Sir plese tell me circuit diagram for find ac mains frequency using sonometer & electromagnet

A sound level meter is a hollow wodden box used in the laboratory to study the transverse vibrations of the string. It is also called monochord because it often has only one sequence. In the rectangular box of wood there are two fixed bridges, near the ends, and at one end there is a pulley. A rope, often a steel wire is attached at one end over the bridges and pulley, and attached to a weight support that hangs below the pulley. Weights may be added to the support to cause tension in the wire, and a third movable bridge may be placed below it to change the length of the vibratory section of the rope.
A sound level meter demonstrates the relationship between the frequency produced by a plucked rope, tension, length and mass per unit length of rope.
For small amplitude vibrations, the frequency is proportional to:
A) The square root of the tension of the string,
B) The reciprocal of the square root of the linear density of the string,
C) The reciprocal of the length of the rope.

F = 1 / 2l v (T / m)