Air-coupled ultrasonic transducers offer unique advantages such as non-contact testing, adaptability to complex working conditions, and in situ testing. They are widely used in areas such as ultrasonic ranging, radar, flow meters, and nondestructive testing. Sound field reconstruction holds importance for characterizing the sound field parameters of air-coupled ultrasonic transducers, including beam width and diffusion angle, ensuring the transverse resolution and positioning accuracy of the ultrasonic testing system. The laser method, drawing on the acousto-optic effect, exhibits advantages such as a narrow laser beam, high spatial resolution, extensive frequency range, high sensitivity, and a non-invasive sound field. This makes it an ideal approach for sound field reconstruction. Achieving optimal spatial resolution and superior imaging quality remains pivotal when using laser tomography for sound field reconstruction. Typically, improving the imaging quality involves reducing the translation and rotation step spacing (often to less than half the sound wavelength) to secure more comprehensive sound pressure path data. Given the short wavelength of ultrasonic transducers, the scanning efficiency of ultrasound field reconstruction decreases considerably. This study, therefore, centers on the sound field reconstruction of air-coupled ultrasonic transducers within the 50?200 kHz frequency range and seeks to optimize the scanning parameters. The goal is to enhance scanning efficiency while maintaining the quality of the reconstructed sound field image. We aim for our fundamental strategy and insights to aid in the realization of ultrasound field reconstruction and the enhancement of scanning efficiency via laser tomography.

This study investigated the sound field reconstruction of air-coupled ultrasonic transducers at frequencies of 50?200 kHz based on acousto-optic effects and tomography technology. The spatial resolution of the sound field was optimized through simulations and experiments. Initially, a model was established to simulate the reconstruction effect under various scanning parameters. By exploring the rules and analyzing the outcomes from the perspective of measurement principles and algorithms, optimized scanning parameters were determined. Subsequently, a two-dimensional sound field scanning system was constructed, and a laser Doppler vibrometer (LDV) was employed to measure the integration of the sound pressure along the path. Employing the classic reconstruction algorithm of tomography technology, the filtered back projection (FBP) algorithm, sound field reconstruction of the air-coupled transducer perpendicular to the sound axis direction was completed, and the simulation results were validated. The reliability of acousto-optic effect tomography for ultrasound field reconstruction was confirmed by comparing the reconstruction outcomes of the sound field with the measurement findings from the microphone method.

The translation and rotation step spacings have different effects on the quality of the reconstructed sound field image. When the rotational step spacing remains unchanged, a smaller translation step spacing results in a higher-resolution reconstructed image. The rotation spacing primarily affects the generation of image artifacts (Fig. 4). As the frequency increases, reducing the translation step spacing weakens its impact on improving the quality of the reconstructed images. The range of the sound-field scanning parameters can expand, and selecting the optimized sound-field scanning parameters can increase the scanning efficiency by 2?4 times (Tables 1,2). Laser tomography allows for the capture of sound pressure amplitudes and radial sound pressure distributions similar to those of the microphone method, verifying the reliability of the LDV for sound field reconstruction (Fig. 8). Experiments yield the reconstructed sound field images of the air-coupled ultrasonic transducer under various resolutions, which show strong consistency with the simulation results (Figs. 9 and 10). To compute parameters, such as sound power or sound intensity, fine sound field scanning parameters are essential. For parameters, such as the beam width and diffusion angle, only a high resolution in the central area of the image is necessary, and the requirement for rotation step spacing diminishes. If artifacts appear unexpectedly in the sound field image, then optimal combination parameters exist for the translation and rotation step spacing.

This article investigates the ultrasound field reconstruction of the cross section perpendicular to the acoustic axis of air-coupled ultrasonic transducers at frequencies of 50?200 kHz, drawing on the acousto-optic effect and laser tomography method. Initially, a simulation provides the original sound field distribution of the ultrasonic transducer, considering air attenuation. Radon and inverse Radon transforms simulate the reconstructed sound field images under various scanning parameters, from which optimization strategies for the sound field scanning parameters emerge. This optimization enhances scanning efficiency by 2?4 times while preserving the quality of the reconstructed sound field image. For experimental verification, a measurement system is constructed. The accuracy of laser tomography for ultrasound field reconstruction is first validated by comparing measurement results to those from the microphone method. Following this, acoustic field reconstruction images of the air-coupled ultrasonic transducer at different scanning parameters are experimentally acquired, showcasing strong alignment with simulation outcomes. This research offers an efficient methodology for optimizing the scanning parameters of the ultrasound field based on acousto-optic tomography imaging, holding significant guidance for ultrasound field reconstruction.