Technical Papers
Development of Active Exhaust Noise Cancellation System (Part 1)* - Optimization of AENC System by use of Acoustic Analysis -
by Tohru Yonezawa,** Minoru Okubo,** Hiroshi Kanda,** Shinichiro Ishida**
In recent years, the demand for the reduction of noise emitted into the environment has strongly increased in order to prevent noise pollution. As the major contributor of emitted noise into the environment is exhaust noise in ships, it is necessary to reduce exhaust noise as much as possible.
Conventionally, passive silencer techniques are used, but larger silencers are now needed in order to achieve larger noise reduction. However, because of the limitation of space, it was actually difficult to reduce the exhaust noise on a ship.
In order to reduce the exhaust noise as well as minimizing the occupied space, the active noise cancellation technique was applied to the exhaust noise silencer for diesel engines. To reduce the exhaust noise efficiently, at first, the acoustic behavior in an exhaust pipe was calculated by the boundary element method. At first, by optimizing the length of pipes and the positions of microphones and speakers, the active exhaust noise cancellation system (AENC) was applied to the main engine for marine use. Also, by applying acoustic resonance aggressively, the AENC, with only one speaker system, was developed for auxiliary engines, Finally, by assembling a passive silencer for high frequency noise, the active exhaust noise silencer was developed.
1. Introduction
In order to improve amenity and the working environment, reduction of noise and inboard vibration has been done aggressivelyl)2). In recent years, the demand for reduction of noise emitted into the environment has strongly increased in order to prevent noise pollution. For example, ships, which have high noise level, are refused entry into ports at night.
The contribution of outboard noise of coastline operating vessels was investigated. As a result, it was clear that the biggest factor of outboard noise was the exhaust noise and that there was only a small contribution of structure air borne noise from the engine (refer to Fig.1 and 2.1). Therefore, for the purpose of reducing the outboard noise, the exhaust noise should be reduced.
Presently, techniques such as the passive silencer are used, but a larger silencer is needed in order to achieve larger noise reduction. However, because of the limitation of space, it is actually difficult to reduce the exhaust noise of the ship. In order to compensate for this, the active noise cancellation technique was applied.
There are many applications of the active exhaust noise cancellation system. Ishino developed the active exhaust noise cancellation system for gas engines which are used for the co-generation system3). Hayashi described the active exhaust noise cancellation system for the auxiliary diesel engine generator for marine use4). Arnold developed the active exhaust noise cancellation system for the diesel engine generator5) However, they could only be applied to the constant speed engine, and they occupied a large amount of space.
With the aim of reducing the exhaust noise efficiently and minimizing the occupied space, the mixing method of acoustic wave, together with the position of microphones and speakers were optimized. Finally, the small and high-efficient active exhaust noise muffler was developed.
