空气耦合超声换能器声场计算与测量研究

Investigation of Acoustic Field Calculation and Measurement Method for Air-coupled Ultrasonic Transducer

利用空耦超声换能器发射孔径的空间脉冲响应计算平面换能器的声场分布特性。该方法基于线性声学理论的假设,利用孔径表面发射的球面脉冲波的叠加来获得空间中任意点的脉冲响应。由于空气介质中超声的吸收衰减对于声场分布影响较大,并且衰减随着频率的增大而增大,所以在频域计算中引入传播衰减函数,并通过与换能器激励脉冲的卷积获得该场点的声压分布。利用多轴扫描系统及微小孔径接收器空耦换能器的声场分布情况进行试验测量,并与声场的理论计算结果进行对比,两者取得很好的一致性。试验研究不同尺寸的接收孔径对声场测量结果的影响,结果表明,由于空气耦合中转换效率、界面损耗等原因,其声场测量时的接收孔径选择是分辨率和信噪比的权衡结果,在接收增益和信噪比允许的情况下尽量减小接收孔径,可以提高声场测量的精度。

The field characteristic of an air-coupled planar ultrasonic transducer was investigated using the spatial impulse response of transducer transmitting aperture,which is based on the assumption of linear sound theory.The impulse response of arbitrary point in field space is calculation by the summation of spherical wave from all of the small area on the transmitting aperture.Attenuation is one of important factor impacting acoustical field distribution for the air-coupled ultrasonic transducer,and which increases with the frequency,a frequency-dependent attenuation function is considered in the field calculation of frequency domain,and then the pressure distribution is obtained by convolution integral of the impulse response with excitation impulse.Compared field measurement experiments are implemented using a multiple-axis scanning system and a miniature aperture detector.The results of theoretic analysis and that of experiment are in a good agreement.Finally,the effects of the receiving aperture size are also considered in sound field measurements,the results indicate that because of the low efficiency and interface loss in air couple,a compromise between resolution and signal to noise ratio（SNR） should be taken into account to choose the receiving aperture size.And a smaller aperture could improve the measuring resolution on condition that the receiving amplification coefficient and the SNR are valid.