being a stage manager for festivals the circular stage idea would be a nightmare for getting artists and kit to the relevant stage as access is normally through the back stage area for vehicles etc... using closed marquees can greatly reduce sound spill as well using a short throw PA rather than long throw. also making sure that the PA's are over specked means that they can be run at low levels with good sound quality rather than being pushed at their max, something that can give a overdriven feel to the music.

a double skin of breeze blocks with sand filling the cavity and a natural grass roof can give good sound proofing. one thing to remember with this technique is to reinforce the bottom few levels of block work to make sure the sand does not buckle the wall at the base (due to the sheer wieght). combine this with hard wearing carpet for the interior walls and a sound proof bunker you will have (with a marginal outlay of cash money)

sound proofing is inherantly expensive but sand and blocks are by far the cheepest option