[JohninCR]

In my small HT, my mains and surrounds will be OB line arrays covering down to 28hz where my one remaining longitudinal room mode will exist.  Below 28hz I will use a boxed alignment sub.  My thought is to make it a quasi dipole arrangement as follows:

2 identically tuned subs.  One at the front of the room under the screen and one about 4m away immediately in front of the seating position in coffee table form.  I will wire the front sub out of phase resulting in a net dipole with a huge distance separating the front and rear waves.  Doing the math, I will actually have reinforcement of the output down to about 15hz where the 6db per octave dipole cancellation will start and be virtually meaningless on axis.  

While the effect would be pretty strange walking through the room with a low frequency tone running, I believe this will enable me to have extreme output at the seating position.  At the same time, the net output in the room as a whole will be zero which should drastically reduce what tries to escape from the room.  Helping even more is a concrete wall at the back of the room.  I also plan to prevent transmission on the front wall, so the bass waves won't be able to escape the room on axis with the resulting dipole sub.  Off axis to the sides is where my real benefit will be because bass responce will be significantly reduced.  In fact, at any point halfway between the subs the output will be zero in theory.  In addition the longitudinal room mode will be a non-factor because I can deal with it using the crossover.

Is there any reason this idea won't work ?

[ScottMayo]

2 identically tuned subs.  One at the front of the room under the screen and one about 4m away immediately in front of the seating position in coffee table form.  I will wire the front sub out of phase...

Push-pull subs. John Casler (Summit audio) has done something like this; I think he used *4* VMPS Larger subs. He reported fantastic results.

My concern with the approach is based on a concern about delay. Yes, you can drive the subs 180 degrees out of phase, and the subs will dutifully produce push and pull on demand. But 4 meters apart is over 10ms of difference; you might get sonic artifacts from that kind of delay. If you use a sound processor that can assign different delays to the subs, you can probably solve this. But it's something you'll want to experiment with.

To be honest, most subs put out enough power as it is. On a dial running 0-10, my sub never gets turned above 3 unless I'm testing for room rattles. And it's not a small room. If you want to rattle your teeth, you might look into a Buttshaker-type device...

[JohninCR]

Scott,

The sub is really only going to come into play for HT.  The only reason I want to use 2 is create dipole dispersion and keep things in room to a much greater extent than a normal sub.

I'm not sure what affect the delay will have, especially since the signals are out of phase anyway, but I am going to do some experimenting.  10-11 milliseconds of delay is pretty small compared to the group delay of typical ported subs where a 25ms group delay is a pretty good target, so I'm fairly confident that I'll be ok.  Plus at the seating position the coffee table sub's output should drown out the front sub by about 12db due to the difference in distance.

It's great to hear that an expert had fantastic results with something similar.  If it works anything like I envision, the effect of walking in and plopping down in the seating area should be like putting headphones on because the entrance and walkway are right through the dipole null.

[ctviggen]

But won't you get a 3dB reduction in sound of the sub that's away from the wall?  (Doesn't one wall add 3dB while two walls add 6 dB?)  I have three subs, two VMPS largers and one SVS.  I have the SVS wired out of phase of the two VMPS.  I split the LFE signal (and any bass going to the rears and center) to the VMPS largers and the SVS.  I'm going to redo this, as I think the bass is lacking in movies, and I think the reason is the out-of-phase relationship of the VMPS largers and the SVS.

[JohninCR]

I'm going to absorb as much at that output as possible, but yes I'm likely to still get some boundary reinforcement from the front sub and will probably need to set the volume lower on the front sub.  We'll see what happens once the room is ready and I start dialing things in.

[bpape]

I guess I'm confused.  The topic is alternatives to bass absorbtion but in order to make what you suggest work, you have to absorb the bass to avoid boundary reinforcement....

[ctviggen]

For me, I see these rules of thumb (such as "near one wall = 3dB increase") and I really don't know how close to reality they are.  I do know that pushing a speaker nearer to one or two walls does increase the bass, but I've never done any testing to see exactly what happens with the bass.  Furthermore, supposedly what happens with a sub placed on the floor near is wall is that it can load certain axial modes (does the term "axial" encompass both front-to-back and floor-to-ceiling bass modes?  It's too early in the morning for me and I have a cold).  I just purchased, but have yet to play around with, an ASC subtrap (although I purchased the largest subtrap, which is rated for lower frequency bass trapping than what's discussed in the following article).  See the following:

http://www.stereophile.com/musicintheround/904music/index1.html

So, by placing one of the subs near a wall and one out from the wall, you're going to be modifying with which room modes each of the subs interact.  I can't tell whether that would be good or bad, and the answer may (or more likely is) room and placement dependent. I've read some sub placement guides that say that the subs should not be placed near a wall and instead should be into the room a distance based on probable room mode locations.   I think the idea (I can't find the site right now) was to determine where the front-rear and side-side room modes would be a null and place the sub there, such that the sub interacts less with those room modes.  Since a corner has all room modes, placing a sub in a corning means that the sub will interact with the bad room modes.

Plus, I'm not sure what the goal of this "push-pull" type of sub arrangement is.  What is the goal of that arrangement?  Is it smoother bass everywhere in the room, reduction of the effect of room modes, more bass at a particular position, some combination of these, or something else?

[JohninCR]

The purpose of the alignment is to create a dipole using 2 subs, not unlike having 2 drivers push/pull on the end of a 4m pipe.  Just replace the ends of the pipe with closed boxes and leave the space in the middle open.  This should completely eliminate the side wall and floor to ceiling modes, leaving me with one remaining mode at 28hz, which is also my expected XO point making it easy to deal with.

My main goal is to keep response in room, which should prove drastically easier with dipole.  Otherwise trying to contain say a 100db 15hz wave would be next to impossible, since the room is above ground.

This leaves 2 areas of full output.  One at the seating area and one at the front wall.  Due to the very large separation of the front and rear waves, the dipole effect on overall output will be negligble although I will have a down sloped response that I'll have to deal with.  Unfortunately the alignment sacrifices room gain, which would nicely offset the sloped response.

Since the rest of the house is on the other side of the front wall and the rear wall is concrete, that leaves me with just one surface requiring extreme measures to achieve very good isolation from the rest of the world.  What I am able to dissipate there will also help offset that sub's gain due to wall placement.  It doesn't matter if that sub is against the wall or not, that wall must be addressed, but the concrete columns along that wall have exactly 4ft of space between them, so even extreme measures will be very easy to construct.

[JohninCR]

Initial tests prove that the concept works exactly like the theory indicated.  The amount of bass at the sides even parallel with the 2 enclosures is even less than I anticipated.  That means the effects of the floor/ceiling and side wall room modes is eliminated along with the bulk of the reflections, leaving the bass clean and powerful on axis.  This alignment is going to provide a substantial savings in construction cost for addressing bass transmission and treatment.

Once I get the room completed, I'll do measurements and post the results, so bear with me a few weeks.  The testing I did today is very very encouraging.

[bpape]

I'd be very interested to see your setup and measured results.  Sounds like you got what you needed.