1. The formula power (P)= intensity(I) x area(S) can be used at any location away from a source e.g light source or sound source. If you are at a distance r away from the source , the energy per unit time (P) is the IS at the surface of area S.
2. Another way the formula can be applied to find the power of the source (P.)e.g light bulb or loud speaker is to take the intensity at r (I)and the area of the sphere of radius r(S .).
3. Intensity is also directly proportional to the amplitude of the wave. This relationship is separate from the above and to be used independently.
A question containing all these information are to be sorted out into two different approach. Use 1, 2 or 3 to solve by comparison and mostly involve case 1 and case 2 type of problem solving technique.
Good luck
