| Remote non-contact sound monitoring is one of the important research contents in the field of counter-terrorism and security.Using the laser as a carrier is an effective monitoring method to measure the surface deformation of the object caused by sound,and then the sound content can be extracted.The signal-to-noise ratio of receiving laser signals depends on the material characteristics of the vibrating object,the laser scattering characteristics of the object surface and other factors.How to effectively improve the signal-to-noise ratio has become a problem that needs to be considered in depth in the application of this technology.It should be noted that this is a multi-physical field coupling problem involving sound field,light field and material vibration.So far,the research in this area is not very deep.This paper focuses on the signal-to-noise ratio of the final extracted sound signal and introduces two laser vibration speech detection methods and their principles.The signal-to-noise ratio of the two methods is analyzed,and the sound field model of sound vibration,the interaction model between sound field and planar objects of different materials,and the modulation model of vibration plane to the laser are established.The actual speech signal is selected for conversion to obtain its spectrogram and time domain map,and some frequency components in the spectrogram are selected as acoustic signals.The selected acoustic signal serves as the acoustic field in the vibration simulation system.To simulate the acoustic field vibration,the ANSYS finite element analysis method is used to segment the grid of the object model,and the simulation is conducted accordingly.The vibration process of the object model in the sound field and the surface shape variables of the object are obtained,and the vibration process of the vibration model is imported into MATLAB.The simulation of the object model is derived from the theory of the laser measurement vibration,which focuses on the influence of the material,the laser angle of incidence and the acceptance angle of the photodetector on the signal-to-noise ratio of laser speech vibration measurement system.Three key factors of the receiver angle of the detector affect the signal-to-noise ratio of the laser speech vibration measurement system.In the case of plastic,glass and marble,the simulation results show that for the vibrating object material,the signal-to-noise ratio of plastic material is higher than that of the other two materials,followed by glass material,and the signal-to-noise ratio of marble material is the smallest.It shows that the smaller the elastic modulus of the material,the greater the surface deformation driven by the same sound field,and the higher the signal-to-noise ratio of the received signal.Under the condition of material selection,the incident angle of the laser and the receiving angle of the detector are changed respectively.The simulation results show that: 1.For direct detection systems,under the condition that the incidence angle and exit angle are both 45°,the signal-to-noise ratio of the laser signal modulated by sound is the highest,which can reach 0.03577.As the angle increases or decreases,the signal-to-noise ratio tends to decline monotonously.2.For the heterodyne detection system,the increase or decrease of the laser incident angle and the acceptance angle of the detector have little impact on the signal-to-noise ratio of the system.At the same time,a laser non-contact sound monitoring experimental system for direct detection and heterodyne detection was built,and the simulation results are experimentally verified The experimental results are consistent with the simulation research results,which proves the accuracy of the simulation research results.The results of simulation and experimental research in this paper have important reference values for how to improve the signal-to-noise ratio of monitoring signals and obtain clearer monitoring content in the practical application process. |
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