I'm seeking help in understanding Kirchoff-Helmholtz Integral. Actually what i am facing the problem here is, i don't understand certain things about Green's 2nd identity which stated that two scalar function can be interchanged, and forming the force [tex]F = \phi\nabla\varphi - \varphi\nabla\phi[/tex], however, i understand that [tex]\phi\nabla\varphi[/tex] represents the velocity of sound vibration across the surface to an observer point. For say, if i take [tex] \phi [/tex] as Green's function and [tex] \varphi [/tex] as Sound potential / pressure. So the problem comes, how would i understand [tex]\varphi\nabla\phi[/tex]? distribution of sound pressure with impulse unit at the observer point? Then why do i need to subtract it ? Are they equivalent? How does reciprocal theorem applies here at the [tex]\varphi\nabla\phi[/tex]? It just seems very confusing to me, hope someone could point out as i'm really stucked in this topic for months.