The Quieting Charm is used whenever a witch or wizard needs to undo the effects of the Sonorous Charm. Its only known use in the novels occurs after the final match of the 1994 Quidditch World Cup. Ludo Bagman, the Head of the Department of Magical Games and Sports, used the Sonorous Charm to project his voice across the entire stadium while providing commentary for the Ireland versus Bulgaria match. Following the match and the chaotic appearance of the Dark Mark in the sky, Bagman used the Quieting Charm on himself, casting `Quietus` to return his voice to its normal state before he could Disapparate from the Top Box (GF9).
The specific difficulty of learning the Quieting Charm is not detailed, but it is likely taught in conjunction with the Sonorous Charm as its direct counter. As a functional, non-combat spell, it is presumed to be of average difficulty. The charm itself is the specific and only known counter-spell for `Sonorus`. There is no known counter for `Quietus`, as its purpose is simply to end a pre-existing magical effect rather than impose a new one that would need to be reversed.
The incantation, Quietus, is a Latin word meaning “quiet,” “calm,” “peaceful,” or “at rest.” This directly reflects the spell's function, which is to restore quiet to the user's voice by removing the magical amplification. In English, the word “quietus” can also refer to the act of settling a debt or, more archaically, to death, but the spell's meaning is tied to the literal Latin definition of becoming quiet.