Obstruents frequently occur in pairs—one voiced (vocal folds vibrate) and one voiceless (no vibration)—such as /s/ vs. /z/ or /t/ vs. /d/.
Obstruent vs. Sonorant Consonants in Phonetics ... - Facebook obstruent
💡 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 if it feels smooth (like "mmmm")