Non-destructive testing techniques, such as ultrasonic testing and acoustic emission analysis, commonly employ piezoelectric sensors for monitoring and detecting defects in structures. The quality of data acquired using these sensors is highly dependent on the coupling layer between the transducer and the structure's surface. In this context, this study compares the signal response of four different couplant materials, namely acrylic adhesive pads, honey, vaseline, and hot glue, applied to a steel surface. For this purpose, experiments were conducted using a laser Doppler vibrometer and acoustic emission analysis to investigate the signal transmission of the couplant materials and their impact on the signal response of a coupled piezoelectric sensor VS30-V. The repeatability of the experiments was statistically analyzed. The findings indicate that acoustic emission measurements with acrylic adhesive pads exhibited the lowest relative standard deviation of 11.4%, followed by honey (13.2%), hot glue (21.9%), and vaseline (32.1%). The investigated couplant materials exhibited different effects on the signal response of the piezoelectric sensor. Specifically, acrylic adhesive pads and hot glue demonstrated more reliable signal transmission in the frequency range of 50 kHz to 65 kHz. In contrast, honey and vaseline had better performance within the frequency range of 65 kHz and 80 kHz. Considering the frequency-dependent characteristics of signal transmission and the ease of application, acrylic adhesive pads and honey are considered the preferred couplant materials for the frequency ranges of 50 kHz to 65 kHz and 65 kHz to 80 kHz, respectively.