Historical clue formulations

• Principal means of address? — 2015-08-01
• Medium for school announcements — 2014-05-08
• It gets the word out — 2012-03-17
• Way to get one's message across — 2003-01-03
• It's used for announcements — 2001-12-25
• Announcer's medium — 2000-07-19
• Medium for announcements — 1999-11-28
• Auditorium fixture — 1999-02-23