Sound Absorption Properties Of Wooden Finely Perforated Panels And Sudden-changed Cross Section Structure

Wooden perforated panels as resonance sound absorbing materials are widely used in indoor acoustical design.As the acoustic impedance of common perforated panels are small,the absorption properties are relatively poor.In order to improve the sound absorption properties of perforated panels,the perforation diameters are diminished.While the large panel thickness can bring large impedance,wooden finely perforated panels with sudden-changed cross section,whose perforation diameter lies between 1mm and 2mm,is studied in this research.In this paper,the sound absorption properties of straight perforation structure were studied initially and Maa MPP model was utilized for theoretical simulation.Finely perforated panels of different parameters were made and tested to demonstrate the absorption regularity and model feasibility theoretically and experimentally.Taking the wood characteristics into consideration,the influence of panel-type vibration and wood porous structure were investigated.The absorption properties of different sudden-changed cross section perforated panels and the composite structure of panel and porous materials were also studied.The main results are concluded as follow:(1)As the end additional length of common perforated panels is largely affected by perforation parameter,the accurate impedance model for perforated panels is still not found.The absorption regularity of finely perforated panels can be predicted by Maa MPP model and Maa-Flex model.Full factorial experiment was design and measured with an impedance tube.The measured and theoretically simulated results of sound absorption coefficients and relative impedance were compared to figure out the absorption regularity.The resanation frequecy lied between 200 Hz to 500 Hz.The highest absorption coefficient could be nearly 1.With the perforation diameter increasing,the peak values were decreased and moved towerd higher frequency,the absorption bands were widen.With the panel thickness increasing,the peak values were increased and moved towards lower frequencies,the absorption band width was narrowed.The results show that there are both Helmholtz resonant absorption and panel-type vibration absorption in wooden finely perforated panels.The absorption characteristics can be better predicted with Maa-Flex model.The mass-spring vibration can enhance Helmholtz absorption.While there is an extra absorption peak near 1000 Hz for thin samples,which is induced by eigen-mode vibration.Taking the extra absorpion peak into consideration,the half absorption band width could be 1000 Hz.(2)Panel-type vibration is a key factor that affects the absorption properties of wooden finely perforated panels.There is both perforation Helmholtz resonance absorption and panel-type vibration absorption in perforated panels.And they can transform to each other when perforation rates are changed.The eigen-mode resonance absorption characteristics are affected by mechanical properties.With the damping coefficient decreasing,the peak value of eigen-mode vibration absorption is increased.With the dynamic Young's modulus decreasing,the peak value frequency moves towards low frequencies.Wood porous structure has little impact on the absorption properties.The vibration absorption characteristics are affected by the mechanical properties along the grain and across the grain.It is proved that the relative dynamic Young's modulus and damping coefficient of tangential and radial direction have power function relationship by DMA measurement on different wood species and direction.The variety of wood anatomical structure is one of the factors that affect wooden perforated panel vibration absorption characteristics.(3)Sudden-changed cross section structure is beneficial to widen the absorption broadband,the absorption properties of which is largely affectedby tiny hole perforation parameter while different cross section structures have little influence on absorption properties.As to the sudden-changed structure(the tiny hole parameter t=2.5mm,d=1.5mm,b=8mm)and the strange perforation board with same parameter,the sound absorption band width was enlarged with about 260 Hz.The back holes have little impedance contribution on the whole structure and they divide back air cavity to enhance the Helmholtz resonance effect.The composite structure of sudden-changed cross section perforated panels and porous materials can improve the end impedance and widen the absorption broadband about 50 Hz.