Antiformant

The articulation of nasal sounds creates antiresonances within the vocal tract. These antiresonances or antiformants are frequency regions in which the amplitudes of the source signal are attenuated because the nasal cavities absorb energy from the sound wave. The effects of these antiformants are more marked in nasal consonants than in nasal or nasalized vowels because consonants are articulated with a complete occlusion of the oral cavity.

Link
Utrecht Lexicon of Linguistics