This law sounds like the same passed by congress which was adopted by 40 states so far. Looks like the interpretation of "fully enclosed" varies from state to state.This is what google provided.
HELMET LAW TN Code annotated. Section 55-9-302. Crash helmet required for driver and passenger — Exceptions. —
(a) The driver of a motorcycle, motorized bicycle, as defined in chapter 8 of this title, or motor-driven cycle, and any passenger on any of these, shall be required to wear either a crash helmet meeting federal standards contained in 49 CFR 571.218,
or, if such driver or passenger is twenty-one (21) years of age or older, a helmet meeting the following requirements:
(1) Except as provided in subdivisions (a)(2)-(4), the helmet shall meet federal motor vehicle safety standards specified in 49 CFR 571.218;
(2) Notwithstanding any provision in 49 CFR 571.218 relative to helmet penetration standards, ventilation airways may penetrate through the entire shell of the helmet; provided, that no ventilation airway shall exceed one and one-half inches (1 ½″) in diameter;
(3) Notwithstanding any provision in 49 CFR 571.218, the protective surface shall not be required to be a continuous contour; and
(4) Notwithstanding any provision in 49 CFR 571.218 to the contrary, a label on the helmet shall be affixed signifying that such helmet complies with the requirements of the American Society for Testing Materials (ASTM), the Consumer Product Safety Commission (CPSC), the Southern Impact Research Center (SIRC), or the Snell Foundation.
(b) This section does not apply to persons riding:
(1) Within an enclosed cab;
(2) Motorcycles that are fully enclosed, have three
(3) wheels in contact with the ground, weigh less than one thousand five hundred pounds (1,500 lbs.) and have the capacity to maintain posted highway speed limits;
(3) Golf carts; or
(4) In a parade, at a speed not to exceed thirty (30) miles per hour, if the person is eighteen (18) years or older. [Acts 1967, ch. 45, § 2 (T.C.A. (supp.), § 50-944); Acts 1976, ch. 758, § 2; 1979, ch. 247, § 10; T.C.A., § 59-934; Acts 2000, ch. 606, § 3; 2005, ch. 430, § 1; 2005, ch. 459, § 1; 2006, ch. 535, § 1.]