Depends on where the arrow was shot from as well. If from a crossbow at a range of say 20 meters, then that arrow is moving pretty quick, and it would take a lot to stop it. But if it was shot from a range of 200 meters, then after a while the arrow would lose momentum and slow down, thus not causing as much, if any, damage. Take note, this is with a crossbow, where is fires pretty straight with not much arc.
Now a bow, fired upward would go up into the sky, lose momentum, fall down, and gain momentum and thus cause a lot of force on impact. Again, thats assuming you got lucky and hit one of the scattered enemies.
At any rate, getting hit in the head with an arrow, protected or not, would suck.