| A late model and applied distribution mode loudspeaker (DML) was invented by the Verity Group (NXT) of the U.K in 1996. . The distributed-mode loudspeaker essentially consists of a vibrating panel that is set into vibration by means of a special electro-acoustic exciter. The force of exciter excites the vibrating panel of exciter setting place, the vibrating panel of exciter setting place transfers vibrating to the whole vibrating panel. Each small area of the panel vibrates independently of its neighbors. The panel can be thought of as a whole series of individual radiators, each radiating sound effectively independently of its neighbor but summing in the far field to give the desired response. Because of particular structure and special distributed-mode principle of DML, DML exhibits numerous benefits. Uniform power response makes for even distribution of energy within the given space. Wide directivity provides uniform tonal balance with listening position. DML is diffuse radiation. Diffusivity makes the reflected sound from boundaries less destructive. With distance, the fall-off in sound pressure level is slow. Swing of the vibrating panel and distortion of DML are low. DML is panel shape, super slim and lightweight. It has no use for sound box and can be produced into fine pictures. It has universal applications in diverse fields from IT to multimedia, personal information and communication tools, home theatres, automobiles and architecture.The vibrating of exciter setting place is transferred by the vibrating panel. So the correlation of each small area of the panel exerts great influence on vibration.In another word, the composition and structure of the sound panel directly effect randomly vibrating which play an important role to acoustic performance of DML. Acoustic specialists generally predict vibrating character and acoustic performance of DML in terms of macro- performance of the sound panel in theory. But there is little report that vibrating character and acoustic performance of DML is related to the composition and structure of the sound panel. Therefore, this studies haSS focused on the relationship between the composition and structure of the sound panel and acoustic performance of DML. Rigid polyurethane (PU) foams were synthesized. A series of sandwich composite panels that PU foam, honeycomb and Poly (vinyl chloride) (PVC) with holes were core, paper and epoxy laminate were skin were prepared. The frequency response, the swing-frequency, the acceleration-frequency and harmonic distortion of DML and the nature frequency of the sound panel were tested. The internal relationships between the composition and structure of the panel and acoustic performance of DML, which provide materials and structure design of the sound panel with theoretic reference, were studied.The study on test method of acoustic performance of DML showed that acoustic testing environment could be improved by baffled panel measurement and porous layer and enclosure measurement. To a certain degree, this simplified testing results that facilitated analysis. Employing baffled panel measurement that the sound panel is close to baffled panel for the first time can reduce interference of reflected wave and avoid "sound wave short", so as to make an ideal testing result. The environmental noises and reflected waves from surrounding objects were avoided by the frequency response in anechoic chamber that facilitated analysis.The study on the frequency response performance of DML showed that sensitivity, frequency area and smoothness degree of the frequency response were influenced by weight, density, moduluCor rigidity)and damp. The internal reasons of above influence are the composition, cell ratio, cell open ratio, cell size, thickness and skin of the panel. The more flexible the polymer molecule is, the more excellent the damp is, the more power is worn down, the lower sensitivity is and the less flat frequency response is. The flexible of polymer molecule induces modulu of thepanel to decrease and the frequency area is narrower. The sensitivity depends on the damp of the core more than on density of the panel. The sound power depends on the damp of PU foam sandwich panels at 200-1000HZ and depends on the rigidity of PU foam sandwich panels after 1000HZ. The sound power depends on the rigidity of honeycomb sandwich panels after 2000HZ. When the cell ratio of PU foam is 98.57%, the damp is weaker, the sensitivity is 83.5dB and the frequency area is 60-6700HZ. When honey cell sizes are 25mm, the resistance between gas and hole wall are weaker, the sensitivity is 84.6dB and the frequency area is 140-7500HZ. The larger cell sizes are, the lower mode density is, the little number of micro-loudspeaker is, the weaker un-correlation of radiated wave is, the less flat frequency response is and the narrower frequency area is. With the increase of the cell open ratio of the core, reflected wave reduces and the sensitivity enhances. As PU foam becomes 9.92mm, the damp improves, the path between holes gets longer, reflections in the holes increase, the power is worn down, the sensitivity is 60.4dB and the frequency area is 110-10000HZ, the frequency response is less flat. As honeycomb becomes thicker, gas in the panel increases, the density of the panel gets lower and the sensitivity improves. As the core becomes thicker, the frequency of producing acoustic radiation increases. The matching of acoustic resistance of skin and core is the main cause of the difference in flatness of the frequency response at 60-100HZ and 200-400HZ. Because mode density is low, the sound power is low before 100HZ. The first apex of the frequency response at 50HZ is caused by the change of exciting force of exciter. The sensitivities of honeycomb sandwich panels are higher and the frequency areas are wider.The study on vibration characteristics of DML showed that the maximum swing at 50HZ is in correspondence with the first apex of the frequency response and distortion at 50HZ. It is non-uniform magnetic field of exciter that causes the change of exciting force, independent of the composition and structure of the sandwich panel. The swing and acceleration were influenced by rigidity and damp of the sandwich panel. The internal reasons of above influence are the cell size, thickness and skin of the panel. As the panel becomes 3mm, the rigidity and the damp of thesandwich panel decrease, the strain increase and the swing is 120 u m. The comparatively large acceleration of PU foam sandwich panels at 200HZ-1000HZ and honeycomb sandwich panels at 130OHZ-200OHZ show the damp and the resistance are better than that of low- frequency. With the increase of the cell sizes, the rigidity and the damp decrease, the swing increase and the acceleration decrease. Modulu of epoxy laminate is higher than that of paper, so the rigidity of the sandwich panel with epoxy laminate skin is higher than that of paper. The swing of the sandwich panel with epoxy laminate skin is little. The difference in modulu between epoxy laminate and PU foam is larger than that between paper and PU foam. This difference shows the different displacement between skin and core after being exerted force, which forms friction that produces damp and increases acceleration.The study on the natural frequency of the panel showed that the first natural frequencies of PU foam sandwich panels from 3 mm to 5 mm ware mostly 50-60HZ. The double of the first natural frequency was 100HZ. The adequate mode density sufficed acoustic radiation after lOOHz. The natural frequency was influenced by the core thickness, skin and core modulu of the panels, weight of the panels. The internal reasons of core modulu are the flexible of polymer molecule, cell ratio, cell size and cell open ratio. As the core becomes thicker or skin modulu increases, the rigidity improves and the natural frequency enhances. With the increase of the flexibility of polymer molecule, cell ratio and cell size, the modulu, rigidity of the panel and the natural frequency decreases. When DML panel vibrates, the vibrating type is complicated, which forms diffusing radiated wave and wide direction acoustic field.The study on distortion of DML showed that distortion below 250HZ that is caused by non-linear of the exciter and resonance at the natural frequency of the panel is the more than 20%. The distortion above 250HZ that is caused by non-linear of the panel, which depends on the rigidity and the damp, is the less than 10%. The internal reasons of above influence are the composition, cell ratio, cell size, thickness and skin of the panel. Non-linear of the panel and the distortion of DML are produced by relaxation and viscosity friction of polymer that is related to the composition of polymer. With the increase of the cell ratio, cell open ratio and cellsize of polymer in core, modulu of the core decreases, non-linear of the panel and the distortion increases. As PU foam sandwich panel becomes thicker, the damp is improved, non-linear of the panel and the distortion increases. As honeycomb sandwich panel becomes thicker, the rigidity of the panel increases and the distortion decreases. The rigidity of epoxy laminate is higher than that of paper, so the distortion of DML with epoxy laminate skin is lower. The distortion of PU foam sandwich panel at 200-1000HZ and PVC sandwich panel below 500HZ are affected by damp, the distortion of PU foam sandwich panel above 1000 HZ and honeycomb sandwich panel above 2000 HZ are affected by rigidity.The partial designing elements of DML panel are that the panel should be super thin; the rigidity of the core and skin should be high; modulu and density of skin should match that of the core, the damp of the core should be low; the cell sizes of core should be small to assure enough mode density and un-correlation; in the situation of low distortion, cell ratios can be increased appropriately; ^he honeycomb sandwich panel is better for DML panel.
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