Frequency of (aural) language exposure and productive (oral) language use, context, and interlocutors are important factors for language learning. Applied linguists and study abroad (SA) researchers have studied these variables mostly retrospectively through questionnaires and learnersâ self-reports which have unknown correspondence with learner's actual language use and social networks. Surprisingly few applied linguists have attempted to capture quantitative aspects of day-to-day language use through systematic long-term observation even though methods to do so have been used in psychological science for some time. In this article, we present arguments for the application of acoustic behavioral observation in applied linguistics and SA research. We introduce the Electronically Activated Recorder (EAR; Mehl, 2017) and illustrate its application in a case study about the use of first, second, and additional languages by two German high school students during their year abroad in the USA. The EAR is a digital audio recorder, currently available as an app for Android devices. It can be installed on smartphones worn by their users as they go about their daily activities. The app intermittently (e.g., 5 times per hour) records brief (e.g., 30 s) snippets of ambient sounds from the wearer's moment-to-moment sonic environment. EAR users are unaware of when exactly the device is recording. We describe the method, data collection, transcription, coding and analyses, address ethical concerns about its application, and report findings from the case study about the SA studentsâ language use to illustrate the potential and limitations of the method for applied linguistics research.