L2 learners are known to be sensitive to acoustic cues beyond the bare minimum required to distinguish target language phonemic contrasts. This paper examines changes in this sensitivity to such extraneous details in the acoustic signal following extended exposure to L2 input. Our Category Activation Threshold Model (CATM) posits that the acquisition of new phonological categories brings about inhibitory mechanisms that actively suppress the processing of acoustic cues not needed for phonemic distinction. To assess the use of acoustic detail at different levels of proficiency, we administered a similarity judgment task to two groups of Japanese learners of English: university students with no study-abroad experience (NoSA: n = 22), and students with 3–12 months experience living and studying in an English-speaking country (SA: n = 18). SA participants judged the acoustic distance between pairs with one native and one non-native segment, /s/~/θ/, /z/~/ð/, to be greater than NoSA participants did. However, the SA group rated the acoustic difference between two utterances of the same stimulus item containing non-native /f/ or /v/ to be smaller than the No-SA group did. This result accords with the CATM, exhibiting the inhibitory effects of category formation.