Two sound sources of same frequency produce sound intensities $I_{0}$ and $4 I_{0}$ at a point $P$ when used separately. Now, they are used together so that the sound waves from them reach $P$ with a phase difference $\phi$. Determine the resultant intensity at $P$ for
(a) $\phi=0$
(b) $\phi=2 \pi / 3$
(c) $\phi=\pi$