The High Frequency Free-field Reciprocity Calibration Of Condenser Microphones

Acoustic measurement is an important part of the measuring science,Mainly divided into air metering, ultrasonic sound measurement, acousticmeasurement. Air sound pressure measurement is closely related to ourdaily life. Air pressure on the values of quantities of repetition andtransfer is mainly through laboratory standard of the sound pressuresensitivity and microphones acoustic sensitivity of the realization ofcalibration. Air sound pressure datum includes mutual coupling cavity tosound pressure datum and the free-field reciprocity calibration to soundpressure benchmark. As a result of free field sensitivity of microphonecalibration has represented a national air acoustic measurements of themaximum level. And in the light of the microphones free field to eachcalibration device, mainly uses the discrete type analogue instrument, theefficiency is lower, and direct measurement of the acoustic signal way inwaves hard purify the influence of the disturbance signal, so it isnecessary to establish and perfect the microphones free field to eachcalibration device.Our country is still did not participate in the international keycomparison of the laboratory standard microphones free-field sensitivity, and at the same time to improve our sound air traceability system, thispaper studies the free-field reciprocity calibration technology of LS2Fmicrophones and WS4microphones to build a20kHz-100kHz ultrasonicfrequency band free-field to each calibration device, relying on threeLS2F respectively microphones and three WS4microphones conducted aseries of free-field reciprocity calibration experiment and completed theuncertainty.First, the free-field calibration system is divided into hardware andsoftware system. Hardware system including PULSE system, sound box,microphones positioning system, etc. the software system is divided intodata measurement part, data processing part and data computation part.Data measurement part realized communication of PULSE system andPC’s real-time, effectively extract the signal of transfer impedance; Dataprocessing using Fourier inverter and time choice technology to waveisolated, got the pure electric transfer impedance; Data computation partrealize the sensitivity of the free field data automation.Secondly, based on the three LS2F microphones and three WS4microphones conducted a series of free-field reciprocity calibrationexperiment. Through the comparison of the experiment results afterselection technology and IFFT time, find out LS2F microphones throughthe Fourier inverter change and time selection technology, the free-fieldsensitivity can reduce the error of about2.0dB in high frequency, the free-field sensitivity of WS4microphones can reduce about the error of4.0dB in high frequency.Finally, the LS2F and WS4microphone free field reciprocitycalibration conduct uncertainty evaluation. LS2F microphones in1.0kHz-51kHz frequency range, the uncertainty index compared to theinternational advanced metrology institutes is a same level; WS4microphones in10kHz-100kHz, frequency range uncertainty index metthe requirement.High frequency free-field reciprocity calibration of condensermicrophones technology research, established a high frequency of freefield reciprocity calibration device and expanded on the air pressure ofthe value of transmission range, met the demand of the air ultrasoundmeasurement, perfected the sound of the China air quantity traceabilitysystem.