Noise is unwanted sound. Traditionally noise is controlled by passive methods like enclosures, barriers and silencers. With the advent of ultra high bypass engines, the space available for passive acoustic treatment is becoming more limited and noise regulations more stringent. Active noise control holds promise as a solution.
Most noise generated in air distributing system is caused by fan operation. Noise attenuation devices are usually designed to reduce transmission of fan noise through ductwork. These devices restrict airflow thereby reducing the pressure drop of these systems. Active noise control devices cancel noise without restricting airflow. The principle of active noise control is to cancel an undesirable noise, which results in destructive interference of pressure waves. These electronic systems use a microphone to sense noise, a processor to determine the cancellation sound and a speaker to produce cancellation sound.
Creation of an opposite sound requires use of atleast one noise source. Sensors are also needed to gather information about acoustic field. Characteristics of sources and sensors are closely related to the Application concerned. The first active noise control test on low speed fan test bed was designed to control either the exhaust or inlet fan tone. This system consists of a "Ring Source" an induct array of error microphones and a control computer. Fan tone noise propagates in a duct in the form of spinning waves. These waves are detected by the microphone array and computer identifies their spinning structure. The computer then controls ring source to generate waves that have same spinning structure amplitude but 180o out of phase with fan noise. The other commercial products, which also use active noise control, are:
· ACTIVE HEADSET: Provides strong noise attenuation at medium and high frequencies.
· ACTIVE MUFFLER: For ventilation and air conditioning networks.
· ELECTRONIC CONTROLLERS: To achieve real time active control of noise and vibrations
