The comparison between the sound pressure level in rated power, at 1m and the exhaust pipe outlet, is shown in figure 17. The dashed line shows the Aweighted sound pressure level of the exhaust noise at the outlet of the engine. The broken line shows it at 1m from the exhaust pipe outlet with the speaker off. The solid line shows the AENC with the speaker on. By using the passive silencer, the noise over 500Hz could be reduced. The noise up to 500Hz could be reduced by AENC and the noise reduction level could be achieved by about 16dB(A).
The exhaust noise levels, reduced by AENC at each propeller loads, are shown in Fig.18. Even though both the engine load and the speed was changed, the exhaust noise level could still be reduced. So, a large noise reduction by AENC could be achieved at all load and speed ranges.
In the case of the propulsion engine, the load and speed of the engine should be changed transiently. In order to investigate the responsiveness of AENC, the noise level was measured on an engine accelerating from low Idle to rated power shown in Fig.19. The exhaust noise, with the speaker on, was always lower than with it off, even when the engine was accelerating.
6.2 Reduction of Occupied space
Fig.20 shows the comparison between the schematic diagram of the AENC and the passive silencer, which have the same noise reduction level. The occupied space of AENC is 50% smaller than that of the passive silencer. So, it is possible to install AENC in ships.
7. Conclusion
The major conclusions of this study are summarized as follows:
1) The three-dimensional boundary element method can be applied to the acoustic field, which has the distribution of flow and temperature, as the exhaust pipe.
