New piezoelectric polymer for air-borne and water-borne sound transducers.
Acoustic transducers made of a charged cellular polymer called EMFi have been designed and investigated with respect to air-borne and water-borne sound. The longitudinal transducer constant is around 90 pC/N, strongly exceeding the values of other piezoelectric polymers. This is mainly attributed to the very low Young's modulus of about 2 MPa. The acoustic impedance is only 2.6 x 10(4) kg/(m2 s) and results in good matching to air but strong loading under water. Due to this strong loading, a pronounced reduction of resonance frequency from 300 kHz in air down to 17 kHz under water is observed. The experiments indicate that fluid loading is not only mass-like but also compliant, reducing the transducer's sensitivity below the resonance frequency of about -63 dB re 1 V/Pa (0.7 mV/Pa) in air to -71 dB re 1 V/Pa under water. This compliance is attributed to the medium's compressibility. Piezoelectricity of EMFi films is limited to temperatures below 70 degrees C; above, irreversible discharge of trapped charges takes place. Furthermore, a second type of EMFi, called "OS" was investigated, having a piezoelectric constant of 15 pC/N and a Young's modulus of 6 MPa. In quasi-static sensor measurements, the piezoelectric constant increases with the applied load. This nonlinearity explains the higher values reported in other publications on the same materials.
Kressmann
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Institute for Telecommunications, Darmstadt University of Technology, Germany. kre@nt.tu-darmstadt.de