Circumflex         The stress put on the marked syllable is drawn out. Always pronounced LONG. In theory the voice goes up, then down again, leading to a singsong sort of intonation. Which is difficult to do unless a 4th century b.C. Athenian comes along and lets us imitate him. I may not do it all that well.
But keep in mind that it most probably SHOULD be pitch going up, then down again. Which will explain why words ending in stressed -ων take an acute accent, unless they are genitive plural or otherwise the result of a contraction. In which case they DO take a circumflex.
As we'll see later on. Just keep an open mind about the seeming vagaries of pronunciation and spelling.
Where do we find it?
on ultima (last syllable)  bla-bla-bla-blâ Examples
on penult (only if última is short)          bla-bla-blâ-bla Examples
  NEVER on antepenult