A frequency-domain synthetic aperture focusing technique for orthotropic media (FD-ORTHO-SAFT) is developed for the in-situ inspection of wood products. The developed algorithm uses the fast Fourier transform to manipulate an ultrasonic array signal in a frequency-wavenumber domain for fast computation speed. In order to take into account the orthotropic property of the media, the group velocity curve information of the media is applied to the developed algorithm. The FD-ORTHOSAFT algorithm is verified with the simulated ultrasonic array signal by the mathematical model and the finite-difference time-domain method. The performances of the algorithm with and without the noise are tested with the simulated signal by the mathematical model and the finite-difference time-domain method, respectively. The FD-ORTHO-SAFT algorithm is compared with the conventional time-domain synthetic aperture focusing technique for orthotropic media (TD-ORTHO-SAFT) in terms of the array performance index, performance map, and the computation time. The results of both algorithms are analyzed using the potential damage curve as well as the operating time. Also, the feasibility of the FD-ORTHO-SAFT is tested using measured ultrasonic array signals obtained by the commercial ultrasonic array device A1040 MIRA from wood specimens. The FD-ORTHO-SAFT algorithm shows a higher detectability than the synthetic aperture focusing technique algorithm for isotropic media and a faster computation speed than the conventional TD-ORTHOSAFT. The results show that the FD-ORTHO-SAFT has a better performance for wood products than the time-domain method at the center of the array and has strong suitability for the in-situ inspection of wood products as well as short computation time.
