I don’t mind ads per se. I’m a fan, even. They allow sites to serve their content for free*. It’s the deafening prerolls that I take issue with.
* Yea, yea, “if you’re not paying, you’re not the customer but the product”. Yet how many news sites are you ditching because they require that you subscribe to them? And how many do you use because they don’t?
Compression is a bit of it. The other bit is commercials usually mix in stereo and negate the 5.1. By not using the subwoofer, it gives an extra 10dB of immediately available headroom, before any compression or limiting occurs. 10dB is... you guessed it, more than double the amount of perceived loudness.
The solid rectangle block is the result of using a limiter. Basically a hard ceiling compressor, for simplicity.
Yeah, having worked on ads you can be sure we check our levels to make the most of the audio's range. It's a technical issue not an intentional one. The people to be annoyed at are the content creators who don't check their levels and have quiet audio so you get blasted when properly mixed audio in an ad kicks in.