A ringer employs a low-cost miniature transducer, having a reverberant cavity, mounted in a cylindrical housing which has a second reverberant cavity in front of the miniature transducer. A very large acoustic output power is achieved when the ringer is employed in known cordless telephones, using the...http://www.google.com.tw/patents/US4728933?utm_source=gb-gplus-share專利 US4728933 - Miniature dual cavity ringer
