*.2 sound. Configuration thoughts?

[MaturityDodger]

Hi,
This is more of a hypothetical for me as I don't have space for a second sub. But I've been thinking recently about 5.2/7.2 setups recently and the config of the two subs.
I'm hoping to get my thoughts out, hear your thoughts on my thoughts, and maybe even be of use to others who do have space for two subs!
I'll start from the very beginning...


In, say, a 5.1 system, your AVR will be doing the bass management. It sets a crossover point, and sends low frequencies from all channels (along with the signal from the LFE channel) to the single subwoofer.

In a true 7.2 system, as I understand it, the AVR would be processing the low frequencies to a left sub and a right sub - I would assume that low freq's from left speakers go to the left sub, low freq's from right speakers go to the right sub, and the LFE channel audio goes to both subs.

I also understand that some slightly cheaper AVRs which have two subwoofer outputs, but they actually both just output the same signal.

The above is all my understanding of how things are right now. If I'm wrong then let me know, as my thoughts are based on this.


Reasons I can see for somebody wanting to have an extra sub is for:
1) sheer extra SPL,
2) L/R directionality,
3) to fix room-related issues, bass boom etc.

My thoughts on the above:
1) Surely there are subs out there that easily cater for enough SPL in a standard-sized living room in a British house?
2) Minimal directional effect at these frequencies.
3) Fair enough, two sources could help out for some users over just one source.


But what about this as an alternative?

In the sub market, it appears that you can get fast and responsive subs (e.g. BK XLS200), or you can get loud, low, rumbly subs (e.g. BK Monolith+). Both have desirable features, but you don't usually get both sets of properties from one sub without spending an awful lot of money.
So instead of getting two matching subs of your choosing, how about getting a big loud ported sub and a smaller sealed sub, and running both off the same sub channel? (i.e. not in a left/right config) Would you get the best of both worlds?

 

[Member 639844]

In a dual sub setup, there is no left and right, only sub 1 and sub 2, where they are placed is entirely optional. Both subs would still get the lfe and the redirected bass, but the redirected bass is quieter than the lfe channels, so really just gets lost. The only bass from the speaker channels that gets redirected to the sub is bass below the speaker crossover setting, which is typically only 80hz and under. All bass under 80 hz is omnidirectional so you will never get any directionality, and more subs actually helps reduce the possibility of directionality occurring.

In theory, having a big ported sub mixed with a punch sealed sub could work very well, but they would need to be properly integrated meaning you would need to take the time and care to get them eq'd properly. The main advantages to having two subs, is more max output capability, a better more accurate in room response, and better response over a greater seating area.

 

[Jase]

I'd say it depends on the room. My Amp has several combinations of sub management depending on how many are connected (up to 3). When I had dual subs I tried both set to LFE (both got the same signal) then Stereo Left/Right and finally Front/Back. Front/Back worked well as did LFE. Stereo Left/Right didn't although I couldn't get the subs positioned optimally for that configuration.

Didn't have three subs so couldn't test Stereo Left/Right + LFE or Front/Back + LFE.

 

[MaturityDodger]

In a dual sub setup, there is no left and right, only sub 1 and sub 2, where they are placed is entirely optional. Both subs would still get the lfe and the redirected bass, but the redirected bass is quieter than the lfe channels, so really just gets lost. The only bass from the speaker channels that gets redirected to the sub is bass below the speaker crossover setting, which is typically only 80hz and under. All bass under 80 hz is omnidirectional so you will never get any directionality, and more subs actually helps reduce the possibility of directionality occurring.

In theory, having a big ported sub mixed with a punch sealed sub could work very well, but they would need to be properly integrated meaning you would need to take the time and care to get them eq'd properly. The main advantages to having two subs, is more max output capability, a better more accurate in room response, and better response over a greater seating area.

Thanks for that.
When you say that the big/small sub combo would have to be well integrated and EQ'd properly, are you just anticipating another crossover point, so that all of the signal goes to one sub or the other (although maybe with a longer rolloff than usual)? Or something more complicated, sending everything up to 80 to both, then applying an EQ curve to each, tailored to that sub?


Re. what's happening with the redirecting bass to the subs...
So if it's sub 1 and sub 2, are you saying they actually get the same signal? I've never had 2 subs so not had chance to play with it. So why do some AVRs have two discrete channels rather than two output terminals on one channel? Is it different EQ, level and phase from the AVR or something?

And (going a little off-topic I think)... I was aware of the standard crossover being 80Hz. But it just made me think... if that's the industry standard, then why does the multichannel soundtrack have any data in the non-subwoofer channels below 80Hz to begin with?

 

[Member 639844]

Thanks for that.
When you say that the big/small sub combo would have to be well integrated and EQ'd properly, are you just anticipating another crossover point, so that all of the signal goes to one sub or the other (although maybe with a longer rolloff than usual)? Or something more complicated, sending everything up to 80 to both, then applying an EQ curve to each, tailored to that sub?


Re. what's happening with the redirecting bass to the subs...
So if it's sub 1 and sub 2, are you saying they actually get the same signal? I've never had 2 subs so not had chance to play with it. So why do some AVRs have two discrete channels rather than two output terminals on one channel? Is it different EQ, level and phase from the AVR or something?

And (going a little off-topic I think)... I was aware of the standard crossover being 80Hz. But it just made me think... if that's the industry standard, then why does the multichannel soundtrack have any data in the non-subwoofer channels below 80Hz to begin with?

How the big/small combo works best is a matter of suck it and see. With all the different combos out there you would have to take them, try them in your room (another infinite variable), and see what works best. Both subs running the full lfe range may work well, or it may not and you might be better running one sub at say 10-30 hz, and the other 30-80 hz, or any other possible combination. Only during setup could you say for sure what would work best for you in your room, and so on.

There is only ever one lfe channel. You could have ten subs, but if they are all fed the lfe channel only, they all get the same signals. If you run a high level connection, it is possible to have a left and right sub, but you would have to run them a fair bit higher than only 80hz to notice any directionality. The normal channels have the full range encoded in them for people that dont run subs, or might have unusual setup, like say full range speakers and perhaps only a rotary sub. The lfe channel is encoded 10db hot as well don't forget. Dual sub avrs that are true dual sub, have the ability to set the distance, level, phase and eq separate for each sub, which maximizes the integration if the subs with the room, each other, and the rest of the system.

 

[gilesy5]

The 80Hz crossover frequency is merely a guidepoint to indicate where speakers should start to roll off and the sub should take over. This obviously depends on your speakers which could easily create sound at lower frequencies... or maybe not!

THX sound processing has always been an advocate of limiting L, R, C and surrounds to 80Hz and directing any frequencies lower than that to the subs... but it's not the only method!

In reality all 5 or 7 channels of your system are technically full range. This means the blu-rays/dvds are encoded with this sounds from 20KHz down to (roughly) 20Hz, and if your speakers can take it, then they can reproduce it. In my system (7.3), I have one sub connected to the main LR channel (high and low level), one sub connected to the LR surround channel (high and low level) and one sub connected to the centre channel (high level only). The only channel I do not have on "full range" in my system is the surround back channels.

 

[Member 639844]

There is a very specific reason for the 80hz setting.

 

[Riverbank]

My sub seems to sound better at 120Hz crossover, I'm presuming it's because it's such a crap sub it can't handle 80Hz . In music Bass players use this trick a lot recording in different channels at different rates of signal or the same notes doubling up on the Bass at different positions down the neck, I'm guessing two subs would have the same affect if integrated properly at the listening position, if not Eq'd a horrid mess.