Have you ever been watching television and suddenly the commercials come on and you’re jostled by the booming voice of someone trying to sell you something? Why do commercials sound louder than the television show?
Advertising is typically talking head (one or two speakers and not a lot of music or sound effects) content that benefits from audio compression. Bringing everything to an even tone allows to speaker to sound consistent and avoids imbalance between two speakers that use their voice at varying volumes.
Audio present during television shows has more dynamic range, and rightfully so. Dramatic pauses, sudden surprises, and hushed whispers all need to have varying volume levels in order to draw the viewer in and keep their attention. If audio is compressed during a dramatic show, it’s done at varying degrees.
The audio levels between a television show and a commercial are actually matched as they’re sent over the airwaves (or cables) to your television. This is part of a limit required by law (at least in the US). The change in volume you notice is actually a perceived volume difference because where a television show saves the higher volume levels for action scenes, the commercial maintains that level constantly throughout the program. It does this thanks in part to compression present in the audio channel.
Here’s where your television comes in. Automatic gain controls and other nifty little tools allow the television to unintentionally do the advertiser’s work for them by automatically boosting a signal to a high point if it appears to be hitting a ceiling below the expected line. Your television will actually take audio levels that are set to the average amount of the television show and boost it all by itself.
While the FCC is flooded with complains from people claiming that the network is boosting commercials intentionally, this is actually the result of some clever engineering by the makers of the commercial and an unfortunate side effect of television technology intended to make everything sound even.