The Corrie Cone of Silence

Remember the Cone of Silence from the 1960s sitcom Get Smart? It varied in type and technology and it never really worked very well. They should have consulted Coronation Street. All you need is two rooms. Person A in one room, and one or more in the other. Have an open door between the rooms and the one person will never overhear a conversation held 10 feet away in the other room, either between the other two or it could be a phone conversation. Don't believe me? It happens all the time! You can read about it over here.