(On YouTube at:  https://youtu.be/TUe0C9g-ueg )

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To do the sign for "whisper" cup your passive hand at the side of your mouth and use your dominant hand to do the sign for "TALK" (The variation that uses a 4-hand at the mouth).

 

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Notes: Another way this sign is done is by leaning forward a bit, and cupping your dominant hand at the side of your mouth (without needing to do the "TALK" sign).