Pretty spendy...
http://www.popsci.com/gear-gadgets/article/2008-03/how-it-works-littlest-subwoofer
http://www.popsci.com/gear-gadgets/article/2008-03/how-it-works-littlest-subwoofer
Ultimate Ears UE 11 Pro custom earphones pack in four separate speakersincluding a subwooferto create sound as realistic as if you were hearing it live...