Final answer:
One downside of EMAT transducers is their relatively low efficiency (option D) , as they must be adhesively attached to a surface and generate sound waves electromagnetically with less efficiency compared to traditional ultrasonic methods.
Step-by-step explanation:
The downsides of EMAT (Electromagnetic Acoustic Transducer) transducers include that they must be adhesively attached to the surface they are inspecting and they display relatively low efficiency in sound generation and reception, which is option D. Unlike traditional ultrasonic transducers that require a liquid couplant to facilitate the transmission of sound waves, EMATs generate ultrasonic waves directly in the material through electromagnetic induction, thus do not require liquid couplant.
Since EMATs rely on a magnetic field to excite and receive ultrasonic waves, their effectiveness is limited when working with nonmagnetic materials. However, the statement 'they do not work with magnetic piping' is incorrect as it's actually one of their application areas, due to their ability to induce and detect ultrasonic waves in ferromagnetic materials.