The combination of a steel wire (length 80 cm , area of cross section 1 ~mm ^2 ) and an aluminium wire (length 60 cm , area of cross section 3 ~mm ^2 ) joined end to end is stretched by a tension of 160 N . If the densities of steel and aluminium are 7.8 ~g / cc and 2.6 ~g / cc respectively then, the minimum frequency of a tuning fork which can produce standing waves in the composite wire, with the joint as a node, i