Answer
A string that is wrapped with another wire.
Work Step by Step
The frequency of the wave on a string is equal to $$f_n=\frac{nv}{2L}=\frac{n}{2L}\sqrt{\frac{F_T}{\mu}}$$ Once the linear mass density increases, the frequency decreases. So, if the linear mass density is larger, the frequency is lower. Therefore, multiple strings would be wrapped around the string to increase the linear mass density and to decrease the frequency.
