Svilainis Linas, Chaziachmetovas Andrius, Kaskonas Paulius, Gomez Alvarez-Arenas Tomas E
Electronics Engineering Department, Kaunas University of Technology, Kaunas, LT 51368, Lithuania.
Electronics Engineering Department, Kaunas University of Technology, Kaunas, LT 51368, Lithuania.
Ultrasonics. 2023 Aug;133:107025. doi: 10.1016/j.ultras.2023.107025. Epub 2023 May 4.
An acoustic field distribution investigation in air requires a small receiving sensor. Needle hydrophones seem to be an attractive solution, and it has previously been demonstrated that needle hydrophones designed for use in water can be used in air. The metrology problem is that an absolute sensitivity calibration is needed, because needle hydrophones are not characterized in air, especially for frequencies below 1 MHz, which is of interest for air-coupled ultrasound. Conventional, three-transducer/microphone reciprocity calibration requires measurements to be done in the far field. However, when transducer diameter is large and the frequency is high, the required measurement distance becomes very large: 3 m for a 20 mm source, transmitting at 1 MHz. Large propagation distance leads to high attenuation and nonlinear effects in air propagation, and distortion and losses accumulate. Small needle hydrophones have low sensitivity, so that high excitation amplitudes would be required, which can lead to transducer heating and increase nonlinearity effects. A derivative of the three-transducer reciprocity calibration method is proposed, where a large aperture transducer is focused onto a hydrophone, using hybrid of plane wave and spherical wave reciprocity. Use of a focused source minimizes the frequency-dependent diffraction effects, and the spherical wave approximation is valid at the focal distance, and low level excitation signals can be used. Focusing is accomplished using a parabolic off-axis mirror. Calibration is in transmission, which reduces the complexity of the electrical measurements. The corresponding equations have been derived for this setup. Calibration of the transducer and needle hydrophone absolute sensitivity is obtained.
在空气中进行声场分布研究需要一个小型接收传感器。针状水听器似乎是一个有吸引力的解决方案，并且此前已经证明，设计用于水中的针状水听器可用于空气中。计量学问题在于需要进行绝对灵敏度校准，因为针状水听器在空气中没有特性描述，特别是对于低于1MHz的频率，而这正是空气耦合超声所关注的频率范围。传统的三换能器/麦克风互易校准需要在远场进行测量。然而，当换能器直径较大且频率较高时，所需的测量距离会变得非常大：对于一个20mm的源，在1MHz频率下发射时，测量距离为3m。大的传播距离会导致空气传播中的高衰减和非线性效应，并且失真和损耗会累积。小型针状水听器灵敏度较低，因此需要高激励幅度，这可能导致换能器发热并增加非线性效应。本文提出了一种三换能器互易校准方法的衍生方法，其中使用平面波和球面波互易的混合方法，将一个大孔径换能器聚焦到一个水听器上。使用聚焦源可将频率相关的衍射效应降至最低，并且在焦平面处球面波近似有效，同时可以使用低电平激励信号。聚焦是通过一个抛物面离轴镜来实现的。校准是在透射模式下进行的，这降低了电学测量的复杂性。已经为这种设置推导了相应的方程。获得了换能器和针状水听器绝对灵敏度的校准结果。
