The combination of a steel wire (length 80 cm, area of cross section $$1\ \text{mm}^2$$) and an aluminium wire (length 60 cm, area of cross section $$3\ \text{mm}^2$$) 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, is