Sonorant and Obstruent
Sonorant and obstruent are the two major classes of speech sounds defined by the degree of constriction in the Vocal Tract and the resulting airflow.
Sonorants
Sonorants are produced with a vocal-tract configuration open enough to allow continuous, non-turbulent airflow and spontaneous voicing. The class includes all vowels, nasals /m, n, ŋ/, laterals /l/, the rhotic /r/, and approximants /w, j/. Because the airstream meets no significant obstruction, the vocal folds vibrate freely without the speaker doing extra work, so sonorants are typically voiced. In acoustic terms they show clear formant structure — visible energy bands on a spectrogram — rather than aperiodic noise.
Obstruents
Obstruents are produced with sufficient constriction to obstruct the airflow and generate either complete blockage or audible turbulence. The class comprises plosives /p, t, k, b, d, g/, fricatives /f, v, s, z, ʃ, ʒ, θ, ð, h/, and affricates /tʃ, dʒ/. Obstruents support a voicing contrast — voiceless versus voiced — because the constriction raises supraglottal pressure and voicing requires the speaker to actively manage the pressure differential across the glottis. This is why voiced obstruents are cross-linguistically rarer and less stable than voiceless ones.
Phonological consequences
The sonorant/obstruent split predicts many phonological patterns. Final-obstruent devoicing, a process found in German, Russian, and Vietnamese L2 English, targets only obstruents because the voicing contrast is unstable there. Sonority-sequencing principles use the divide as the largest step on the sonority scale: syllables typically rise from obstruent onsets through sonorants to a vowel peak and fall back through sonorants to obstruent codas.
ELT implications
Vietnamese learners of English routinely devoice English final voiced obstruents (bag → [bæk]) but leave final sonorants intact. Naming the class makes the pattern transparent and the remediation targetable.