It depends what sort of noise and how much noise you're wanting to block out.

Autism spectrum sites usually have good information about different types because of sensory issues.

http://www.forbes.com/sites/geoffrey...ng-headphones/
Noise Cancelling Vs. Noise Isolating Headphones - Forbes

There's also interesting info on this site below about Hyperacusis.
There's a small section there talking about Recruitment too - in the section on Hearing Loss
http://www.entnet.org/content/hypera...veryday-sounds