This study investigates the acoustic realizations of English phonemic contrasts by Korean EFL learners, comparing their productions with those of native English speakers. Focusing on the segmentally correct production, the research aims to determine if Korean learners' acoustic properties align with those of native speakers, influenced by second language (L2) proficiency and specific consonant involvement. A total of 20 Korean university students were divided into two proficiency groups and compared with native English speakers. Acoustic analyses were conducted to measure vowel ratios, frication ratios, and formant frequencies. Results indicated that the learners' acoustic properties typically differed from those of native speakers and were mediated by proficiency levels. Even within the high group which showed more correct segment pronunciation than the low group, its acoustic realizations differed from those of native English speakers. In addition, the low group at times illustrated acoustic realizations similar to the native English group. The study underscores the complexity of L2 phonological acquisition in that accuracy at the phonemic and acoustic level may be dissociated. The study thus highlights the need for pedagogical approaches that address accuracy at both segmental and acoustic levels.