What is voice onset time (VOT)? How does it help us to distinguish speech sounds like ba and pa?

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What is voice onset time (VOT)? How does it help us to distinguish speech sounds like ba and pa?

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A /p/ sound is made by trapping air behind your lips and then releasing it. A /b/ sound is made by doing this while simultaneously starting to vibrate your vocal cords. An /a/ sound is made by vibrating your vocal cords (well, with the back of your tongue in the right position) without trapping any air.

So /pa/ and /ba/ both start the same way (air trapped, vocal cords not vibrating) and end the same way (air flowing, vocal cords vibrating). The only difference between them is how much time passed between when you released the trapped air and when you started vibrating your vocal cords. This length of time is called the “vocal onset time” (VOT).

Note that the border between what counts as a /pa/ and what counts as a /ba/ is different in different languages. One language might say “it’s a /pa/ if the VOT is greater than 15ms and a /ba/ if the VOT is less than 15ms”; a different language might say “it’s a /pa/ if the VOT is greater than 25ms and a /ba/ if the VOT is less than 25ms”.