Low-impedance and low-loss customized materials for air-coupled piezoelectric transducers

For efficient operation of air-coupled piezoelectric transducers (frequency >0.5 MHz) new materials to produce quarter-wavelength matching layers are required. Low acoustic impedance, low attenuation coefficients and the possibility to produce thin membranes attachable to the transducer surface are the main requirements. Two candidate-materials were selected, manufactured and characterized: SiO/sub 2/ aerogel and highly porous PMMA. From the obtained results the suitability of such materials for this particular application is discussed and compared with other materials currently in use.