The purpose of the present study is to observe the acoustic insulation by wearing the earplug and to analyze error responses on a articulation test for the Korean speech in terms of phonetic structure. 1) Three types of earplug were designed to permit speech to be heard through a canaliculus, measuring 8 mm in diameter, which kept equal air pressure in the external auditory canal. 2) The No. 3 earplug, which canaliculus is refracted as a symbol of III, could obtain a good acoustic insulation particularly for the high frequencies. 3) Acoustic insulation provided by the No. 3 earplug was shown to range from 10 db for the low tones to 30-40 db for the high tones. 4) Sound confusions and omissions in error responses occurred most frequently in the initial consonants. 5) In the initial consonants, greater percentage of error occurred on the ""ㅅ""(s), ""ㅆ""(tz), ""ㅎ""(h), ""ㅍ""(p), ""ㅌ""(t) belonging to the unvoiced consonants, while the voiced consonants ""ㄴ""(n), ""ㄱ""(g), ""ㅁ""(m) were shown with lesser percentage of error. 6) In the vowels, the percentage of error was greater on the diphthong ""ㅚ""(oi), ""ㅘ""(oa) than the open vowels. 7) The final consonants ""ㅍ""(p), ""ㄴ""(n) showed relatively greater percentage of error.