Effect & Incantation
The Amplifying Charm is a spell that makes the caster's voice or any other target's sounds louder. The incantation of the Amplifying Charm is Sonorus.
From the Story
Discovered in Book 4, Chapter 8, The Quidditch World Cup
"Ludo Bagman uses this Charm to amplify his voice when he gives his welcome speech at the Quidditch World Cup. He directs his wand at his throat and mutters the incantation, allowing his voice to reach every corner of the stands. The counter spell to Sonorus is Quietus."
1. Read the following link: http://www.pottermore.com/en/book4/chapter8/moment1/amplifying-charm
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