First, my approach to the effects of helms on surprise, hear noise, and similar abilities is more freeform. I've mentioned before how I go with a +1 bonus for the side which has an advantage over the other. There's no set list of when to apply the bonus: it's a matter of interpretation. Here are some examples:
- listening at a door: wearing a helm that covers the ears places you at a disadvantage.
- surprise during an ambush: narrow eyeslits place you at a disadvantage for ambushes you could detect with peripheral vision; helms that cover the ears place you at a disadvantage for ambushes you could detect with hearing.
That leaves attacks aimed at the head. Attacking the head means using the head's AC instead of the body's AC, and there are three kinds of helms: non-rigid (leather or padded headgear,) semi-rigid (chain coif,) and rigid (helm.) A successful hit requires a Con save instead of general hp loss, with a penalty based on helm type (no penalty for rigid helms.) Failure means loss of the body part -- immediate unconsciousness, for blunt attacks to the skull; decapitation, for edged attacks to the neck.
The difference between a Roman-style metal helm with an open face and a great helm is that an attacker can attempt a blow to the AC 9 chin/jaw instead of a blow to the AC 3 skull. Blows to the jaw aren't killing blows, but can cause serious damage and put an opponent at a disadvantage. Similarly, an attacker can aim a thrusting weapon at the eye (AC 9 for most helms, AC 5 for great helms with a wire mesh over the eyeslit.) A failed Con save results in blindness.