this is that the acoustic resonance modes were equivalent to each other, and this occurred at both the path from the inlet of the exhaust gas (shown in Fig.3-A) to outlet of the exhaust pipe and the path from the speaker (shown in Fig.3-B) to the outlet, because the distance from the inlet of the exhaust noise to the mixing area (shown in Fig.3-P) and the distance from the speaker to the mixing area were equal.
In an actual exhaust pipe, the temperature of the exhaust pipe is different from the secondary speaker pipe. Therefore, the length of pipes should be decided by Eq(2).
L1 / L2 ＝ C1 / C2 ... (2)
3.2 AENC for Anxiously Engine
By using SYSNOISE, the shape of the exhaust pipe was optimized acoustically. In case of the constant speed engine, the frequencies of the major order of the exhaust noise were constant. Hence, it was considered that the number of speakers be decreased, by applying the acoustic resonance aggressively.
3.2.1 Angle of the secondary speaker pipe
The basic number of speakers was one. The influence of the angle θ of the secondary speaker pipe which was shown in Fig.8, was investigated. At each angle, the magnitude and phase of the secondary noise should be optimized to maximize the reduction of exhaust noise under conditions in which the magnitude and phase of particular velocity of exhaust noise was constant (Exhaust noise level was constant). The relationship between the reduction in noise level and the angle θ is shown in Fig.9. The reduction noise level was increased within -20 degrees through +20 degrees. This was caused by the acoustic resonance, because the sound emitted by the secondary speaker could be reflected within the small angle. Finally, the secondary speaker pipe was installed at right angles.
