a open 还是 an open

an open，例如：

1.Night duty, a person surrounded by a car which had an open voice.夜晚值班，一个人，四周是汽车一开而过的声音。

2.An open table tennis tournament is a tournament which is open for entry to the players of all associations.乒乓球公开赛是所有乒乓球协会的球员都可以参加的比赛。