| In recent years,with the continuous improvement of micro-nano technology,thermo-acoustic loudspeakers with micro-nano films as sound devices have gradually emerged and been recognized by people.Compared with traditional electro-acoustic(EA)transducers that generate sound by mechanical vibration,graphene film TA transducers show a lot of attractive merits----no vibration parts,small in size,light in weight and flexible,which can improve the situation in the vehicles where the loudspeakers are too big in size to be installed flexibly.They can also be used as secondary sound sources for active noise control(ANC)in vehicle cabins.The electrothermal-acoustic model and the optical-thermal-acoustic model are the two most important theoretical models of the thin film thermoacoustic effect.However,these two models have some shortcomings and deficiencies,which limit the application field of the thin film thermoacoustic effect.For example,in the electro-thermal-acoustic model,the wire and electrode materials cause contact errors in the output acoustic signal of the thin-film speaker;in the optical-thermal-acoustic model,a high-quality controllable light source is required,and there must be no obstacles between the film and the light source,otherwise the conversion between light-heat-sound cannot be completed.In order to expand the application field of thin-film thermoacoustic loudspeakers,based on the theory of eddy current effect of excellent conductors under alternating magnetic field and the theory of thermoacoustic effect of solids,indium tin oxide films with micron-level conductive layers are selected as sound-emitting devices.And firstly,a magnetic-thermal-acoustic theoretical model is established and theoretical calculation of thin film acoustics is carried out.Then the correctness of the theoretical model is verified by experiments,and the controllable parameters of thin film sound pressure level(SPL)are analyzed;finally,from the perspective of heat transfer,the heat transfer characteristics of the magneto-thermal-acoustic effect of ITO thin films are analyzed,and the law of transient temperature rise is analyzed.And,the mechanism of the thin film producing acoustic signals—temperature oscillation is analyzed.The research in this paper comes from the general project of the National Natural Science Foundation of China.The main work and results are as follows:(1)Based on the eddy current effect theory of excellent conductors under the alternating magnetic field,the heat transfer theory of substrates and the influence of heat capacity of films,a magneto-thermal-acoustic model of indium tin oxide films is established,and the near-field sound pressure formula and far-field sound pressure formula of the film are derived.An experimental platform for the sound pressure test of the thin film is built.The experimental results show that the theoretical calculation value of the indium tin oxide thin film is in good agreement with the experimental test value as a function of frequency,which verifies the correctness of the magneto-thermalacoustic theoretical model.The acoustic signal output by the indium tin oxide film has a linear relationship with the eddy current thermal power of the film;the relationship between the sound pressure in the far field and the measurement distance is inversely proportional.(2)On the basis of verifying the correctness of the ITO film magneto-thermoacoustic theoretical model,the relationship between the coil-related parameters and the ITO film sound pressure level is quantitatively analyzed.Then,the influence of gas parameters on the film sound pressure level and the effect of film’s parameters on the film performance are analyzed.The results show that:1)The turns of the coil increases by 30 turns,and the sound pressure level increases by about 4.2dB.The circle radius of the cross section of the coil increases by 7mm,and the sound pressure level of the film decreases by about 3.6dB.When the distance between the center of the film and the coil axis increases by 9mm,the sound pressure level of the film decreases by about 4dB.2)The sound pressure level of the film speaker is negatively correlated with the constant pressure heat capacity of the gas,and positively correlated with the thermal conductivity and density of the gas.3)When the thickness of the film much smaller than the radial depth of the film that the magnetic field can reach,that is,with the skin depth as the boundary condition,the sound pressure level of the film increases with the increase of the film thickness;the mechanical strength of the film increases with the increase of the film thickness,and the service life of the film will increase accordingly,too.(3)The heat transfer model of the magneto-thermal-acoustic effect of ITO film is established,considering the heat transfer loss of the substrate、the natural air convection and other factors,the transient temperature rise formula and temperature oscillation formula of the surface of ITO film are deduced,and the oxidations of different areas are tested.The correctness of the temperature oscillation of the thin film is verified by an indirect experimental method.The research results sh ow that the steady state response time of the thin film is about 50s,and the steady-state temperature of the thin film is related to the initial ambient temperature,but the steady-state temperature response time is independent of the ambient temperature.The temperature oscillation value of the indium tin oxide film increases with frequency,which explains why the output sound pressure of the indium tin oxide film has good amplitude-frequency characteristics in the high frequency range from the perspective of heat transfer. |
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