Abstract
This paper describes an active sound control system using an FIR digital filter, which attenuate low-frequency noise in an air-conditioning duct system. This system is a monopole type with two identical microphones located on upstream and downstream sides at equal distances from the cancellation loudspeaker in the duct. The output signals of the two microphones are mixed 180° out of phase with one another, and are fed to the input terminal of the digital filter. This reduces acoustic feedback in the system, offering high performance for duct noise caused by a centrifugal fan in a range of 70 Hz to 600 Hz.
