L C of Norway Voigt Pipe -quick fix - diyAudio
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Old 9th January 2006, 01:53 PM   #1
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Default L C of Norway Voigt Pipe -quick fix

I've been sorting through my notes that have been building up on the hard drive over the past 18 months or so and thought this might come in handy / be of interest for anyone out there. Nothing overly new of impressive, but someone might find it of some use, hense the post.

The Lowther Club of Norway 'Voigt Pipe' (Not that poor old P.G.A.H Voigt had anything to do with it). It's the entry for many people into the world of full-range DIY speaker-building -not sure why, probably because it looks dramatic and it's easy to construct.

Pity that, as it stands, it is, quite frankly, rotten. Below is the MathCad simulated response for this enclosure with the Fostex FE206E, which, rightly or wrongly, I suspect is the most popular driver that finds its way into these enclosures. (If you're interested, the response looks almost identical with the FE207E, FF225K, FE166E, FE167E and FF165K. With the Lowther PM6C drivers it was designed for, it looks even worse.)

But what if you've put a lot into your LCofN Voigt pipes, both in time and effort, and don't want to scrap them? Well, you're never going to have the best enclosure in the world I'm afraid. But I can suggest something that might help a bit: see next post.
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Old 9th January 2006, 02:06 PM   #2
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Still with me? OK, now if you ask anyone who is used to modelling quarter-wave enclosures with Martin's MathCad worksheets to pick one single thing they hate about the LCofN 'Voigt Pipe' above anything else, I would put money that it will be this:

They are an So=0 design. For those who haven't a clue what I'm on about at this point, So is the area of the top of the pipe. Now normally, when designing a Mass-Loaded TQWT enclosure, we say never use a top area less than the surface area of the drivers.

Thing is, in this case, that isn't the major problem. The drivers that frequently find their way into these things tend to be low Q designs. And they actually appear to be less affected than the higher Q drivers in this regard -in some ways, they seem to like the extra gain it provides. Gregreferred to this some time ago I believe?

What is a major issue is that although these things are mass-loaded (although I seriously doubt the thought even occured to the designer: he just set the vent area to the same surface area as the intended Lowther driver, or as close as he could get) -it's not enough.

Here's the same enclosure, same driver, everything the same. But instead of the large vent, there's a 3" wide, by 3" long circular port, 4" up from the bottom. OK, it's not perfect by any stretch of the imagination, but it's a heck of a lot better than having big peaks and holes all over the place.

This works wonders for every driver I tried it out with. The lowest Q types need a bit of series resistance as you can see: I'd say 2ohms or so, to bring the response down, as it rises a bit. Ripple is mostly flattened, and on average, you suddenly get an extra 20Hz of bass. These assume 0.25lbs ft^3 of stuffing from the top to just below the driver by the way.

Oh yes -a BSC filter would be of use too, in-room. I'd try the one from Martin King's Project 2 MLTQWT which should do the job fine.

Hope some of this is of use. I figured I'd put it up here before I delted it all.

Best
Scott
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Old 9th January 2006, 04:54 PM   #3
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Scott,

very nice! Thanks for the effort. :-)

Best,
Oliver
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Old 9th January 2006, 05:17 PM   #4
steenoe is offline steenoe  Denmark
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Thanks a lot for the nice writing, Scott. I had my eyes on those Voigt pipes from Norway, among others It's very nice to have the additional info you provides here.

Steen
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Old 9th January 2006, 05:30 PM   #5
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Konnichiwa,

I'd note a few things about the LCONVP.

1) LF extension is limited, but at high efficiency, basically that thing is 95db/W/m down to 70Hz, not bad.

2) The first really nasty mode is at 300Hz, where a simple counter resonator or even likely just the right kind of stuffing could limit the "big dip" and likely flatten out the feever curve higher up.

3) Given the kind of FR evidenced by the FE206 toeing these in strongly or pointing them straight on so that the listener is off axis would probably give a quite even tonal balance.

So, with a little luck in stuffing and listening off axis the LCONVP could make a surprisingly decent speaker..

For the modified design we see that efficiency is traded for bandwidth, by reducing the resonant behaviour of line, the result is a rather early but shallow LF rolloff, which despite having a lower -6db point (~40Hz) than the LCONVP (~55Hz) would likely sound rather strident and forward with lean tonality, unless the speakers midrange is pulled down by around 5db to bring it in line with the LF.

I suspect persnally I would probably prefer a reasonably sorted version of the original to the "fixed" one.....

Sayonara
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Old 9th January 2006, 09:24 PM   #6
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You're welcome guys -hope it's of use to someone. Note: I do not recommend constructing this cabinet. If you wish to build a TQWT for your driver; do yourself a favour and design a decent one, from scratch, in Martin's MathCad worksheets. It will reap major rewards. This mass-loading is just a boot-strap, quick fix for people who've already constructed the wretched thing and discovered the frequency response is all over the shop, and bass is serverely lacking.

Kuei -You make some very fair points. I agree with you regarding the toe-in, or listening off-axis. This would likely mitigate against requring series resistance; though I'd still advise using 24AWG or 30AWG wire with a low-damping factor amplifier, and a resistor with a high damping-factor amp or eveything under 120Hz or so is going to vanish.

Good luck if you think you're going to sort out the massive response issues this cabinet has via stuffing however. No chance. Promise. Stuffing exists only to damp harmonics that cannot be otherwise engineered out of a cabinet. There is a 20db hole in the response and that's just the one at 285Hz or so: you won't loose that by shoving more stuffing into the pipe, and for the love of heaven don't believe the myth that all additional stuffing does is clobber the efficiency. It will start to absorb the Fundamental, which is the whole reason for using a QWR in the first place. The mid-Q drivers like the FE207E and FE167E don't have this rise. Me, I'd rather have a flatter response than 20db holes and savage comb-filtering.

Be aware that whatever you do, this cabinet needs baffle-step correction. Active or passive, take your pick. Without it, you're going to run into even more problems. Passive will need a 2mH inductor with 4ohm resistor, approximate (minimum) values.

Best
Scott
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Old 13th January 2006, 08:59 PM   #7
steenoe is offline steenoe  Denmark
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Well, I was wondering... How come those Cain's Abby's got so fancy reviews? They look an awfull lot like those norwegian "pipes" Except for the port, being made like Scott suggests Well, more or less at least?

Steen
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Old 13th January 2006, 09:23 PM   #8
MJK is offline MJK  United States
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Quote:
How come those Cain's Abby's got so fancy reviews?
If you look at my Project #2, you will find the design details behind this type of ML TQWT enclosure. My earlier design has a strong ressemblance to the Abby. I would assume the performances of both are very similar.
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Old 13th January 2006, 09:30 PM   #9
Previously: Kuei Yang Wang
 
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Konnichiwa,

Quote:
Originally posted by steenoe
Except for the port, being made like Scott suggests
The port appears the depth of the front wall and appears to have a width nearly of the whole pipe and sign ificant hight. Looks more like a cosmetic cahnge than an acoustic one....

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Old 13th January 2006, 09:38 PM   #10
steenoe is offline steenoe  Denmark
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Thanks a lot for responding, MJK. Just visited your homepage and realised that I have some reading to do I see what you mean, your project 2, looks a lot like like the Abby's. Just shaped a little different. Come to think of it, I guess I could pop my Coral Flat 6-II into those boxes? They shouldnt be too different from those Fostex, should they? Sorry if this is a stupid question, but I really dont know that much about speaker-units

Steen

Kuei... Crossposting I love that
Quote:
Looks more like a cosmetic cahnge than an acoustic one....
So you mean that fancy looking venthole in the Abby's are more or less a cosmetic showoff? Well, being in the process of planning a pair of speakers for my Pass F2 clone, I sure would like to get this just right from the start!! Could the venthole be just circular as in MJK's lovely box?? With the correct diameter, ofcourse
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