how to transfer energy through the metal wall ??

Dear all,

I am doing a project. I would like to transfer several watts of
(electrical) energy through a metal wall (about 1 cm thick) and use
the energy to activate an electric device. I could put a coil on one
side of the aluminium wall, feed the coil with AC current, and pick up
AC current from the coil on another side of the aluminium wall. But
this method does not work with the steel wall efficiently enough.

I am thinking about using sound waves to transfer the energy. For
example, I could fix an ultrasound transducer to the wall, feed it
with several watts of electrical energy, have almost all of it
transferred through the steel wall of 1 cm thick, and pick up a large
part of the sound energy on the other side with another transducer. Do
you think it is feasible ? The only problem could be that the
receiving transducer would pick up the energy from the noise
vibrations of the metal wall caused by other sources, and I would like
to avoid this. (E.g., I am not sure if the ultrasound receivers are
highly selective in frequency...).

The piezo sound transducers sold by e.g. Farnell or RS Components seem
to be not powerful enough. I saw on the Internet the high-power
ultrasound transducers used in metal welding, but I do not know where
to get the piezo ceramic devices itself.

I would greatly appreciate your considerations.

Regards,
Va1erian