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Old 2nd July 2005, 07:28 PM   #1
sol is offline sol  United States
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Default Advice: a fence post for a full range?

Hi all, first post.

I'm constructing a small set of speakers with a 3" fullrange driver. Design goals are as follows:

Floorstanding full range with very small footprint, easy to construct. Paired with a sub, active hi-pass filter.

I started out intending to clone Zaph's Hi-Vi project linked here: http://www.zaphaudio.com/audio-speaker18.html

Then I found an interesting material at the home store: a composite decking corner post, 4.5" by 4.5" by 42" long, hollow in the center, with 3/16" thick walls. The material is molded in one piece. Knocking on the sides produces pipe-like resonance, but the walls themselves sound fairly dead like MDF or particle board.

This material is attractive for a number of reasons. 42" is an ideal height. Construction should be simple, cutting a driver hole and fastening on end caps. The narrow width fits perfectly where the speakers need to live. The total volume inside the column is more than a traditional dimensioned box sized to sit in the same small place.

This raises a number of design questions, as to what alignment to use:

1. Clone Zaph's design parameters, a volume of about 0.12 cu ft, by plugging the middle of the column. Or filling it with sand (in a bag) so I can play with different volumes later.

2. Keeping a sealed alignment, cap both ends for a total volume of .32 cu ft, stuffing the bottom of the enclosure to try and kill any paper-towel-tube reverberations.

3. Something more interesting. This is what I'd like to know: what novel alignments could I get out of this long, slender enclosure? Open it up at the bottom and let the whole pipe resonate? Try wedging a baffle inside for a mini- QWTL? Bipole? The cyburgs-needle enclosure really piqued my interest, but alas the box is too deep, and the construction process too involved for my small apartment.

4. Is this a bad idea? Will the long narrow enclosure sound boomy, with no way to fix it? Should I stick with a traditional box?

The full-range thing has really got me back into diy. The simplicity of a single driver appeals to me. If I have any sucess with this, I'll probably investigate other drivers, Aura, TB W3-871, fostex. I spend a lot of time working from home, and realized I rarely use the big stereo, but spend my time in the kitchen with the laptop, listening to this all day:
http://www.tivoliaudio.com/product.p...55&cat=&page=4

-Sol
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Old 3rd July 2005, 12:14 AM   #2
squalor is offline squalor  United States
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Sounds like you could make a nice Line Array with that size material. In your home, can you position the sub directly between the two posts , err cabinets ?
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Old 3rd July 2005, 01:57 AM   #3
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Default K Slotted TL

Sounds perfect for a TL terminated by a K slot to broaden the resonance of the pipe, a good alternative to BR for getting more lows from a FR driver, and with gentler roll off.
Here's my version using 150mm pipe:

Pete McK
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Old 3rd July 2005, 06:57 AM   #4
sol is offline sol  United States
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Squalor: A line array might be something for the future. But for now I'm sticking with the single drivers. It's all about the simplicity, to increase the odds of it actually getting done.

And yes, the good news is the sub will be squarely between the posts. I was thinking of a crossover at 150hz.


PeteMcK: Hmmm. Your photo is intriguing. I have two questions:

1. what is a K slot? I've searched and found a couple of references, but none that make sense to me. I understand sealed, ported, even bandpass, but TL stuff is all new to me.

2. could you include a photo of the bottom of your tubes, or a sketch of internal construction? That is, after all, where the line is terminated.

Working through the math, this is what I get for a quarter-wave TL, including my numbers in case I mess up:

Frequency = speed of sound/ 4*(lenth of pipe)

Freq. = 13397/4*42

Freq. = about 79hz

Hmm, that's right about where response starts dropping off on the sealed box alignment.

I think I'll build one sealed, and one T-line. I can use a wedge-shaped wood batten inserted from the bottom to narrow the line, and elevate the post off the base to let the sound out. If the sealed alignment sounds better, I'll pull out the batten and screw the base tight.

So now I get to explore a full range project and a T-line project at the same time.

-Sol
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Old 3rd July 2005, 09:12 PM   #5
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Default Karlson site

Hi Sol,
Check out the theory page on the Karlson site:
http://home.planet.nl/~ulfman/

Sorry, don't have a full sized photo, essentially I've put a baffle down the middle of the pipe, so it's a folded TL.
(Sealed at the bottom and at the top behind the baffle)
If you do this with your fence post, you should be able to get down to around 40Hz

Cheers,
Pete McK
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Old 3rd July 2005, 11:37 PM   #6
GM is offline GM  United States
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Greets!

The fencepost doesn't have enough Vb to allow this driver to 'breathe', so to KISS it, a straight ML-TL would be my choice: 19 mm no-void plywood caps, driver down ~6", with a downfiring 1.5" dia x 1.5" long vent. Really stuff the top above the driver, then as little as sounds good to you down the rest of the pipe. Due to the small vent/pipe base area, it only needs to spaced up off a solid surface ~0.10".

GM
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Old 5th July 2005, 05:16 AM   #7
sol is offline sol  United States
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Wow, new concepts. Thanks for the insights.

PeteMcK, thanks to your link, I know much more about this Karlson thing. Unfortunately, it is beyond my resources (time, skill, neighbors lovin' the jigsaw all night).

GM, the kiss principle is in full effect, so I like your suggestion. Three questions for further refinement:

1. I plugged the volume and port tuning data into ISD and I got a graph with a big bump at 75hz. Now I know that the simulation is primitive, especially for a driver this small and a TL type alignment, but it still concerns me. Should I be concerned? The graph is pasted in below.

2. The enclosure is so narrow that it could easily be used as a port itself (for a ginormous subwoofer box). Total cross sectional area is the same as a 4" diameter tube. In an enclosure that is a tube itself, whose close-pipe resonance and port tuning frequency are about the same (as best I can simulate) what is the purpose of the smaller port? Does it filter out higher-harmonic resonances of the line?

3. Why place the driver 6" from the top of the enclosure? Does the line resonate differently with the driver at the end? Should I calculate the harmonic null points to place/not place the driver at one of them (all these questions start me liking the sealed alignment again)? This is one area where I cannot experiment, once I cut the hole that's where it is.

So now I'm considering three alignments: sealed, TL with taper insert, and open TL with a port. I'd love to get it down to two, so I can build one of each and do a side by side test.

-Sol
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Old 6th July 2005, 02:57 AM   #8
GM is offline GM  United States
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Greets!

Box programs assumes a uniform particle density (no wave motion), so the vent would need to be somewhat longer to get the same Fb as the pipe, though the sim will still be off a bit. You'll need to sim it in MJK's ML-TQWT MathCad worksheet to see the results. If it performs as simmed, I believe you will be pleased, and yes, if it had the response you posted I wouldn't recommend it.

Yes, the vent mass loads it for smoother performance.

Yes, pipe performance can be tuned somewhat by the position of the driver and/or vent. Ideally, the driver should be where it damps the 3rd harmonic, or ~12.606" down in this design, but this puts it too far below a typical ear height, so chose 6" as an acceptable alternative when combined with vent slot loading.

Anyway, if it performs remotely like it sims, it will 'blow the doors off' the sealed cab, with solid output to 50 Hz, and the folded pipe doesn't have enough Vb for its length, so for me it's a no-brainer.

As always though, YMMV.

GM
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Old 8th July 2005, 05:27 AM   #9
Pallas is offline Pallas  Pakistan
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GM,

Alas, there's no OSX version of MathCAD Explorer to play with myself, so I have to ask: would the above enclosure specs (same tube, bottom-firing vent, 0.1" standoff - about the thickness of a quarter and a nickel, right? - from a hard surface, driver placed ~6.3" from the top of the pole, 1.5" id x 1.5" long bottom-firing port) also work for the Aura NS3? (In short, the NS3 has a lower F3, lower Qts but less mechanical damping, and a larger Vas.) Also, any reason to use plywood instead of, say, ceramic tile for the endcaps? (It'll be resting on a 6"x6" tile plinth.) From what I've read on Martin King's quarterwave.com website, what matters is the line length and not so much the driver, but I know you've built these things before and I haven't so I want to make sure I'm on the right track.

Also, thanks sol for pointing out such a cool (almost) insta-enclosure for small drivers! Apparently these posts are pretty easy to paint, too, and inexpensive at ~$23/pair at the local HD. (Lowes has a different model that's thinner walled, slightly more expensive, and lacks the three ribs per wall.) Until I read this thread I was planning to use PVC pipe but I think a pair of painted decking posts will blend in much better in my office than PVC towers. Maybe I'll build lamps on top of them, too.
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Old 8th July 2005, 02:19 PM   #10
GM is offline GM  United States
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>GM,

Alas, there's no OSX version of MathCAD Explorer to play with myself, so I have to ask: would the above enclosure specs (same tube, bottom-firing vent, 0.1" standoff - about the thickness of a quarter and a nickel, right? -
====
Correct, though obviously you can experiment with more/less loading.
====
>from a hard surface, driver placed ~6.3" from the top of the pole, 1.5" id x 1.5" long bottom-firing port) also work for the Aura NS3? (In short, the NS3 has a lower F3, lower Qts but less mechanical damping, and a larger Vas.)
====
Yes, it's actually a much better 'fit' than the B3S, though a lower tuning with up to a 3.5" long vent will extend/flatten/'tighten up' the response more to my liking, but experiment to find what sounds best to you.
====
>Also, any reason to use plywood instead of, say, ceramic tile for the endcaps? (It'll be resting on a 6"x6" tile plinth.)
====
Not really, marble and slate is my preference, but these are exterior mounted, you still need an insert, so I recommend 19 mm no-void plywood. Just so we're clear, the ~6.3" is i.d., so with a 19 mm insert the driver cutout is down ~7.06" from the end of the fencepost.
====
>From what I've read on Martin King's quarterwave.com website, what matters is the line length and not so much the driver, but I know you've built these things before and I haven't so I want to make sure I'm on the right track.
====
Can't say as I agree with this per se if that's what he said. With my fading short term memory I don't recall any details from his docs. I mean that if you make the pipe acoustically large/long enough then the driver really doesn't much matter, but this wouldn't be a very optimized design. The way I 'designed' (I use the term loosely) cabs before T/S (and still do in many cases) was a simple, but effective 'formula' I derived based solely on the driver's frame diameter, tuned it to Fs with up to a ~'Sd' area vent and critically' damped it for best blend in-room. Were they T/S 'optimum'? Simming the ones I still have my notes of, hardly. Did they perform/sound good? All who auditioned them thought so.

I see you list Hotlanta as 'home', so if you came to the DIY Meet several years ago when I brought the unfinished 40-1354 towers (now ML-TL) or last fall and heard the little folded pipes I brought, they were both designed using my theoretically antiquated 'formula', both of which were quite well received.

GM
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