# Kirchoff-Helmholtz Integral problem

1. Nov 2, 2009

### danong

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 $$F = \phi\nabla\varphi - \varphi\nabla\phi$$,

however, i understand that $$\phi\nabla\varphi$$ represents the velocity of sound vibration across the surface to an observer point.

For say, if i take $$\phi$$ as Green's function and $$\varphi$$ as Sound potential / pressure.

So the problem comes,
how would i understand $$\varphi\nabla\phi$$? 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 $$\varphi\nabla\phi$$?
It just seems very confusing to me,
hope someone could point out as i'm really stucked in this topic for months.