Sound reflection and absorption by piezoelectric polymer films

The principle to control the elastic coefficient of piezoelectric materials by connecting negative capacitance circuits is described. The stress-induced charge is amplified by the circuit and the feedback voltage is applied on the materials. The direct and inverse piezoelectric effects are thus superposed. By marching the capacitance and loss factor of the circuits to those of piezoelectric materials, the apparent elastic coefficient of the materials are arbitrarily increased or decreased depending on the kind of the circuit. A film of copolymer of vinylidene fluoride and trifluoroethylene (P(VDF/TrFE)) was instilled in the middle of an acoustic tube and connected to the circuit increasing the elastic coefficient. Using a standard two-microphone method, the transmission loss of the film was determined as a function of frequency. The complete isolation of sound was realized at tuned single frequencies. With the improved circuit, about 15 dB increase of transmission loss was obtained over a frequency range from 250 Hz to 1 kHz. A piezoelectric ceramic PZT plate was fixed between a shaker and a mass and connected to the circuit decreasing the elastic coefficient. The transmissibility of vibration through the PZT plate was determined as a function of frequency. The complete isolation of vibration was achieved at tuned single frequencies. With the improved circuit, about 20 dB decrease of transmissibility was obtained over a frequency range from 800 Hz to 4 kHz.