We all know that by galvanizing a plate with a silver coating, this plate will have a very high reflection coefficient and act as a mirror for EM radiation (not for all $\lambda$ ranges of course) with specular reflection. This is nicely elaborated by John Rennie in a recent post here.
Now I would imagine that one could conceive a mirror for any type of wave, be it mechanical or of EM nature. I would imagine that the same line of reasoning with Huygens principle as for a light mirror should be applicable to acoustic waves.
Knowing that sound is a mechanical longitudinal wave, does this fact make the properties of a sound-mirror very different than that of a light-mirror?
How would one go about building a sound mirror (let's say for our own audible range)? Should the material used be very elastic or instead very hard? How should it be galvanized?
Last question: wikipedia's page on acoustic mirrors gives a few pictures of acoustic mirror used in WW1 made of concrete, they all seem to be concave and not flat, why do they have to be concave?
I have tried to make the questions as related as possible to one another, so that the post remains coherent and answerable. Note that I'm just trying to gain an understanding and don't intend to build an actual acoustic mirror myself, this should spare repliers from the possible in-depth technical details.