2. Acoustic Field Analysis
2.1 Contribution of noise sources for outboard noise
The factor of noise sources for outboard noise in anchorage condition was investigated in the following way.
2.1.1 Exhaust Noise
First, the acoustic transfer function between the outlet of the exhaust pipe and quay at 50m away from the ship was measured by the speaker excitation method. Next, the exhaust noise at the outlet of exhaust pipe was measured during engine operation. Finally, by multiplying the acoustic transfer function by that exhaust noise, the exhaust noise at the quay could be calculated.
2.1.2 Air Borne Noise of Engine.
The acoustic transfer function between the engine room and quay was measured by the speaker excitation method. Next, the noise inside the engine room was measured under engine running conditions. By multiplying the acoustic transfer function by that engine noise, the air borne noise at the quay could be calculated. In order to avoid the effect of background noise, the transfer functions at several points, 5m from the ship's surface, were measured. According to the theory of surface noise source, the transfer function 50m from the ship could be calculated by using those transfer functions.
2.1.3 Structure Borne Noise of Engine.
At first, by vibrating the floor of engine room, the transfer function between the vibration at the engine floor and noise at quay was measured. Next, the vibration at engine floor was measured in engine running condition. By multiplying the transfer function by that vibration at the engine floor, the air borne noise at quay could be calculated. Also, the effect of background noise could be rejected by the previous method (see 2.1.2).
2.2 Requirement for second noise source
Mikami showed that the difference between the exhaust noise and the secondary noise, which was emitted by the speaker, should be less than 1 dB for reducing the exhaust noise significantly at the mixing point6). Therefore, the effect of the differences of the sound pressure level and the phase between the exhaust noise and the secondary noise for reducing noise level was simulated as shown in fig.2. In the case of adaptive control, the experiments clarified the point that the noise reduction control progressed according to the arrow sign.
1) The noise reduction control is switched on under the conditions of zero secondary noise level, and in which the phase difference is too large.
2) The controller increases the noise level of the secondary noise at first. Then, the secondary noise level becomes larger than the exhaust noise level (so called Overshoot).
3) The controller decreases the noise level of the secondary noise while correcting the phase gap. Finally, when the secondary noise level is close to the exhaust noise, the exhaust noise can be reduced.
By these processes, the speaker for the secondary noise should emit 3 to 5 dB, which is enough to produce a larger noise than the exhaust noise at all frequencies.
2.3 Simulation of acoustic field by boundary element method(BEM)
In order to calculate acoustic behavior in the exhaust pipe, the 3D indirect varied boundary element method7) of the SYSNOISE software package was used. Figure 3 shows one of the BEM models. By adding the particular velocity on area A in fig.3, the sound pressure levels at arbitrary points could be calculated.
