Broadband EMFi ultrasonic transducer for bat research

By utilizing the EMFi material, ultrasonic trans- mitters with a diameter of 1.5 cm were developed for emitting a chirp signal with a sound pressure level up to 90 dB at a distance of 1m for the whole frequency range of 20-200 kHz. With the same material, a broadband ultrasonic receiver with a sensitivity of 500 µV/Pa and a low equivalent acoustic noise level of 45 dB was set up. For an optimization of the transmitter and the receiver we need a deeper understanding of the physical behavior of the polymer material. Therefore, we applied a 3D finite element simulation by using a piezoelectric material model for a macroscopic description of the EMFi material. However, vibration measurements of the transducer surface show a nonlinear inhomogeneous vibration behavior at and above resonance frequency. One reason for this is the inhomogeneous structure of the foil. Inside the polymer film, the number of cavities as well as their size strongly varies. Because the resonance frequency of each point of the surface depends on the average cavity size at this point, the whole surface vibrates inhomogeneously. Therefore, the EMFi material cannot be described with a homogeneous piezoelectric material model and we developed a more complex microscopic model for the precise numerical simulation. To solve this problem we computed the electrostatic and mechanical partial differential equation coupled by the electrostatic forces (Coulomb forces) including the complex geometric structure of the cellular ferroelectric film. To investigate the influence of the geometric structure on the vibration behavior, models with different void shapes and sizes have been taken into account.