3.2.2 Optimization of Length of Pipes
For the next step, the length of each part, shown in Fig.10, was optimized. The ratios of each length were important, in order to apply the resonance. It also clarified the point that the most significant factor was the length of the pipe of the secondary speaker L2. The "Power Ratio" can be defined by dividing the power level of the secondary noise, which is required to reduce the exhaust noise, by that of the exhaust noise. The relationship between the power ratio, for obtaining the same noise reduction level, and the length of the secondary speaker pipe is shown in Fig.11. The power ratio was minimized when the length L2 was nearly equal to one-forth of the wavelength of the major order of exhaust noise. It was assumed that the secondary speaker acted on the acoustic resonator. There is a special name for such kind of secondary speaker pipe, called the acoustic pipe. In the acoustic pipe, the sound radiated from the secondary speaker was amplified at odd resonance frequency orders. The disadvantage of this method was that the radiated sound at even resonance frequency orders was de-amplified. In order to reduce the even order of exhaust noise, a much larger capacity of speaker would be required, meaning that the resonance type of passive silencer should be installed before AENC (refer to 4.1).
In addition, Fig.11 leads to an interesting idea. It is generally explained that the AENC can reduce noise by letting noise interfere with noise. Both acoustic pipe and side branch type silencers have the same structure. The difference between both of them is that the end of tye acoustic pipe is the speaker, and the side branch silencer is a rigid wall. In AENC, it is considered that the location of the point in which particular velocity should be zero is similar to the rigid wall in the side branch silencer and the point was changed by controlling the particular velocity at the speaker. In other words, AENC is the system to absorb noise by equivalent length control.
4. Compornent of Active Exhaust Cancellation System
The schematic diagram of the active exhaust noise cancellation system with optimized shape is shown in figure 12. In this figure. Type A is AENC for variable speed engines such as a main engine, and type B is one for constant speed engines, such as an anxiously engine.
