Active Control Of Sound Radiation Through Openings Of An Enclosure

Noise radiation through openings is a very common problem in our life,and the most often used method to reduce it is passive noise control,including closing the window or implementing specific sound absorption material or structures,which is effective in middle to high frequency range while does not work for low frequency noise.For the purpose of access,natural ventilation and lighting,the openings have to be kept open,which will deteriorate the passive noise reduction performance.This paper investigates the active control of noise radiation through openings and the main work of this paper includes four parts:The planar virtual sound barrier is proposed based on the Huygens' principle.The secondary sources and error microphones are evenly distributed over the entire opening to reduce the noise radiated from inside the cavity through the opening to the outside.A theoretical method based on modal superposition is proposed to calculate the sound pressure in and outside the cavity with five rigid walls and a baffled opening and it is verified with numerical simulation results obtained with the commercial software SYSNOISE.The performance of the planar virtual sound barrier system is investigated with the acoustic transfer functions calculated with the theoretical method.The sound power of the primary and secondary sources with and without control is calculated to explore the noise control mechanism of the planar virtual sound barrier.Experiments are carried out in the anechoic chamber to demonstrate the feasibility of the planar virtual sound barrier to achieve global control of broadband noise.Some of the secondary sources are distributed in the middle of the opening,which are difficult to implement in practical applications and affect the natural ventilation,lighting and access through it.A single-layer secondary source system at the edge of the opening is then proposed.Its global noise reduction performance is investigated with numerical simulations and it is found that it is effective only within a limited frequency band.The local control performance of the single-layered secondary source system at the edge is then investigated and experiments are carried out with an open wooden box and a semi-closed meeting room in an open plan office to create local quiet zones.To improve the global control performance of the virtual sound barrier system at the edge,a double-layer secondary source system is proposed,in which secondary sources are at the edge of the opening at two different heights.It is found that its noise reduction performance is significantly improved compared with the single-layer secondary source system at the edge,and it is demonstrated with experiments.The reason why the double-layer secondary source system outperforms is explored with the modal decomposition method.It is found that secondary sources at the same height cannot effectively excite some modes,which makes it difficult to suppress these modes with the single-layer secondary source system at the edge.The upper limit frequency of effective control of the system with boundary installed secondary sources is also investigated.Theoretically there exists no upper limit frequency for a planar virtual sound barrier system.The error microphones are now evenly distributed in the planar,single-layer and double-layer secondary source system at the edge of the opening,which affects the functionalities of the opening as well.The error sensing strategy with double-layer error microphones at the edge is proposed to solve this problem and both numerical simulations and experiments demonstrate that the noise reduction is higher than that achieved with single-layer error microphones at the edge.The reason is that the double-layer error microphones enlarge the effective noise reduction area near the error points while noise is attenuated within a very limited area along the boundaries with the single-layer error microphones.The active control of noise radiation through openings is investigated in the paper.For the purpose of access,natural ventilation and lighting,the planar virtual sound barrier,single-layer and double-layer virtual sound barrier at the edge are proposed and their noise reduction performance are compared with each other.The double-layer error microphone strategy is also investigated.The double-layer secondary sources and error microphones at the edge achieve higher noise reduction than the single-layer system,while help keep the functionality of the openings.There are still some problems to be solved before the active noise control system can be applied in practical applications,including the acquisition of reference signal,the design of small secondary sources and the stability of multi-channel active control systems,which will be investigated in the future.There are four main contributions in the paper:1.A planar virtual sound barrier is proposed to achieve global control of sound radiation through openings.A modal superposition method is proposed to calculate the sound field in and outside the open cavity.The noise reduction performance of the planar virtual sound barrier and the mechanisms are investigated and experiments are carried out to verify the simulation results.2.A single-layer secondary source system at the edge of an opening is proposed and it is found that there exists an upper limit frequency for global control.Experiments with an open wooden box and a semi-closed meeting room demonstrate its feasibility of local control.3.A double-layer secondary source system is then proposed and it is found to outperform the single-layer system with the same number of secondary sources.The modal decomposition method is used to explain the reason.The effective upper limit frequency for a virtual sound barrier at the edge and its relationship with the opening size is investigated.4.The concept of double-layer error microphones at the edge is proposed.It is found that they perform better than single-layer error microphones at the edge because they enlarge the effective noise reduction area around the error points.