Watch how you bond the transducer, as this can give spurious results, and provide high losses. A crimped design mght be superior. SIlver solder will be better than epoxy, if the transducer can take a little heat (and you base material can wet silver solder). Metals have the lowest "absorption coefficient". The hardest metals usually have the lowest of the absorption coefficients. Thickness represents the ability to absorb, and geometry to deflect the signal, so keep the thickness short. I'd pick a metal that can be implanted. You'll know those better than me.
And engineers occasionally get to make noise too.
David A. Smith