Muggles' Guide to Harry Potter/Magic/Sonorus

Overview
Sonorus is a spell that is used by a wizard to make his voice louder, used when he needs to address a large gathering.

Extended Description
The Sonorus spell is first seen used by Ludo Bagman when he is called upon to commentate the Quidditch World Cup. Having charmed his own voice, it then is loud enough to reach every one of the hundred thousand or so wizards in the stadium. When the game is finished, Ludo ends the spell by invoking Quietus.

Ludo uses this spell again when commentating the events in the Triwizard Tournament.

Analysis
From a simple viewpoint of physics, one has to wonder about how this charm actually works. Specifically, if Quidditch matches are anything like football (soccer) matches, the fans will be loudly singing their team's songs, and so sound that would reach them would have to be extremely loud. Muggle stadiums have multiple sound sources, set up in the air well away from the spectators, so that they can be made arbitrarily loud without deafening anyone in particular. Yet Ludo is in the top box, along with all the dignitaries; sound loud enough to reach the cheap seats at the far ends of the pitch, over all the shouting that the fans are going to be doing, would have to be literally deafening to other occupants of the Top Box, and rendering the Minister for Magic deaf, along with the Minister's Bulgarian opposite number, cannot be good for any wizard's employment prospects. On that basis alone, it is most likely that the spell works partly by boosting the Charmed wizard's volume, and partly by selectively sensitizing the onlookers' ears to his voice, so that they hear him partly through the air, partly by direct mental impression. This would explain both the ability of Ludo's voice to reach the entire Quidditch World Cup stadium, and also be dimly audible to Harry while he is waiting to enter the arena for his First Task – in the latter case, Harry was not in the area where Ludo had chosen to make his voice audible, so could only hear the amplified audio portion.