Audoguru has a point. If you go with a peizo speaker, a lot of manufacturers don't live-up to the specs they claim.
There are some other good ideas in this thread that I haven't heard before.
When I selected the peizo speakers I used way back when, I made sure I got ones that were good at 20Khz.
My application was for use in a confined, enclosed patio. I didn't think what I built would work in an outdoor application. Some of the other ideas I've read here might be better for you if you intend an outdoor application.