For HT, how important is extension south of 20 Hz?

I think it is more for bragging rights than anything. Although I have heard some systems with huge twin subs and it FEELS great to have such super low bass. But I personally wouldn't bother trying to be flat below 20Hz.

I think that is is necessary for movies. The bass down that low causes your clothing to move and you really feel like you are witnessing the event. A few subs that I have used for music worked well but ran out of gas with movie soundtracks. By the way, I'm in Gretna.

I too think it's a personal choice. I have a pair of subs that measure below 20Hz (due to in room gain) and it's nice, but they are actually only rated to 20 Hz.

My opinion is that 20 Hz is fine.

Though, I just read a Dolby Digital White Paper that says the Sound Engineers will send LFE info from 3 to 120 Hz to the .1 channel.

Good question and I don't know the answer. I've never measured the low freq output in any movie I've watched so I'm not sure how low they go?

I'd guess that the lower the sub will go...the less problems it may have out putting the freq's above at high SPL's?

My room is 4600 cubic ft.... 3 VMPS subs (17hz)...each has an active 15" and 12" driver, and a 15" passive,...nearly bone crushing bite when called for, extended low freq harmonics in SiFi and Horror type movies are amazing in their ability to pressurize the room.

That said...what freq's are these? I'm not sure.

Dave