Efficient transduction of a circumferential (C-) Lamb wave by a pair of noncontact air-coupled ultrasonic transducers (NAUT) was presented. A line focus type (LFT-) of the NAUT was employed for the efficient transduction. Characteristics of the LFT-NAUT were theoretically and experimentally shown in comparison to those of the conventional plane type (CPT-) NAUT. The efficiency of the C-Lamb wave generated and detected by the LFT-NAUT took about 20 times higher than that by the CPT-NAUT. As for an application of the C-Lamb wave generated by the LFT-NAUT, a novel method of an accurate estimation of pipe wall thickness was introduced and evaluated with a theoretical model. It was confirmed that the averaged error between the experiments and the theoretical model was at less than 10 μm.
