Abstract
A survey to reveal social structures to determine what kind of contents of public announcement systems are seen as noise by societies are conducted at neighboring areas whose characters are very different from each other : one is a boom town and the other is a old community. In conclusion, (1) Two types of common understandings of contents are exist : understanding as necessary information for the members of a community, and understanding as not necessary information for them. These common understandings make the contents whether noise or not. ; (2) In spite of necessities, scales of commonality of these understandings are variously from as small as small colony to as large as whole city, depending on contents. ; (3) In a boom town, there exist some residents who do not know the contents of public announcement or existence of the public announcement system.
