Research On Theories And Methods Of Active Sound Absorption

Active noise control is achieved by introducing a canceling "anti-noise"wave of equal amplitude and opposite phase using a secondary source. It is controlled with incident sound wave, yet active sound absorption is controlled with reflected sound wave and the absorption coefficient is maximal for the reflected coefficient is reached to zero. Active sound absorption is vacant in china till now, yet it is researched in foreign country from 1990's and the correlative achievement is seldom reported, so the main content of the dissertation is focused on the study with active sound absorption. It presents the development of modern techniques about active sound absorption and then puts emphasis on the study about theoretical calculation absorption coefficient of the multi-layer absorber, the active absorption system based on the air stream, the active absorption system based on the characteristic impedance, the active absorption system based on the piezoelectric materials and the principle of the active absorption and active isolation based on piezoelectric materials. A theoretical method to calculate the absorption coefficient of the multi-layer absorber is proposed in this dissertation. For the multi-layer absorber composed of perforated plate, composed of perforated plate and cavity, composed of perforated plate, porous absorption material and cavity, the absorption coefficient is theoretically calculated. At last, the absorption coefficient of the different assembled absorber is calculated using the method mentioned in this dissertation. The results denoted that the method to calculate the absorption coefficient mentioned in this dissertation is feasible. The theory of active absorption using perforated plate is proposed in this dissertation. The perforated plate is used as the material of active absorption. The frequency is measured in order to get the resonate frequency and the depth between the perforated plate and the rigid wall, then the rigid wall is moved in order that the depth between the perforated plate and the rigid wall is satisfied with the resonate frequency, so the absorption coefficient is maximal and the aim of active absorption is reached. Numerical calculation denoted that the absorption effect is not obvious, so, on the condition of the resonate frequency of the perforated plate, the air stream is passed through the cavity between the perforated plate and the rigid wall so that the absorption coefficient is about 1.0. The theoretical calculation results denoted that the active sound absorption method is feasible. The method of active absorption based on the characteristic impedance is proposed and two systems are designed in this dissertation. One of the systems is consisted of the absorption material and the moved rigid wall, the depth between the absorption material and the rigid wall is adjusted to make the surface characteristic impedance of the absorption material match with the air characteristic impedance so that the absorption coefficient is maximal. Yet this system is effective for the single frequency sound wave and noneffective for the wide frequency sound wave, so the second system is posed which is consisted of two microphones, the absorption material and two loudspeakers, the incident wave is measured by the two microphones, the sound impedance is controlled in order to make the surface impedance match with the air impedance to reach the aim of active absorption. Numerical calculation denoted that the second system is effective for the low frequency sound wave and noneffective for the high frequency sound wave in which the absorption material is effective. In this dissertation, the active sound absorption system is posed. The piezoelectric material is applied as the absorption material in active sound absorption system. Two PVDF (Poly Vinylidene Fluoride) films with equal distance are placed in front of the piezoelectric material in order to measure the sound pressure of the incident wave. The voltage is applied on the surface of the piezoelectric material in order to make the reflected wave zero according to the method of electro-acoustic analogy. At last, the experiment is carried out. The results denoted that the active sound absorption method is feasible. On the base of the theory of active sound absorption using one piece of piezoelectric material, the active sound absorption and the active sound isolation using two pieces of piezoelectric materials is posed in this dissertation. Two pieces of piezoelectric materials are used as the main materials of active sound absorption and active sound isolation. The voltageis applied on the surface of the two piezoelectric materials respectively in order to make the reflected wave and the transmission wave zero according to the measured sound pressure of the incidence plane wave using two pieces of PVDF (Poly Vinylidene Fluoride). The experimental results denoted that the active sound absorption and active sound isolation method is feasible.