The paper presents a spectrographic study of the acoustic phonetics aspect of Bengali nasal speech sounds in three different cases namely, nasal murmurs , nasalisation of vowels due to contextual effects of nasal murmurs and purely nasal vowels. Data base consists of 100 and 14 Bengali words pronounced by 3 male and 1 female informants. The analysis of formant structure of nasal vowels and nasal murmurs are presented A formant frequency shift is noted due to nasalisation in pure nasal vowels. Additional nasal formants are also found to occur in few cases. Specific formant frequency region of nasal murmur exhibits some discrimination with respect to the place of articulatioa Keywords: Formant; Antiformant; Nasal murmur; Nasal vowel; Spectrographic analysis.