Dual Subwoofer Question

[FSUguy]

A few questions concerning dual sub placement, LFE, and line in/out options on subs...

1) From my understanding, if you run your sub through LFE, the sub only gets a predetermined LFE signal from the receiver and not always the full range of bass. Conversely, if you run the sub through line in, and then line out to your fronts, etc. the sub gets the full range of bass and takes out the frequency response according to its internal crossover. I have heard to maximize this, you want dedicated subs for line in/out for your fronts, and another sub for the LFE. Is my understanding correct, or any of this true.

2) When placing dual subs I have heard that you can place them front and back, diagonal across the room, on opposing sides of the room facing each other, or just both in the front. I was then told by an installer that placing them anywhere other than the front, or preferably stacking them, causes cancellation of the low frequency waves, resulting in less bass. I have read on many sites that it is ok, and sometimes better to place them in different places; what's the truth? Also, regardless of placement, would you still expect to see some negative interference of waves, causing cancellation?

I know these are some big questions, but any help on understanding would be greatly appreciated....

thanks

 

[MDS]

1) From my understanding, if you run your sub through LFE, the sub only gets a predetermined LFE signal from the receiver and not always the full range of bass. Conversely, if you run the sub through line in, and then line out to your fronts, etc. the sub gets the full range of bass and takes out the frequency response according to its internal crossover. I have heard to maximize this, you want dedicated subs for line in/out for your fronts, and another sub for the LFE. Is my understanding correct, or any of this true.

This is tricky because you are actually asking about multiple different concepts.

First...LFE has a specific meaning despite the fact that many people use the term as all encompassing for 'bass'. LFE is the .1 channel of a DD or DTS track and is limited to 120 Hz and below. When you say 'run the sub through LFE' do you mean using the subwoofer pre-out on a receiver/pre-pro to an input on the sub marked LFE? On subs with an input specifically designated LFE, the internal xover is automatically disabled. The sub will get the frequencies at (roughly) and below the xover set in the receiver - that means the .1 channel AND 'regular' bass from channels set to Small in the receiver.

If you use the sub pre-out on the receiver and connect to a normal line-in on the sub (one that does not automatically disable the sub's internal xover), then you have two xovers in succession aka 'cascading xovers'. This is not good.

If you were to use any form of line out from the receiver (other than the sub pre-out - say tape out or the front channel pre-amp outs) and connect to the regular line-in of the sub, then the receiver will do no filtering of the signal. The sub's xover will do the filtering.

2) When placing dual subs I have heard that you can place them front and back, diagonal across the room, on opposing sides of the room facing each other, or just both in the front. I was then told by an installer that placing them anywhere other than the front, or preferably stacking them, causes cancellation of the low frequency waves, resulting in less bass. I have read on many sites that it is ok, and sometimes better to place them in different places; what's the truth? Also, regardless of placement, would you still expect to see some negative interference of waves, causing cancellation?

You will have some peaks and nulls in the frequency response no matter where you put the subs. The goal is to find the best placement that yields the flattest frequency response and that is easier said than done.

 

[Jack Hammer]

The issue of them cancelling each other out you refered to is probably the phasing. Try one at 0 and the other at 180. Play with the phase switch to get the most bass. It helps to have a friend flip the switch or knob while you stay seated in the primary listening position.

Jack

 

[AVRat]

A couple of articles:
http://www.audioholics.com/tweaks/get-good-bass/subwoofer-connection-guide
http://www.harman.com/wp/pdf/multsubs.pdf