The world is getting louder. Scientists define "noise" as unwanted sound, and the level of background din from human activities has been doubling roughly every three decades, beating population growth. Road traffic in the United States has tripled over the last 30 years. By 2032, the number of passenger flights is expected to be nearly double the 2011 figure—at peak hours, planes are even audible overhead 70 percent of the time in the remote backcountry of Yosemite National Park.
