Abstract
The theory of active absorption of a perforated plate is considered in this paper. The perforated plate is the material of active absorption and the frequency of the incident sound wave is measured. According to this frequency the depth of the cavity between the perforated plate and the rigid wall, is moved in order that resonance occurs so that the absorption coefficient is maximal. According to the numerical calculation, when the perforated plate is resonant, the distance moved is large at low frequencies and the absorption coefficient is low in some conditions. It is effective for a single frequency of incident sound wave, yet it is difficult for a wide frequency range. Hence active absorption based on airflow is considered and calculations and experiments an carried out. The results denote that this method of active absorption is practical.