There is an economic answer: The United States has moved from a farming/manufacturing economy to a service economy, and more jobs "demand higher levels of concentration, reflection and creativity." This leads to a logistical answer: With 70 percent of office workers in cubicles or open work spaces, it's more important to create one's own cocoon of sound. That brings us to a psychological answer: There is evidence that music relaxes our muscles, improves our mood, and can even moderately reduce blood pressure, heart rate, and anxiety. What music steals in acute concentration, it returns to us in the form of good vibes.



That brings us finally to our final cultural answer: Headphones give us absolute control over our audio-environment, allowing us to privatize our public spaces. This is an important development for dense office environments in a service economy. But it also represents nothing less than a fundamental shift in humans' basic relationship to music.



A SHORT HISTORY OF PRIVATE MUSIC

In 1910, the Radio Division of the U.S. Navy received a freak letter from Salt Lake City written in purple ink on blue-and-pink paper. Whoever opened the envelope probably wasn't expecting to read the next Thomas Edison. But the invention contained within represented the apotheosis of one of Edison's more famous, and incomplete, discoveries: the creation of sound from electrical signals.



The author of the violet-ink note, an eccentric Utah tinkerer named Nathaniel Baldwin, made an astonishing claim that he had built in his kitchen a new kind of headset that could amplify sound. The military asked for a sound test. They were blown away. Naval radio officers clamored for the "comfortable, efficient headset" on the brink of World War I. And so, the modern headphone was born.

The purpose of the headphone is to concentrate a quiet and private sound in the ear of the listener. This is a radical departure from music's social purpose in history. "Music together with dance co-evolved biologically and culturally to serve as a technology of social bonding," Nils L. Wallin and Björn Merker wrote in The Origins of Music. Songs don't leave behind fossils, but evidence of musical notation dates back to at least Sumeria. In 1995, archaeologists discovered a bone flute in southern Europe estimated to be 44,000 years old.



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.

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