Analysis of crosstalk between fluid coupled cmut membranes

Crosstalk between array elements is an important and intensively discussed topic for CMUTs. For operation in fluids crosstalk effects significantly influence the dynamic range at pulse-echo mode, directivity pattern and frequency response and are highly undesired for medical application. On the other hand, crosstalk effects between the membranes and the surrounding fluid are used for other CMUT applications like pumping and mixing in micro fluidic channels. So far, coupling between the membranes and the surrounding fluid was mostly described as Stonely or Scholte waves at the CMUT surface. This paper presents an analytical description of this crosstalk effect between CMUT membranes. The basic idea is that the elastic membranes act as a complex load onto the fluid. Waves at such an interface are comparable to gravitational waves at a free water surface. Considering a homogeneously stiff and mass loaded interface, the speed of sound of an evanescent fluid wave at this interface will be described analytically with a simple formula for a fluid half space bounded by a complex acoustical impedance. The approach was validated with FEM simulations. Investigations with respect to the influence of discretisation and size of typical membranes are discussed as well as the influence of membrane stiffness, membrane mass and inactive area between membranes onto the speed of sound and the intensity distribution of this surface wave. The wave is highly dispersive, its speed of sound increases with increasing stiffness and with decreasing frequency. Both effects are monotonic but nonlinear for frequencies above the membrane resonance. Near membrane resonance the speed of sound approaches zero. FEM simulations of typical CMUT membranes show that these surface waves have a significant influence onto the array behavior and can be well described by the presented analytical approach.