What is the .1 in 5.1?

[philipf]

I see a lot of mention of 5.1, but it isn't clear to me what that means, particularly the .1. 

[HiFi_Sid · Accepted solution]

The .1 refers to a dedicated low frequency channel usually handled by a subwoofer which it includes its own amplifier and is dedicated to that purpose. In good systems, it will handle frequencies below 80 Hz, which includes the very lowest base tones, as well as LFE or low frequency effects in movies. These are the ones that can rattle the walls.