[Chops]

Did you see the pictures of JohnK's scale model?

http://www.diyaudio.com/forums/multi-way/195734-nelson-pass-slot-loaded-open-baffle-project-20.html#post2700050

Yes, I've seen his model. I've tried following info in that thread, but there's too much constant bickering about it not working as promised, 1m measurements vs 3m measurements, 9dB vs 3dB, yada, yada, yada... Unfortunately, because of the bickering, that thread is rather annoying to look through.

I guess making each "chamber" as small as possible will be the best way to go judging from JohnK's model. At least that would push the resonant frequency up a lot higher than I'll ever be running those 8" drivers, about 125Hz.

[Chops]

I am sure Nelson provided all of the details he had on hand. He probably designed the slot loaded OB based on his vast knowledge, experiences, and inherent understanding of loudspeaker design. Then he built it and listened. Maybe he tweaked later, I don't know.

So when it comes to building a different design using different drivers you are starting at square one, you have to do all the work yourself. Nobody has prior knowledge to share. If it were my project I would write the software required to do a trade-off study to optimize the geometry and driver choice, then I would build and test. But that is just my method.

So you are left blazing a new trail, getting frustrated that people are not helping seems pointless since they probably have nothing to offer. You are the design leader. You can either do the engineering work or drop back and go in a different direction, your call.

I guess the thing that gets me the most is absolutely no reply of any kind. If I were just to get something saying "I have no other info at this time" or something like that, I wouldn't be too bothered. But to be completely ignored is just wrong I think.

[JohnR]

Seems to me that you are being rather unfair to Mr Pass, given that he replied to you twice in your thread.

[MJK]

But to be completely ignored is just wrong I think.

He owes you nothing, he shared what he had and is obviously busy. If you were building his exact design using his drivers and had a question about some minor detail of his construction a response might be expected. But you have changed everything so he probably does not have a simple answer. I think it is wrong for you to keep complaining that people are not doing enough for you.

The problem with the thread you mention is that most of the people are just speculating and arguing, many of them do not understand the basic physics. Like any thread on one of these forums you need to sift through the responses to figure out which are of value and which are just BS, most are just BS.

[planet10]

The pics of the slot loaded open baffle are very vague. There's no well lit, close up shots of the slot. All you see is a black slot on a light bare baffle. No details to speak of.

JohnK's model is very good. The only thing you need to know about the slot is that the smaller the volume, the higher in frequency things go before you have cutoff (and not so small that you scompletely choke off the air flow)

dave

[Poultrygeist]

My TB/H-frame Alphas take up less space that the Zu's they replaced.

[scorpion]

OK Chops, you are cornerd !  (may be a new word, I don't know)

But why not ask, what shall I do then with my 12 Peerless Indian 8" woofers and my two Alpair 10.2 units if going OB ? 

/Erling

[Chops]

Maybe I have been unfair, and for that I apologize to Mr Pass.

I just get easily rattled about things at times. I'm more of the type that needs to see something in order to be able to build off of it. Knowing how something is designed/built and then add my own to it so to speak.

From what I can tell, this slot loaded design is really nothing more than a modified ripole with an open back. I may consider going that route and build a stacked ripole. The only thing that concerns me there is the effects of the resonant frequency of the rear chamber which would be much larger than the slot. That's one of the reasons I like Nelson's design so much. There IS no resonance effect from the rear, and the slot would be so small that its resonance frequency would be up near 500Hz or higher, nowhere near the 125Hz crossover point I'd be running at. From what I gather, the woofer section should be down nearly -40dB at 400Hz.

[Rudolf]

The only thing that concerns me there is the effects of the resonant frequency of the rear chamber which would be much larger than the slot.

Neither experienced that in my own "ripole" builds nor heard it from anybody else. If the "ripole" is built in the usual way, the resonant frequency of the rear chamber should be the same as in the slot chamber.

[JohnR]

From what I can tell, this slot loaded design is really nothing more than a modified ripole with an open back.

Oh hang on, I'd claim that it's really nothing more than my design but with the woofers moved closer together:



 

In the end, you just have to build something and try it.

[Chops]

Oh hang on, I'd claim that it's really nothing more than my design but with the woofers moved closer together:



 

In the end, you just have to build something and try it.

What exactly is "your" design? Any threads about it?

[JohnR]

Not that I know of.

[Chops]

I think I'm pretty much set on the slot design and such and will probably start cutting up some wood in the next couple of weeks. Nelson replied to my email finally and I apologized to him a couple times as well as dropping him a tidbit of info as well as a question or two.

As far as the Alpair 10.2's are concerned, I said the heck with the little Dayton T-amp and hooked them up to the Monarchy Audio SM-70 Pro. I have them running from 40Hz on up, and the subs are running from 125Hz on down, all crossed over via my Onkyo TX-NR809. Surprisingly, you still can't tell where the bass is coming from. Just seems like it's coming from the Alpair's. speaking of bass, those little driver can move! Not that I've pushed them at all, but still... They're quite the driver for sure.

Obviously, there's a gap in the frequency range, but depending on the music, it sounds pretty damn good! I noticed a vast improvement moving to the Monarchy amp. Much fuller, smoother and 3 dimensional. There's definitely no need for tweeters and they are not harsh at all, though sometimes they do sound a little bit honky. That might be due to the small, perfectly square 12" baffles they're on though, and the fact they're probably rolling off somewhere upwards of 200-300Hz or more.

And on a side note, I've been ripping my CD collection to Flac all day like a madman while re-reading Nelson's entire "SLOB" thread on the DIY forum. I realized that in the midst of filtering through some of the BS in that thread, that I also accidently skipped over some vital information in the process. I've got about another 23 pages to go!

[Chops]

I'm wondering how or if something like this would work. And please excuse the horrid SketchUp work. It's my first time playing with it...

[JohnR]

It seems that the "waist" will act to to reduce the effective area of the "slot." The increased distance from the driver to the opening might also act to reduce the frequency of the slot resonance.
 

[Chops]

I'll be the first to tell you that this picture is in no way near what I was actually shooting for. The drivers would be pushed out more towards the edge of the slot and those "lobes" wouldn't be anywhere near that size.

I'll have to have a second go at it tonight and see if I can come up with a rendering closer to what I'm thinking of.

As far as the design is concerned, I was thinking that the "waist" as you say, would somewhat act like the compression chamber in the throat of a horn. Maybe not the best idea for this type of application, but that's why I'm posting it here, to get some input on it. 

[JohnR]

From what I've read, it's simply the ratio of the slot area to Sd that matters. Nelson used a ratio of 1:3, I think I saw someone say 40%. I don't think I've seen anyone comment that it matters how the slot area is arrived at, I'd be curious to know if it did matter. With regard to parallels to horn theory, I have no idea

[Chops]

I wonder how it would effect the resonant frequency as well, as there would be no parallel walls at all at that point, lengthwise at least.

[Rudolf]

From what I've read, it's simply the ratio of the slot area to Sd that matters. Nelson used a ratio of 1:3, I think I saw someone say 40%. I don't think I've seen anyone comment that it matters how the slot area is arrived at, I'd be curious to know if it did matter.

If Chops would follow people with SOME "slot-loading" experience and not a once-in-a-lifetime experiment by Papa Nelson, he would have found that the general recommended slot area is 1/4-1/3 Sd for drivers with Xlin < 5 mm, and no less than 1/3 Sd for drivers with Xlin = 5 mm and more. Rear slot opening should be 1-1/2 Sd. Optimal numbers depend on Vas values and need a simulation with AKABAK or another precise simulation program to be calculated with sufficient precision.
AFAIK the idea is that drivers with a "loose" suspension should see more air resistance from the slot than drivers with a stiff suspension.

Chops' waist would effectively lower the slot cross area, but would do nothing WRT resonances.

Rudolf

[scorpion]

I think I should answer my question above myself. For what matters I agree enterily with Rudolf that the Nelson Pass baffle is more or less a gimmick, not really thought over in the light of proven knowledge. You won't get around the EQ problem and effeciency gain would probably be small. So what you are set with in this design is more or less a flat baffle with some introduced problems in how frequency response will be. So it is not really an advisable way.

I suppose what Rudolf is hinting at in his answer is the Ripole concept which deals with pressure loaded units but that in it's really useful application is more of a subwoofer solution. Now you settled on the Alpair 10.2 and some reasonably slim baffle width, I assume. And you are also quite right given your recent hearing experience that the fullrange response will begin to decline about 300 Hz with those reasonable baffle widths.

So here is one kind of answer: I simulated 4 (6 is not possible with the MJK   software) Indian Woofers arranged in two groups with two units parallel in each group and then these groups are serially conected. I simulated those in an U-frame MJK-way in a 7.5 " deep (baffle excepted) U-frame with inner measures 26 x 18 ". That is to make place for another pair of woofers to use all 6 per side. To add the extra woofers just connect one each in the parallell groups to make two 3 groups.

On top of the U-frame is placed a flat baffle of 14 x 18 " dimensions the Alpair put 10" up in the middle. Crossover was set to 100 Hz 2 nd order LR for the woofers LP and 250 Hz HP also LR for the Alpairs, a kind of standard MJK simulation. I, given Chops equipment, would advocate to use 6 woofers per side for greater margin. I don't think that adding the woofers would alter anything but giving more bass output, (about 3.5 dBs) so it is just to adjust bass SPL to the Alpairs.

Now here are the results, Baffle picture first:



and then response:



/Erling