Hi Abdes,
In order to be able to help you at all I think we need a bit more information about what you are trying to do. This could include which formula you're using (wave_equation.vti?) and what kind of output you are expecting.
It kinda sounds like you want something like the inverse-wave-equation, which might even be a thing! (ripples that ripple-inwards? negate the addition in the update step? might work.. but might just explode too.. not sure).
Either way, more details about the problem you're facing would be very helpful :D
Dan