The most important example of this type is the tuning-fork, which may be regarded as consisting of two parallel bars clamped together at the base.
A metal or brass tube will serve as such a pipe, and may be excited by a suitable tuning-fork held at one end.
1834 Hullah's tuning-fork.
If, at the same time, a tuning-fork of known number of vibrations per second be made to trace its own line close to the other, a comparison of the two lines gives the number corresponding to the sound under consideration.
A tuning-fork or a bell-glass, a silk or cotton thread, the other extremity being either fixed or passing over a pulley and supporting weights by which the thread may be stretched to any degree required.