Terry Cain's BIB -why does it work and does anyone have those Fostex Craft Handbooks? - diyAudio
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Old 17th October 2005, 07:14 PM   #1
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Default Terry Cain's BIB -why does it work and does anyone have those Fostex Craft Handbooks?

OK guys, thinking hats on, because, since the idea's recently come up again, I think it's about time we revisited this design and figured out why this monster TQWT simulates so poorly, yet usually performs well in-room -far better than conventionally accepted wisdom would suggest. I admit I have a liking for this box -I built a rough pair in response to a challenge from Terry a few months ago using the FE166E, and they worked pretty well -in fact, astonishingly so, considering all the apparant anomolies. We're looking at a driver ill-suited for TL loading with a low Q, a pipe with an So of 0 (so a theoretically high F3), and open at the other end, which isn't going to provide much any damping, and a driver position that seemingly dictates a massive hole circa 100Hz.

But it's never as bad as that. There's usually a dip at 100Hz[ish] but ony around 4 db in my experience. I got 28Hz loud and clear from the boxes I built -lower than the model says it should go. And while there's ripple, again, it's never as bad as predicted. So what's going on? Clearly, it's not the fault of Martin's software: his MathCad worksheets don't lie, though they don't take room-gain into account. So it must be what we (or I!) am / are inputting into them.

I understand that this is basically a Fostex factory design from their craft handbooks, which seem to be unavailable, or almost impossible to get hold of -anyone got them, and if so, could you send me an email? How is this thing it sized? I suspect / assume the line length is set to roughly 1/2 the wavelength of the Fs of the driver, but what about everything else? It's a pre Martin design, but what methodology, and how can we figure it out? Whatever it is, I suspect it's quite basic; perhpas it's a case of I can't see the wood for the trees.

That internal baffle particularly interests me for example. Terry's original is 14" deep, yes? Subtract 1 1/2" for the front and rear walls to give us 12 1/2". Now, unless I'm confusing things here (I'm not at my best at the moment, so if I make a stupid error do let me know), I understand that it finishes 5 1/2" from the base, and 5 1/2" from the rear wall. But half of 12 1/2" is not 5 1/2", so surely the expansion of this taper is not constant? If you look carefully at his CAD drawing, it doesn't even look it. It almost seems to be two differentaially tuned pipes, one like a Voigt pipe with a rear vent, venting into a second pipe with different proportions. Thoughts?

Terry commented to me that the FE166E is a great match, and I can't deny it worked pretty well (and I like a warm ballance, believe me), the FE168ESigma is even better, but a friend (you around Dan?) mentioned that in a discussion with Terry shortly afterward, he also suggested the best bet would be a higher Q driver like the FF165K -I assume he's referiing to its 10.92 Qes, because Qts is down at 0.2 which is the lowest of all the Fostex 6" drivers.

Any thoughts and coments are welcome!

Best
Scott
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Old 17th October 2005, 08:33 PM   #2
SCD is offline SCD  Canada
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Default Terry's designs

Hello Scott:
I am also interested Terry's designs. I think you have to add a bit to your first note. I am note really sure just what the heck you are talking about. The Abby, or one of his horns
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Old 17th October 2005, 08:35 PM   #3
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Interesting questions. I have no idea why it works, but it does. It's a pretty simple design. One thing that's nice is there really aren't any obsticals with ceiling propagation.

I'm using mine with the 168ESigma and it works well. I built them to his dimensions for a 5" driver I had, so the 168 is basically shoehorned in. If I were to build again for these speakers, I'd open up the interior dimensions a bit. I think that would help the driver breathe a little. I have some stuffing above the driver and a piece of an old seat cover behind the driver. I should try without all of this, but I have plans for something different anyway.

Who knows, I may come back to this design down the road.

Something to note, I did some emailing with Terry with regards to this design. One recommendation he had was to chope the back of the box down at an angle from the front to open up the mouth a bit. I haven't done it, but would design that into the box if I were to do it again.
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Old 17th October 2005, 10:02 PM   #4
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Default Re: Terry's designs

Quote:
Originally posted by SCD
Hello Scott:
I am also interested Terry's designs. I think you have to add a bit to your first note. I am note really sure just what the heck you are talking about. The Abby, or one of his horns
Neither. It's his 'Bigger Is Better' TQWT which you;ll find in the TQWT DIY projects section of the Single Driver Site. He's never produced it commercially, though I understand he made a few what I suppose can be termed as pre-production-prototypes a few years back using the FE168ESigma. As I understand it, those were slightly shorter, at 64" high, and deeper at 18" (external), which would give the driver more volume and, I suspect, was caused by him increasing the area of So, probably to Sd, though that's just a guess, and where the internal baffle would end or be positioned I can't quite work out. I suspect it would be approximately 8" from the floor and rear baffle, but without having the design method / calculations / equations that's only a guess again.

Increasing the mouth area is an interesting idea I hadn't thought of -I suppose this is where we're getting to the fringe of a design type where you can go in different directions -closer to a horn or remain in the TL camp by restricting the area by mass-loading. Were yours the dimesnions on the Single Driver Site?

Clearly, it uses room gain more, or differently to most other speakers, and driving the vertical mode of a room can often smooth things out which probably helps the in-room response. It's clean too, not having much in the way. As a rule I'm in the Linkwitz camp of avoiding if possible, but if not, get as much advantage as you can! I haven't emailed Terry for a while -I figured he's busy as I haven't seen him on any boards for some time, but I might fire one in his direction on this score soon.

Cheers for the replies guys, anyone else got some thoughts?
Scott
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Old 17th October 2005, 10:13 PM   #5
Tweeker is offline Tweeker  United States
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Madisound offers Nagaoka Tetsuo's Special Techique to Design the Original Speakers. A five book set for $90 if thats what your looking for.
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Old 17th October 2005, 10:33 PM   #6
dmason is offline dmason  United States
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hi Scott

Well, I built the BIB for the 168Sig as well, and it certainly does work, and work well. In a 13X15 room I was getting strong bass down to around 30Hz, solid and fast. The amazing thing is that this driver is in no way optimal for this load, ideally a mid Q design is best. What I like about this design is how effortless and FAST the sound is, lending itself to fast, single stage digital amps, and the room gain is such that with the 168Sig you can rattle the cages with a couple tube watts as well. VERY impressive, I would say my favorite load for wide rangers.

The 8 inch mid Q drivers yield a load so big as to be untenable for any sort of home use around here, mostly ~18" deep, 70" high, the coming HempTones will be interesting here, ...but one driver that has caught my eye and I keep coming back to is the Supravox 165LB, Qts=.45, Fs=62, 95db, and a very healthy Xmax of 4mm linear. Far more suited for this type of thing, measures very well. Perhaps Scott you would like to apply your math skills to this one.
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Old 17th October 2005, 10:43 PM   #7
MJK is offline MJK  United States
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Scott,

Quote:
Clearly, it's not the fault of Martin's software: his MathCad worksheets don't lie, though they don't take room-gain into account.
I think the last part of you sentence is the key. The MathCad worksheets you are using contain a significant number of simplifying assumptions. Once you start removing these assumptions, and calculating a better answer, the predicted response can improve (or get worse) if the real design is dependent on other factors like being close to the rear wall as shown in the article's photo. If the math improved, so does the understanding of the speaker performance possibly revealing why it works better then originally predicted.
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Old 17th October 2005, 10:50 PM   #8
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I like that 4mm xmax. The 0.45 Q looks good too for these boxes. The Fs is a bit high, but with decent excursion I doubt that'd be an issue, especially as the box is tuned so low. Same question as above: were yours the original Single Driver Site Dimensions or did you modify them at all? The hempsters look and sound promising too from what I've seen so far. I'll see what gives in MathCad for those Supravox jobs as well and get back to you tomorrow (a day off! Well, OK, so I have to spend most of it re-drafting a chapter of my thesis, but I think I can find time ;-)

I've got a pair of the original spec boxes half complete at present and a pair of FFs waiting to go in them. Or perhaps not. I don't like the 0.3mm xmax, which seems way too low for my liking, even though I suspect that's Fostex being hyper-conservative, and the overall Q of 0.2 seems very low as well, despite the high 10.92 Qms. No harm in trying I suppose, and I can re-use the drivers if they don't work well, and get the Sigmas or those Supravoxs into them. I'd forgotton about the latter -you could be onto something there, though until I can get my paws on the Craft Manuals and figue out exactly how these things are sized, with the BIB box we'd be working on guesswork as I can't sim in MathCad without accurate dimensions to plug into it. Those books from Madisound might well have them in. I might email them to ask. Things are getting interesting!

Cheers for that
Scott
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Old 17th October 2005, 11:02 PM   #9
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Quote:
Originally posted by MJK
Scott,

I think the last part of you sentence is the key. The MathCad worksheets you are using contain a significant number of simplifying assumptions. Once you start removing these assumptions, and calculating a better answer, the predicted response can improve (or get worse) if the real design is dependent on other factors like being close to the rear wall as shown in the article's photo. If the math improved, so does the understanding of the speaker performance possibly revealing why it works better then originally predicted.

That sounds sensible and would explain it -I don't pretend to be an expert on math or acoustics; what I know I owe primarily to yourself, Siegfried Linkwitz and Bob Brines. I suspected that it might be the case that it's down to elements outside the software's remit, for want of a better phrase (it's midnight here!); that would account for the terrible predicted response of the pipes and their frequently solid in-room performance when shoved into corners or against a rear wall. Never neutral, and they've got a heavy character, but they do some things very well, I have to admit. I'll stick to your Project 2 for my normal listening, but I might finish the pair I have in construction as my 'party speaker' or for use if I ever join the SET brigade from my usually solid-state enclave. They should certainly rock the place down nicely!

Thanks Martin
Regards
Scott
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Old 17th October 2005, 11:43 PM   #10
MJK is offline MJK  United States
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Just got back from my evening walk, I think my wife is trying to kill me, and I was thinking some more about the Terry Cain pipes.

The location of the open end is very interesting and probably the reason the simulation looks so bad. But as you report the real response seems to be much better. Two points probably contribute to this difference.

First, with the open end backed up against the wall the effective mouth area is doubled, due to the reflection boundary condition at the wall, and this will allow lower frequencies to be more efficiently transferred into the room (the resistive portion of the acoustic impedance is doubled). At the same time, less energy is reflected back into the line reducing the magnitude of the standing waves. So more bass output and attenuated standing waves compared to the simulation results.

Second, by pointing the open end up you are listening over 90 degrees off axis. With the mouth output reflected off the wall, the depth dimension effectively doubles while the width remains the same. This shape is going to become very directional with increasing frequency so the ragged response predicted above say 300 Hz is probably grossly over estimated.

Two interesting properties not accounted for in the current MathCad simulations. My newer worksheets are starting to account for these issues, along with the baffle step, and include them in the final calculated results.
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