The National Park Service has just released their "Programmatic Accessibility Guidelines for National Park Service Interpretive Media." It appears to be an excellent document that, with input from Janice Schacter at NYC's Hearing Access Program, accurately describes various forms of assistive listening, while also noting that
Induction loop systems use an electromagnetic coil to create a magnetic field. People who use hearing aids or cochlear implants with a T-coil can receive the sound directly by switching their hearing aid to the "T" position. No additional devices are needed. Receivers with headsets shall be available for people without a T-coil, who do not wear hearing aids, or do not have a cochlear implant.
Thanks partly to Janice's advocacy, hearing loops are now in place at two National Park Service sites: Ellis Island and the Dayton Aviation Heritage site.