In batman arham asylum how do you exsplode the walls

To explode the walls, you need to have the detonation gun or whatever it is called. You get it from the batmobile, early in the game.

On the PS3

go to detective mode Press L2
find a weak wall
press left direction button to choose gun
press L1 and R1 to spray
when you want press L1 and R2 to explode