What effect do ferrites have on speaker cables?

I an running 3strand 12AWG 99.999 pure copper speaker cables. Due to room requirements (wife) I have speaker and sub-woofer cable runs of between 15-18 ft. Would ferrites on the cables help? What effects could I expect. Cables run under the house thru crawl space.
Ferrites will deaden the sound which is very bad.Good cables don't need anything.