Transference Impedance Estimation of IEC60318 Couplers by Image Processing and Finite Element Modelling

In order to know the acoustic transference impedance of acoustic coupler described in IEC60318 standard it is necessary to make measures based on the reciprocity technique. This technique is used for microphones calibration as is defined in IEC61094-2 standard. This calibration method is complex enough to execute, and therefore it would be very interesting to look for alternative procedures that allow knowing the coupler acoustic impedance. In this paper an acoustic impedance calibration method for type I coupler is proposed based on non invasive X-ray inspection, virtual instrumentation image processing application and computer simulation. This new approach requires knowing the coupler\´s geometry and use of finite element model approach for acoustic coupler behavior determination. In order to establish a precise mechanical model of the couplers described in IEC60318 standard it is necessary to know the geometry and key dimensions of the coupler. These dimensions are obtained with non-invasive measurement techniques, based on X-ray inspection. Then the use of a finite element model allows prediction of the coupler acoustic impedance values. This modeling is quite different from the "lumped parameter model" proposed in the IEC60318 standard. Lumped parameter modeling has limitations in its application because the dimensions of the coupler\´s elements are comparable with the acoustic signal wavelength. Moreover, when the sound propagates in narrow cavities and ducts the losses produced by viscosity and thermal effects must be taken into account. All these effects are not reflected with accuracy in the classic lumped model. The result of finite element simulation can provide more detailed information about the interior acoustic behavior of the coupler and therefore provide a more realistic and accurate value of acoustic impedance in order to calibrate the device. The comparison of the data obtained with this new approach and the values defined in the current version o- f IEC60318-1 standard (29/633/CDV) confirm the validity of the method.