Measurement Of Air-borne Acoustic Pressure In Free Field Based On Laser Doppler Velocimetry

Accurate measurement of air-borne acoustic pressure is of great significance for environmental noise control.Realization of acoustic pressure by reciprocity method does not reflect the concept of acoustic pressure unit Pascal.In contrast,the measurement of air-borne acoustic pressure based on optical method can directly reflect the concept of acoustic pressure unit Pascal,and has the advantages of non-invasive and high spatial resolution.It can trace the acoustic pressure to SI unit through the laser wavelength,which is used for flat transmission of acoustic pressure value.In this paper,the air acoustic pressure was measured by laser Doppler technology and Optical heterodyne detection technology.The scattered light produced by tracer particles was captured by photoelectric detector,and the particle velocity was demodulated to realize the direct reproduction of air acoustic pressure in free field.The main contents of this thesis are as follows:(1)The research status of optical measurement of air-borne acoustic pressure was fully investigated,focusing on whether tracer particles are used in the process of measuring.The characteristics of several commonly used optical path modes and data processing methods were studied.Combined with the actual experimental requirements,the double beam double scattering optical mode was adopted in this experiment,and the spectrum analysis method and Photon correlation spectroscopy were used as the data processing methods.The theoretical analysis of free field air-borne acoustic pressure measurement by spectrum analysis method and photon correlation method was introduced.(2)The experimental results showed that the scattering light power of particles increases with the acoustic pressure of acoustic field,which provides a preliminary basis for measuring particle velocity and acoustic pressure of free field by laser Doppler.The form of laser Doppler signal was simulated,the modulation index was calculated,and the feasibility of measuring free field air-borne acoustic pressure by spectrum analysis method was verified.Then the acousto-optic modulator and mixer in the experimental optical path were studied;In view of the existing problems in the experiment,this thesis putted forward some solutions to achieve the goal of acoustic pressure reproduction(3)The autocorrelation function affected by acoustic pressure was simulated.The experimental system was built,and cubic spline interpolation and cubic polynomial fitting were used to improve the accuracy.The factors causing the deviation of the results were analyzed.Finally,the method of microscopic imaging was proposed to obtain more accurate beam angle and improved the accuracy of air-borne acoustic pressure measurement.