Sound from two coherent sources  $S_{1}and{S}_2$ are produced in phase, and detected at point P, equidistant from both the sources. Speed of sound in normal air is 340 m/sec, but in some part in path $S_1$  there is a zone of Hot air having temperature 4 times the normal temperature, and width (d = 85m). What should be minimum frequency of sound, so that minimum intensity can be found at P? (in Hz)