In recent years, a laboratory noncontact excitation method utilizing the focused ultrasound radiation force generated by ultrasonic transducers has been exploited to excite vibrations of small structures, and may potentially be used for experimental modal testing. However, the inability to calibrate, assess the acoustic radiation spot size, and monitor the real time radiation force prevents this method from practical use for measuring a structure's frequency response functions (FRFs) in modal testing. This work aims to gain a full understanding of the acoustic field generated by ultrasonic transducers and to quantify the radiation force by using a recently developed fiber-optic acoustic sensor. A boundary element model is developed to simulate the generated acoustic field. The vibration of the radiating surface of an ultrasonic transducer is characterized, and velocity profiles are obtained and used to calculate the emitted sound pressure. Dual experimental methods, namely, a microphone array and a fiber optic sensor are used to measure the FRFs of the sound pressure vs. the transducer drive voltage for several planar slices in front of the transducer, resulting in the experimental sound pressure profiles. Compared with simulations, the experimental results from both the microphone array and novel fiber-optic sensor show good agreement. Particularly, the pressure mapping functionality of the fiber-optic sensor is demonstrated, suggesting its potentiality in calibrating the imparted force from the transducer, which is necessary for modal analysis of small structures that cannot otherwise be easily excited at very high frequencies.
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