Two coherent point sources $S_1$ and $S_2$ emit sound of frequency $80Hz$. Source $S_1$ starts emitting sound $\frac{1}{960}$ second after the start of $S_2$. Both sources are at same phases when started. If intensity due to each source at the position of detector is $I_0$, then the resultant intensity observed by the detector shown in the figure is (speed of sound $=320m/s$) is $n{I}_0$, value of $n$ is