Marking a significant turn in media and communications studies, this extraordinary sonic history connects the world's most popular format for recording audio to ideas about ears, media, and technology as they have emerged and developed since the early twentieth century. Jonathan Sterne contends that everything we think we know about hearing in the state of nature is a result of the interaction between ears and technologies in twentieth-century science and engineering. The model of hearing built into the MP3 format reveals the extent to which compression, rather than verisimilitude, is a driving force in the history and ongoing development of communication technology, sonic culture, and aesthetics. Sterne argues that formats, standards, infrastructures, and protocols are displacing the boxes that we call "media" as the technological keys to understanding the history and future of communication. Every MP3 bears the imprint of a theory of human hearing or, rather, its gaps and absences. Investigating the theories of hearing built into the format, he sheds new light on twentieth-century ideas about ears and media, the surprising ways that those ideas functioned both technologically and culturally, and their implications for future studies of media, technology, and the senses.