They are marked by the presence of noise or silence during the constriction phase.

Air is blocked completely and then released (e.g., /p, b, t, d, k, g/).

Air is forced through a narrow passage, creating friction (e.g., /f, v, s, z, ʃ, h/).

In phonetics, obstruents are divided into three main "flavors" based on how the air is managed:

Obstruents frequently occur in pairs—one voiced (vocal folds vibrate) and one voiceless (no vibration)—such as /s/ vs. /z/ or /t/ vs. /d/.

💡 If a sound feels like it "squeezes" the air (like "ssss") it’s an obstruent; if it feels smooth (like "mmmm"), it’s a sonorant. Advanced Phonological Features