An electro-acoustic transducer includes a housing enclosing an electro-acoustic transducer device and having an opening by which the diaphragm side of the transducer device is exposed to the outside, and a ring provided to the outer surface of the perimeter of the opening. The outer perimeter of the...http://www.google.es/patents/US4981194?utm_source=gb-gplus-sharePatente US4981194 - Electro-acoustic transducer
