Abstract
A system to attenuate the sound or pulsating pressure in a duct which has a noise source with a variable exciting frequency, such as a compressor or a fan, is investigated. This system consists of a Helmholtz resonator whose cavity volume is automatically controlled by a personal computer such that the system retains the antiresonance. The steady-state and transient responses are investigated theoretically. The experiments are also carried out by making use of the harmonic sound radiated from a speaker and a pulsating flow caused by a centrifugal fan. In this autoadjusting system, the volume of the resonator cavity is controlled by making use of the fact that the phase difference between the pressure in the duct and that in the cavity changes 180degrees at the antiresonance. The sound can be reduced significantly, especially in the vicinity of the resonance frequency. When the tracking for the change of exciting frequency has a delay, the system may pass through the resonance point.
