Efficiency of comb transducers

There are periodic voids at the interface between comb transducer teeth and the investigated sample. These voids are modelled by a periodic system of inplane cracks with solid comb/sample contact between them. When an incident bulk wave is scattered by cracks, a resonant generation of interface `crack waves´ occurs which efficiency can be as high as 25% per single comb tooth. Due to crack wave leakage back to bulk waves, an ordinary steel comb on steel sample is estimated to transform about 5% of the incident power into a Rayleigh wave in one direction and in sample only, depending on the comb geometry