The 20th century did a number on music technology. Radio made music transmittable. Cars made music mobile. Speakers made music big, and silicon chips made music small. But headphones might represent the most important inflection point in music history.
If music evolved as a social glue for the species — as a way to make groups and keep them together — headphones allow music to be enjoyed friendlessly — as a way to savor our privacy, in heightened solitude. In the 1950s, John C. Koss invented a set of stereo headphones "designed explicitly for personal music consumption," Virginia Heffernan reported for the New York Times. "In that decade, according to Keir Keightley, a professor of media studies at the University of Western Ontario, middle-class men began shutting out their families with giant headphones and hi-fi equipment." Headphones did for music what writing and literacy did for language. They made it private.