Ultrasound Research Institute, Kaunas University of Technology, K. Baršausko St. 59, LT-51423 Kaunas, Lithuania.
Sensors (Basel). 2020 Jan 16;20(2):516. doi: 10.3390/s20020516.
At low frequencies, in thin plates the phase velocity of the guided A mode can become slower than that of the ultrasound velocity in air. Such waves do not excite leaky waves in the surrounding air, and therefore, it is impossible to excite and receive them by conventional air-coupled methods. The objective of this research was the development of an air-coupled technique for the reception of slow A mode in thin plastic films. This study demonstrates the feasibility of picking up a subsonic A mode in plastic films by air-coupled ultrasonic arrays. The air-coupled reception was based on an evanescent wave in air accompanying the propagating A mode in a film. The efficiency of the reception was enhanced by using a virtual array which was arranged from the data collected by a single air-coupled receiver. The signals measured at the points corresponding to the positions of the phase-matched array were recorded and processed. The transmitting array excited not only the A mode in the film, but also a direct wave in air. This wave propagated at ultrasound velocity in air and was faster than the evanescent wave. For efficient reception of the A mode, the additional signal-processing procedure based on the application of the 2D Fourier transform in a spatial-temporal domain. The obtained results can be useful for the development of novel air-coupled ultrasonic non-destructive testing techniques.
在低频下，在薄的板中，导波 A 模式的相速度可能会变得比空气中的超声速度慢。这种波不会在周围空气中激发出泄漏波，因此，不可能通过传统的空气耦合方法来激发出和接收它们。本研究的目的是开发一种用于接收薄塑料薄膜中慢 A 模式的空气耦合技术。本研究证明了通过空气耦合超声阵列接收亚音速 A 模式的可行性。空气耦合接收是基于在薄膜中传播的 A 模式伴随的空气中的消逝波。通过使用从单个空气耦合接收器收集的数据排列的虚拟阵列，增强了接收效率。记录并处理在与相位匹配阵列的位置对应的点处测量的信号。发射阵列不仅激发了薄膜中的 A 模式，还激发了空气中的直达波。该波在空气中以超声速度传播，并且比消逝波快。为了高效地接收 A 模式，需要进行基于在时空域中应用二维傅里叶变换的附加信号处理过程。所获得的结果可用于开发新型的空气耦合超声无损检测技术。
