Sound Absorption Performance Of Wood/Polyester Fiber Composites Based On The Biot Model

Synthetic fibers,such as polyester fiber,are commonly used for sound absorption materials,which synthetic fibers main sources are identified from petroleum industry resulting in larger energy consumption and serious pollution.What's more,nature fibers belong to light-weight,low processing energy consumption green-materials,as well as sound absorption.It's okay to substitute nature fiber for synthetic fiber for reducing energy consumption and pollution,even increasing the comprehensive utilization ratio of green plants.In this paper,the polyester fiber and poplar wood(Populus L.)fiber were needle-punched and formed the wood/polyester fiber composite.The composite was equivalent to a certain medium according to Biot sound absorption phenomenon theory.The relationships of between equivalent density,bulk elastic modulus of the certain medium and the composite parameters were confirmed.The correlation and error of between theory and measurement sound absorption coefficients were analyzed.The effect of composite parameters,fiber morphology on its sound absorption performance were revealed according to sound impedance or the relationship of between the composite parameters and densities,bulk elastic modulus of equivalent fluid.The main conclusions were as following:(1)The composite material was equivalent to a certain medium and the model of between density,bulk elastic modulus of equivalent medium and the composite parameters were built.The surface sound impedance and sound absorption coefficient of the composite were calculated according to sound propagation into bounded fluid.The theory and measurement results had positive correlation by nonlinear regression(coefficient of determination,R2=0.72~0.97).Wood fiber morphological difference was one major cause of the theroy value error.(2)The flow resistivity(from 1.3×104Pa·s/m2 to 6.0×104Pa·s/m2),average sound absorption coefficient(from 0.111 to 0.333)and the max-sound absorption coefficient(from 0.404 to 0.99)of the composite were increased with the increase of its density from 0.08 to 0.13g/cm~3.What's more,porosity and viscous/thermal characteristic lengths of the composite material were decreased by 4.5%,213.88?m/427.76?m,respectively,but the tortuosity was increased by 0.027.Those results indicated that the porosity decreased,but pore structure became complicated,and the ratio of the effect pore area and volume with density increasing.The acoustic resistance was decreased following the density increase in the below 1500 Hz,then increased.And the acoustic reactance was decreased,then increased in the higher frequencies(>5500Hz)due to reflected sound wave and the incident sound wave overlaying,this would result in the strength of sound field being increased and sound absorption property decreased.(3)With the increasing of thickness from 12 mm to 25 mm,the average sound absorption coefficient of the composite was increased from 0.367 to 0.569,and the first resonance frequency was shifted to the lower frequencies(from 4000 Hz to 2000Hz)due to the first resonance frequency was inversely proportional to thickness of material.When the frequency exceeded the first resonant frequency,the acoustic peaks and valleies were shown,and the sound absorption curve ups and downs in the higher frequaencies.The elastic property was more prominent when the thickness of the composite was increased.The acoustic resistance of the composite was decreased in the low-middle frequencies,but the acoustic reactance was increased so that the sound absorption was better.However,the changing trend of sound impedance showed in opposite direction duo to the difference of phase position for reflect and the incident sound wave.The sound impedance and sound absorption changed tendencies with air gap thickness increase were similar to composite thickness.When the air gap thickness was odd number of times the 1/4 wavelength,the pressure was zero,the vibration velocity and sound absorption coefficient were the largest.(4)The wood fiber diameter and density were 204?m,0.83g/cm~3,and those of polyester fiber were 34?m and 1.69g/cm~3,respectively.With the increase of wood fiber ratios,the characteristic lengths were larger.Those results indicated that the sound wave viscous friction loss between air and fiber wall was increased,but this tendency was not more and more significant.The average sound absorption property of type II wood fiber was lager 0.02 than type I duo to the type II wood fiber was composited with fine wood fiber which were not separated completely,so more gaps and specific surface areas were existed compared with the type I wood fiber.