This article traces the technological and cultural transformation of car audio from its World War II–era status as an aid to middle-class information efficiency through a process of fragmentation, including the development of “boom cars.” These ultraloud sound systems, enabled by wartime innovations and postwar abundance, increasingly became indexes of opposition for those at the ethnic and economic margins of American life. Boom cars turned the perfected technology of hi-fi car audio outward into the streets, using sound to aggressively contest space and assert identity. These technological practices have been met with resistance from governments, continuing the centuries-long struggle for social power on the proxy battlefield of sound, and offering a template for understanding the possibilities and challenges of technological multiculturalism.