Resonances and nonuniformities in CMUT elements or arrays

We determine the response of individual cells of a CMUT array immersed in water using the small-signal equivalent circuit of a single cell and radiation impedances. Using a numerically efficient technique, we are able to simulate large arrays. The results indicate the presence of resonances at low frequencies where Rayleigh-Bloch waves are excited on the surface of the array. Reflections from the edges cause standing-wave patterns. Above the cut-off frequency of Rayleigh-Bloch waves, no standing-wave pattern exists. However, there is nonuniformity among cell velocities mainly due to unequal radiation impedance seen by the cells. Rayleigh-Bloch waves and nonuniformity in cell velocities do not cause a significant degradation in the point spread function, but the oscillations extend the duration of impulse response, limiting the dynamic range.