Study on frequency tunable resonators via variable mass

This study proposes a frequency tunable ultrasonic resonator based on a variable mass method, in which a metal ring, i.e., a tuning frequency ring (TFR) is mounted on the resonator. The resonant frequency of the resonator is adjusted by altering the position of the TFR, which introduces changes in both inertia and stiffness. LTspice were conducted to investigate the effects of the TFR's size, material, and position on the resonant frequency, with experimental measurements further validating the results. Contrary to the conventional belief that adding mass lowers the frequency, the study reveals that placing the TFR near the displacement node can increase the resonant frequency. The results also demonstrate that the tuning range of the system can be controlled by modifying the TFR's geometry and material properties. For a resonator with a resonant frequency of 28  kHz, when the mass of the TFR is about 36  g, studies show that the maximum tunable frequency range is 9  kHz. Based on the energy method, the equivalent mass and stiffness of the resonator were calculated using the SECM (Segmented Equivalent Circuit Model) and cubic spline interpolation, explaining the physical mechanism of frequency tuning using the variable mass method, providing a practical solution for achieving wide-range frequency tuning in ultrasonic resonators.