A Dictionary of Music and Musicians/Stopped Pipe

STOPPED PIPE. An organ pipe, the upper end of which is closed by a wooden plug, or cap of metal. The pitch of a stopped pipe is one octave lower (roughly speaking) than that of an open pipe of the same length; it is usual therefore, in a specification, to state the pitch of a stopped pipe instead of its length; thus, 'Open Diapason 16 ft.,' 'Bourdon 16 ft.-tone.' etc. By the former it is understood that the longest pipe is 16 ft. long; by the latter that the longest pipe (though only 8 ft. in length) gives the same note as an open pipe of 16 ft. For the acoustic law which governs the pitch of closed pipes, see Pipes, Vibration of Air in, vol. ii. p. 754.

[ J. S. ]