During the First World War large-scale aerial warfare necessitated new methods of acoustic defense: tracking the enemy through listening and acoustic sensing. Referencing now-declassified military reports, military manuals and scientific literature from this period, this presentation investigates the the development of WWI-era acoustic defense technologies including geophones, double trumpet sound-locators, acoustic visors, listening wells, sound mirrors, acoustic goniometers, the Baillaud paraboloïde and the Perrin télésitemetre. It examines new modes of listening that emerged in relation to these devices, including “alt-azimuth" listening and other modes of collaborative and cooperative listening. It further uncovers historical phenomena like the establishment of écoles d’écoute, “schools of hearing” where Allied soldiers received training in operating acoustic defense technologies, and it examines the design of “ear training exercises” for a new class of expert military auditor. It argues that, during this period, the listening act was reconfigured as a complex, fragmented act of data collection in ways that prefigured modern notions of “machine listening.” Similarly, directional listening, which had previously been studied in terms of perceptual psychology, was newly understood in strategic terms: a tactical activity that could determine human and even national survival.