Impedance mismatch and attenuation. The impedance difference between air and the piezoelectric elements of the transducer is too great - this will cause soundwa

Impedance mismatch and attenuation. The impedance difference between air and the piezoelectric elements of the transducer is too great - this will cause soundwaves to reflect on contact (and thus never leave the element). If some transducer innovation changed that variable, then attenuation would be the next hurdle. In general a form of couplant (water, oil, some form of gel, etc) is necessary to eliminate air between the transducer and a material in order for soundwaves to pass.