Research On The Theoretical Model Of Graphene Speaker

As a novel type of speaker,the graphene speaker is gradually replacing the traditional moving-coil speaker because of its advantages of no resonance,wide frequency band,small volume and light weight.Nowadays,the graphene speaker has been used in earphones,intelligent artificial throats and other fields,but there is a lack of theoretical research on graphene speaker,especially for the frequency distortion and energy conversion.Therefore,the theoretical model of graphene speaker is studied in this work.The main contents of this work are as follows:（1）The research background of graphene speaker is introduced.Three main methods for the preparation of graphene films（CVD,LIG and LSG）are reviewed.The research status at home and abroad,basic operation process,advantages and disadvantages of the three methods are compared.It is found that there is no theoretical model of graphene speaker which can directly reflect the relationship of sound pressure with the influencing factors.（2）The theoretical model of graphene speaker is proposed.Firstly,the theoretical model of frequency distortion is established,and the methods of suppressing the frequency doubling effect are studied.It is found that the frequency doubling effect can be neglected when the DC excitation is greater than the AC excitation.Secondly,the theoretical model of thermoacoustic effect based on the thermally induced energy density fluctuations is established.The thermoacoustic effect is divided into two processes:electric-thermal conversion and thermal-acoustic conversion,and the corresponding theoretical models are established,respectively.According to the theoretical model of electrical-thermal conversion,the expression of thermal energy is obtained.The results show that the thermal energy propagated into the air Q_air increases linearly with the effective power P_eff and f^-1/2.And according to the theoretical model of thermal-acoustic conversion,the expression of sound pressure is obtained.The results show that the sound pressure p increases linearly with the effective power P_eff and f^1/2,and inversely with the measuring distance r.Finally,the relationship of the sound pressure level（SPL）with the influencing factors is obtained by MATLAB simulation.The results show that the SPL increases rapidly with the frequency in the low frequency band,and increases slowly with the frequency in the high frequency band;the SPL decreases with the measurement distance;the SPL increases with the input power;the SPL decreases with the substrate heat storage coefficient;the SPL decreases with the thickness of graphene.（3）Graphene film is prepared by laser scribing technology（LSG）,and graphene speaker is fabricated by screen printed silver electrode in this work.The microstructure and property of graphene film is analyzed by scanning electron microscope and Raman spectroscopy.The results show that the graphene film prepared by LSG is porous structure,which is more conducive to heat transfer.（4）The temperature and sound pressure of the graphene speaker are tested.Firstly,the theoretical model of frequency doubling effect is verified by experiments,and it is found that when the DC excitation is greater than the AC excitation,the double frequency sound pressure level component is less than 10 d B,and the frequency doubling effect can be ignored.Secondly,the temperature of the electrical-thermal conversion of the graphene speaker is analyzed by fluke thermal imaging instrument.It was found that when the input power is 20-500 m W,the surface temperature of graphene film ranged from 31.5℃to 55.4℃.The results show that the graphene film surface temperature increased linearly with the input power.And the sound pressure of the thermal-acoustic conversion of the graphene speaker is analyzed by microphone and B&K system.The relationship of sound pressure level with influencing factors is verified.The maximum sound pressure level is about 55 d B when the graphene film area is 3 cm×3 cm and the measuring distance is 3 cm.The experimental results of temperature and sound pressure are consistent with the theoretical results,which verify the correctness of the theoretical model based on thermally induced energy density fluctuations.