Study Of Nonlinear Acoustic Characteristics Of Perforated Plate Structure

A perforated plate construction has been widely applied in thearchitecture acoustics, the control of industry noise , the suppressionof combustion instability and the reduction of acoustic fatigue. As weknow, in order to improve further its performance it is essential tohave better understandings of the acoustic properties of the perforatedplate construction. For decades, perfect theory has been gotten forthe perforated plate construction at low sound pressure levels, butthere are still some unsolved problems at high sound pressure levels.In this thesis, theoretical and experimental investigations are carriedout in an attempt to find out the effect of the nonlinearity of theperforated plate construction to its acoustic properties at high soundpressure levels. In chapter 2, four acoustic impedance theoretical models areintroduced briefly, and the strong points and weak points of each modelare analyzed. In chapter 3, consider the cells of the perforated plate constructionas small Helmholtz resonators, make use of the linear momentum balanceequation built by Melling and RungeKutta method to build a numericalcalculation method. Use the numerical calculation method to simulate the change of acousticproperties of the perforated plate construction with the sound pressurelevels changing, and draw some conclusions: the sound resistance of theperforated plate con-struction increases ,while the sound reactancethe frequency band broadens with the increase of the sound pressurelevels.In this chapter, the change of acoustic properties of the perforatedplate construction with the structure parameters ( hole diameter,porosity, plate thickness) change is predicted, and some conclusionsare drawn: the more small of the porosity ,the more small of holediameter, the more big of plate thick-ness, the more big of soundabsorption coefficient in low frequency, otherwise,the more big ofsound absorption coefficient in high frequency.In chapter 4, an experimental device is designed and built in orderto study the frequency coupling effect caused by the nonlinearity ofthe perforated plate construction at high sound pressure levels. Fromthe numerical calculation and experimental results, some conclusionsare drawn :no matter fix sound pressure levels of the low frequency,change sound pressure levels of the high frequency or fix sound pressurelevels of the high frequency , change sound pressure levels of the lowfrequency, the acoustic resistance of fixed frequency will increasewith the increase of sound pressure levels of the changed frequency,especially , the nonlinearity of the perforated plate constructionappears. In this chapter, the effect of structure parameters change tothe frequency coupling effect caused by the nonlinearity of theperforated plate construction is studied. Some conclusions are drawn:when the sound pressure levels of the high frequency increases ,the moresmall of hole diameter, porosity and plate thickness, the more fast ofthe acoustic resistance of the low frequency increase ,which caused bythe nonlinearity of the perforated plate construction, otherwise, themore slow of the acoustic resistance of low frequency increase.In chapter 5, the difference of acoustic properties under singlefrequency excitation and many frequencies excitation is studied for aperforated plate construction. Some conclusions are drawn: at low soundpressure levels, the difference of acoustic properties between singlefrequency excitation and white noise excitation are very small, but athigh sound pressure levels, the differ-ence are very large: the changedquantity of acoustic impedance are larger at white noise excitation thanat single frequency excitation, besides the line of sound absorptioncoefficient moves 20dB to the low sound pressure levels of singlefrequency. In this chapter, the transmission loss is measured at differ-rent sound pressure levels for the perforated plate construction, andsome conclusions are drawn: the transmission loss peak increases beforedecreases and the frequency band broadens with the sound pressure levelsincrease at the white noise. At last, the effect of the structureparameters changing to the transmission loss are studied, and someconclusions are drawn: the more small of hole diameter and porosity ,the more suitable to low sound pressure levels for the perforated plateconstruction muffler, otherwise, the more suitable to high soundpressure levels;The more large of porosity and the more small ofplate thickness, the more suitable to high frequencies, otherwise, themore suitable to low frequencies.