HiFi Subjectivist ¦ Audio Forum ¦ NVA User Group

some random pondering.

been contemplating building a set of BBcubes for which I already have the requisite Doc Mod drivers

however as they do thoughts cross my mind

I'd already planned to have each driver powered by a seperate amp. And then it occured to me that having drivers in teh same box ust add some compromise, i.e a 12" moving air at frequency x must exert an influence on the mid range which is trying to perfom at different frequencies. So WAF factor aside would they actually perform better in seperate cubes for each driver?

If you are doing a proper nva style speaker with no filters on the bass and mid then i don't see it being such a big problem. The 12" and 5" are getting exactly the same full range signal with the 12" not going as high because of its size and the doping.

However, i have my mid range in a seperate enclosure and i am planning on doing the same in my next version of it. It just seems like the ultimate thing to do imo. If putting the 5"in same enclosure as the 12" then due to the large volume the 5" will be very much over damped (i am guessing this as i dont know exactly what influence the 12" will have on the 5" if in same enclosure) I guess you may prefer that to the 5" being in a correctly sized box for qts of 0.7 or there abouts. Then again you may prefer seperate boxes.

Have you got a hunch about it ?

Well from a sound point of view I can't help but feel removing any effects of driver interaction in the box cannot but be a good thing

As you said it should also allow no compromise on enclosure volume


Finally it should allow the mid-range to also fire upwards which follows the cube principle of the driver opposing the stand to minimize box movement.

All sounds right in theory to me at least

I'm not a speaker designer, but I thought the point of the cubes is not so much how they interact internally but how they interact with the room. Sound isn't created iside the box but by a box within a box (the room) and the interaction of the two (providing we are dealing with room of domestic room height) is managed by moving the speakers in and out from walls (till the balance sounds right) and then toeing in to get the focus).

I use a 12”, 8” & tweeter in same cube
12” up, 8” & tweeter forward- off centre design
Using Nva 8” drivers & tweeters
Amps wired directly to the drivers
Agree with the above that it’s the interaction with the room that’s important, I’ve created a false ceiling above the cubes as original was high, glass & sloping

The large movements of the woofer will cause pressure changes within a sealed cabinet (hence the “acoustic suspension” approach to speaker design).
Those pressure changes will act on the inside face of an exposed midrange cone. Tending to push it out and suck it in.
You probably want to avoid that, or at least check you can’t hear any unwanted effects.
You can avoid it by using an internal cabinet for the midrange (possibly just a sturdy cardboard tube as Goodmans used in their old Goodwoods design), or a midrange driver with a sealed back design (as Goodmans did in their even older Magisters. That midrange sounds utter shit these days, but modern alternatives are available!) Tho it sounds like you have already chosen your drivers.
Tweeters have sealed backs anyway, so need to worry about that.

Edit...Or yes...Build a whole separate cabinet for the midrange.

The Doc always said it was not a problem having bass & mid in the same enclosure. I assume this to be true but even though there may not be a problem perhaps seperate boxes are better. In my OCD head i have always preferrred the idea of seperation but i have no listening test to back that up. The following is only my opinion YMMV.

So you want the mid up firing ? Which direction are you going to point the 12 incher ? I have tried all sorts of combinations. For some reason i don't like side firing the bass or mid drivers. At the moment i am down firing the big bass cube (on 6" legs) with the smaller cab on top with mid up firing. Down firing didn't work for me originally when i only had one 12". Since revisiting it with two 12 inchers in isobaric config i do like it. Front firing the bass also works for me and i will have to choose at some point between that and down firing.

The Doc had to compromise a little with the bbcube imo. The box would have been an odd shape with bass and mid both up firing so he chose bass to fire up and mid sideways. As he always stressed getting the drivers movement in phase with the floor then getting the bigger of the drivers up firing was a priority. The mid is still a considerable mover and does deal with bass also (if no filters) so it must produce some unwanted movement 90 degrees out of phase with the floor ?

There has to be some compromise along the way i suppose. If religously following the in phase with the floor thing then you would either have to have both bass and mid up firing or do as i am trialing now and have bass down firing and mid up firing. It does work. But which works best, i am not completely sure yet. Cubes 1, 2 & 3 need to use a flat wall at the rear to enforce bass. It is usually recommended to have them real close to the back wall for that reason. Doc thought i would prefer bass driver at floor level firing forward for the Roy Allison effect. I originally agreed but down firing is extremely tempting.

I think with a 3 way design using big 12 incher it is less ctritical to get the mid / bass close to the back wall so you have more possibilities. I dont have a flat back wall to work with as mine are either in a bay window or with one window behind each speaker. Having the seperate big cube has helped no end with fine tuning and getting good bass. There is no lack of bass believe me but neither is it over blown or bumpy.

Good look with your project Monte and enjoy the design, build and of course the listening