I read once, can't remember where, that once the sound in a room with people talking reaches a certain level, it will get increasingly loud. The reason being, if sound is below a certain level, we can hear each other while we speak at normal volumes and the other person near us will be able to understand us.

However, once volume reaches a certain background level, we have to raise our voices to be understood over the background noise which increases the overall background level which causes others to have to raise their voices over the new background level ever increasing.