A device consisting essentially of a hollow sounding box with two bridges attached to its top. The string, fixed to the box at one end, is stretched between the two bridges so that the free end can run over a pulley and support a measured load. When the string is plucked the frequency of the note can be matched with that of another sound source, such as a tuning fork. It can be used to verify that the frequency (f) of a stretched string is given by f=(1/2l)√(T/m), where l is the length of the string, m is its mass per unit length, and T is its tension.

Originally called the monochord, the sonometer was widely used as a tuning aid, but is now used only in teaching laboratories.