Not to disagree with WildBill at all, but the density question reminds me of the old college 'safety cars' that had 12" soft foam padding on the dashes. at anything over 15 miles an hour, it was the same as none at all.
I agree that anything designed for a helmet is probably too hard, but it will deform and be better than no padding or stuff that's too soft. Let's face it, that's a steel baseball bat swinging at your head.