Muffliato

Source: The Half-Blood Prince

Fills the ears of target persons near the caster with an unidentifiable buzzing, so that the caster can hold lengthy conversations without being overheard.

References from the canon

• One of the spells Harry learned from the Half-Blood Prince's notes. Hermione disapproved of it and would refuse to talk at all if Harry had cast the spell on anyone in the vicinity (HBP12)
• Ron wished Harry had used this when Sprout caught the trio talking in class during their sixth year (HBP14)
• Cast by Hermione to help cover up her conversation with Harry and Ron on the night of Harry's seventeenth birthday (DH7).
• The corridors at the Ministry of Magic were as hushed as though the Muffliato charm had been cast over the place (DH13).
• One of the protective charms cast by Hermione on the campsite (DH14).