Four flue pipes of a diapason rank.Wooden flue pipes
A flue pipe (also referred to as a labial pipe) is an organ pipe that produces sound through the vibration of air molecules, in the same manner as a recorder or a whistle, in a pipe organ. Air under pressure (called wind) is driven through a flue and against a sharp lip called a labium, causing the column of air in the pipe to resonate at a frequency determined by the pipe length (see wind instrument). Thus, there are no moving parts in a flue pipe. This is in contrast to reed pipes, whose sound is driven by beating reeds, as in a clarinet.
