What to put behind water fall.
 

I have been working on my return stream and the water fall into the pond.
What should I put behind the fall to make it hollow so it will echo and give
it a bit more noise. It looks like the fall will only be about 15". I was
thinking about a SS pot like is used for cooking. I know it makes a heck of
a noise when kids beat on it. I could also make it out of landscape timbers
but I think the wood would absorb the sound somewhat and not echo. I also
thought about brick but not sure if that would echo either. I am open for
suggestions, I think the area will be 10" wide. I need to find a weir
calculator again.