Experimental Study On Modal Recognition And Voice Listening Based On Laser Lateral Displacement

Vibration is a common phenomenon in nature and a common form of movement of matter.Generally,the specific use of vibration depends on its application and the goal of the study.some vibrations in nature are negative and need to be avoided.For example,optical precision measurement needs to reduce the impact of environmental noise^[1,2]on measurement accuracy.Such bad vibrations will affect the performance indicators of the optical system;on the other hand,some vibrations are active and need to be utilized,such as laser eavesdropping^[3-5],laser radar target detection and tracking^[6,7],precise multi-degree-of-freedom guidance^[8,9],detection of atmospheric molecules in the environment^[10],aerospace structural active control technology^[11],modal analysis of automotive structures^[12],acoustic focusing of acoustic lens^[13]and target detection in the underwater acoustic field^[14-16].This article will use the principle of laser lateral displacement measurement to utilize the vibration,measure and capture the sound source signal,so as to realize the modal analysis of the sound signal and the interception of the voice.Vibration measurement methods can be divided into two types:one is contact measurement,and the other is non-contact measurement.The contact measurement method is generally to install a piezoelectric sensor on the surface of the measured object,convert the vibration of the surface of the object into an electrical signal output,and then process the displacement,velocity and acceleration represented by the electrical signal.The advantage of this method is that the system price is affordable,the disadvantage is that it’s additional quality will make the test results deviate,and when it acts on the measured object for a long time,it will cause slight damage to the measured object;the non-contact optical measurement method can measure hard to detect long distance vibration signals.Laser method is widely used in the field of micro-vibration measurement.The measurement of the lateral displacement of the laser is a non-contact optical measurement method,which has a remarkable effect on the measurement of vibration signals in the acoustic frequency range of 20Hz-20KHz^[1].It improves concealment and is suitable for laser eavesdropping scenarios.However,this method also has shortcomings.Laser measurement is easily affected by the effect of the reflective surface^[17-21]and environmental noise^[10],which has been a hot issue which needs to solve recent years.In future experimental research,we can focus on reducing this type of impact while improving the signal-to-noise ratio and accuracy of the measurement system.In summary,the structure of the modal recognition and voice interception system based on laser lateral displacement in this paper is simple and easy to use,and can realize high-precision measurement nanometer level of small lateral displacements.In addition,the detection device of the system also has the characteristics of strong real-time performance and low cost.In this paper,based on the lateral displacement effect of the laser,the laser is focused on the reflective surface,and the vibration source information is obtained by monitoring the change of the lateral displacement signal of the echo.The main contents are as follows:（1）The basic principle of the experiment is introduced.Analyze the basic principle of the commonly used technology of laser vibration measurement,compare the interferometry,coherent Doppler measurement method^[22]and laser reflection modulation measurement method^[23],introduce a method based on laser lateral displacement measurement,by detecting the fine vibration of the displacement perpendicular to the z-axis direction of the Gaussian beam to capture and analyze the sound source information it carriesto realize the conversion from"sound amplitude"to"light lateral displacement".（2）Introduce the basic concepts and properties of light fields.A photoelectric detection method with balanced zero-beat detection and beam-splitting detection is given,which is used for precise measurement of the translation and tilt of the beam.Design and build an experimental device suitable for laser lateral displacement detection of acoustic signals.The system adopts the method of beam splitting detection,and performs acoustic-optical displacement conversion,optical displacement-electrical signal conversion,electrical signal-displacement calculation,and optical displacement-acoustic signal restore analyzing.The detection system includes an optical transmitting end,a reflecting end,a receiving end and a signal processing end.（3）Research on laser lateral displacement eavesdropping system.This experiment mainly uses the light from the 1064nm laser to irradiate a smooth reflective surface with a certain reflectivity,and detects the lateral displacement of the reflected spot through a four-quadrant detector,and then obtains the longitudinal vibration displacement of the reflective surface,so as to complete the modal experiment and to achieve acoustic signal detection.Initially,The system realized the research of laser lateral displacement measurement system based on LabVIEW.Then,the gold-plated diaphragm was customized,and the acoustic signal detection and modal analysis of the micro-electromechanical diaphragm were carried out to prove the feasibility of acoustic signal restoration based on the lateral displacement of the laser.Finally,the listening experiment of double-layer vacuum glass was carried out,the results of the listening experiment were analyzed,and the waveforms of the single-frequency and voice signals were compared with the original sound,and evaluated with the MOS evaluation index.The device successfully proved the effectiveness and feasibility of the laser listening function.