[Danny Richie]

That should work out pretty well.

[skeeter99]

So how is this coming along? Eagerly anticipating your results I might be starting my double tower this weekend using the TB Buyout W3-881SI's with a similar configuration. I'm going to be using mine as surrounds though.

Scott

[Peter J]

I got the test box done today, this is what I'm sending you, Danny.

Final versions will have the riser attached to bottom instead of the plinth I put on this one. Wires are running out port holes, finished speakers will have binding posts:

http://www.parts-express.com/pe/showdetl.cfm?Partnumber=260-275

Sounds kinda weird running stereo signal into common box, little amp doesn't have a way to switch to mono. I'm pleased so far. They'll never rock the house, but weren't intended too anyway.

Question for Danny about stuffing. Should I stick some in there before sending to you and if so how much? The plinth is screwed on so it's easy to change.

[skeeter99]

Nice they're looking good! So are you not doing them ported? I can't remember what you decided.

Scott

[HAL]

Scott,
The wires are coming out the port holes.  They are just not very big.

[Danny Richie]

I would load that thing with some loose poly fill or insulation.

[skeeter99]

Scott,
The wires are coming out the port holes.  They are just not very big.

Haha ok I'm a dork. Carry on 

[jcotner]

I don't know. I wouldn't have thought those were port holes.
My thinking was there was normally only one port in a box
anyway. But it is handy for getting wires out of the box. 
They do look like nifty boxes though. Simplicity is always
a good thing.

[Peter J]

After building the test box that's going to Danny, I've been refining the Sketchup model to simplify box building and facilitate changing riser height to accomodate different needs. Also gives easier access to internal space should one want it. Larger hole in rear is for binding post cup mentioned earlier.










Unless threre are changes that come about after Danny's measuring, this is how I'll build 'em. I'm happy to share the Sketchup model if anyone wants it, send me a PM with your email address. This was created in Sketchup Pro, but I'm pretty sure the freebie version will open it and manipulate any way you want.

[Danberg]

It's very interesting following this project.

In the photos of the box, I didn't notice any nails or screws holding it together.  Did you simply glue the sides and top together?  In the final version will it be necessary to apply some type of caulk on the inside at the intersections of all the joints, to make sure that it is completely air tight?

Never built speakers before myself & am interested in answers to my novice questions.

[Peter J]

It's very interesting following this project.

In the photos of the box, I didn't notice any nails or screws holding it together.  Did you simply glue the sides and top together?  In the final version will it be necessary to apply some type of caulk on the inside at the intersections of all the joints, to make sure that it is completely air tight?

Never built speakers before myself & am interested in answers to my novice questions.

I used Titebond and biscuits on the butt joints, although there are other methods that would be just as good structurally. Even a simple glued butt joint would probably suffice considering appliaction...it's not like we're parking cars on it. I often avoid metal fasteners at the joints 'cuz carbide tooling and steel are a bad mix and I've got some ideas for corners that would require machining.

 I'm the original anti-caulk crusader, so I may be jaded    The only reason I can see to use it would be if joints were somehow difficient and there were gaps that effectively increased port size. This is a ported speaker so airtight isn't really necessary.   

I think this design could be further simplified for a novice by not rabbeting for driver flange. Considering the intended application, I'm just not sure that it would make an audible difference, but Danny could speak to that better than me.

 My altruistic thought is that this would be a fairly easy build for a newcomer, but I imagine that could be good or bad. Once on that train its hard to get off...

[jcotner]

This is a ported speaker so airtight isn't really necessary

My thinking, and maybe it is not that critical, is that the port is tuned and
any other leakage in the box will alter that tuning. Sounds like your box
building techniques probably result is a pretty tight box already but I
don't know if you can just discount box leakage in a ported design. IMHO

And I modify my post because it looks like you somewhat say the same
things in the sentence before. When my sinuses get bad things aren't
always obvious to me.

[Danny Richie]

Okay, I measured your speakers and here is the deal.

The up firing woofer is too far back of the front woofer. In other words the acoustic center of the front woofer is about an inch back (center of the voice coil) while the acoustic center of the up facing woofer is about 4.25" back. So there is quite a bit more delay involved than if the upper woofer were at the front of the cabinet verses at the back edge of the cabinet.

Here is the effect.



The green line is the front facing woofer. The Red line is the up facing woofer. And the blue line is the two of them together. As you can see there is cancellation going on in two places and a big peak at 3kHz.

So I started dropping parts on it. The parts used can correct for response errors and also shift phase.



This one got a notch filter on the front facing woofer. It corrected it's peak, but the cancellation effects are still there and in the same places. The peak is still there too.



Here is one with a little more of a notch filter on the front woofer. It is the light Blue line. It looks really good on its own. The green line is the up facing woofer with a 12mH inductor in line with it. This makes it do low duty only. Unfortunately it adds even more delay. The dark blue line is the two of them together. The upper woofer is actually cancelling out the lower one some. I could reverse the polarity of the upper woofer to cause it to add a little gain, but then they would be fighting against each other in that air space.

The solution looks like requires the upper woofer to be moved to the front edge instead of the back edge. This will leave only about 1.25" of offset delay. Hopefully the area of cancellation will then be high enough in the frequency range that the limited off axis response will limit interaction or cancellation.

[Danny Richie]

You are also going to have to figure out a way to line the walls with insulation while leaving the middle open so that there is a clear path to the ports. A wad of poly fill stuffed down the center isn't going to work.

[skeeter99]

You are also going to have to figure out a way to line the walls with insulation while leaving the middle open so that there is a clear path to the ports. A wad of poly fill stuffed down the center isn't going to work.

Very interesting Danny. I was wondering how the placement of the top woofer would affect the frequency response. I thought the top one looked cooler that way but apparently it isn't working. I'm glad I haven't cut the holes of mine yet. So bringing the top woofer to the front edge should really help out the delay and cancellation?

Scott

[Danny Richie]

Very interesting Danny. I was wondering how the placement of the top woofer would affect the frequency response. I thought the top one looked cooler that way but apparently it isn't working. I'm glad I haven't cut the holes of mine yet. So bringing the top woofer to the front edge should really help out the delay and cancellation?

Scott

Yes. I'll just have to give it a try and see if it will work.

[Peter J]

Thanks Danny...it's pretty cool of you to do this.

 It does raise some questions. Given what you've measured, would you say the idea of an up firing driver in this application is beneficial or is it just adding complexity and compromise?

Because of magnet structure on drivers, front one will need to move down to accomodate top one moving forward. Although it's not a problem from a woodworking standpoint, will it introduce any significant new elements?

I can figure out the stuffing I think, but am wondering if moving ports nearer top would be desireable. Seems like I've mostly seen them down low, but now wondering if drivers actually "see" port position or just cabinet volume.

From a a woodworking standpoint it wouldn't be too difficult to separate the cabinet into two cavities to facilitate  reversing phase on top woofer, but would require moving front woofer down even more, and still keep design somewhat easy to build. Would this make any sense?

Yet another possibility would be to put them both on front baffle and add notch filter...perhaps this would be the path of least resistance?



 

[Danny Richie]

It does raise some questions. Given what you've measured, would you say the idea of an up firing driver in this application is beneficial or is it just adding complexity and compromise?

If implemented properly with the acoustic centers aligned then it should add realism to the presentation. It will give you more of an in the room with you experience verses a from the speakers effect.

Because of magnet structure on drivers, front one will need to move down to accomodate top one moving forward. Although it's not a problem from a woodworking standpoint, will it introduce any significant new elements?

Moving the front one down will add a little more surface reflection to the front woofer output, but it will be real minimum. I wouldn't expect to see much change in the response.

I can figure out the stuffing I think, but am wondering if moving ports nearer top would be desireable. Seems like I've mostly seen them down low, but now wondering if drivers actually "see" port position or just cabinet volume.

In a box this small the location will have a minimal effect. But if you get them too close to the driver then the driver tends to not use the air space in the box as well.

From a a woodworking standpoint it wouldn't be too difficult to separate the cabinet into two cavities to facilitate  reversing phase on top woofer, but would require moving front woofer down even more, and still keep design somewhat easy to build. Would this make any sense?

I'd try the same air space and just move the front woofer down and the upper one forward.

Yet another possibility would be to put them both on front baffle and add notch filter...perhaps this would be the path of least resistance?

I did two of them on the front of the baffle for my kids computer speakers and added an inductor in line with the lower woofer to keep it from playing to high.

The response of these look different than the older version of this woofer that I had. So the best solution for a crossover will be different. 

[Danny Richie]

Looking back on page two at these speakers makes me think that they will have similar response problems.

[Peter J]

First test didn't fare as well as hoped, so on we go to round two. Danny suggested two things to improve upon the basic platform; move top driver forward and line the walls to reduce internal reflections.

This is the new improved version, top driver moved forward and front driver moved down. I'll make another test box so we can see how it measures up.




Lining the cabinet is hampered a little by access to the inside once box is assembled...there's just not much room to work.  Something that can be cut to size, maintains it's shape and can be glued in place. After some head scratching and a couple  of phone calls I came up with this;





It's felt carpet pad, sometimes called rug pad. Often used under throw rugs on hard surfaces to keep them from sliding and give a little cushion. Danny, does that look like it will do the trick?



I edited the drawing to correct a dimensional error which I discovered while building box.