Obstruents frequently occur in pairs—one voiced (vocal folds vibrate) and one voiceless (no vibration)—such as /s/ vs. /z/ or /t/ vs. /d/.
They are marked by the presence of noise or silence during the constriction phase. obstruent
They produce a significant increase in air pressure inside the mouth compared to the surrounding air. Obstruent vs. Sonorant obstruent