Reverb & Sound Leakage?!

Hi, Just finished by build, control rooms are sand/cement rendered and double doors to lobby..... Room is a 6 sided diamond shape with 11ft ceiling. I have put carpet down for now but there is still crazzzy reverb/flutter in there even if you whisper..... Question I want to ask is....The echo can be heard in the next room which is divided by 2 walls of 7N 100mm concrete with 100mm gap.... How does this echo sneak out and is it doubling the sound?? Thanks J

can you post pictures and diagrams?