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-   -   Folded Voigt Pipe (http://www.diyaudio.com/forums/full-range/66827-folded-voigt-pipe.html)

Ropie 27th October 2005 08:02 PM

Folded Voigt Pipe
 
I was searching the 'net for a new fullrange design to try and found this one:

http://www.speakerbuilding.com/conte...1/voight_d.gif
http://www.speakerbuilding.com/content/1031/

It has visual proportions and a size that I like and I've heard good things about the VP design generally. I would like to try it with a low Qts 8" driver such as Fostex 206E.

The thing is I would need to put the driver on the side of the enclosure to fit it in (this would also make for a more living-room friendly speaker too ;) ) Here is my revision:

http://piero.eml.cc//stuff/voight1.jpg

Has anybody got any opinions on this? Will I need to use a baffle-step circuit? Extra stuffing in the line? Any help appreciated :)

Scottmoose 27th October 2005 09:26 PM

TQWTs are almost always interesting, and this one... How very strange. I haven't seen one quite like that before. If you don't mind using hefty values in the BSC circuit, then the FE206E will probably be fine, at first glance -I haven't had chance to look at all the details. If you don't want to use such high values, then go for the FE207E.

It's an interesting design, and that's always refreshing. The only point of concern for me is the upward firing vent. This enclosure isn't going to be particularly tall, so it might struggle to couple to the room as well as a low mounted vent, or a taller enclosure with a vent in the top plate. My gut feeling is that it might be better having a sealed top panel, and moving the vent to the top back of the rear baffle, to keep the same line length. If it hasn't been already, ensure that it's modelled in Martin's MathCad sheets to get the dimensions spot on. Could be worth pursuing though -I'd certainly be very interested to see one and hear your views.

Scott

Panelhead 27th October 2005 10:00 PM

Did not work in my house
 
I built one a couple years back that was close. The cabinet was a simple folded TQWT. Following advice it was placed closer to the middle of the horn than usual. This placed it firing into the bend of the horn.
The totally screwed up the horn loading of the TQWT. It acted more like a BR cabinet.
I think the TQWT needs the driver mounted in a smaller volume area to work properly. I my mind it sees the bend as a large cavity not as a horn per se.
The folded TQWT can be spendid. I have yet to hear any I liked any better.


George

GM 28th October 2005 02:12 AM

Quote:

Originally posted by Scottmoose
My gut feeling is that it might be better having a sealed top panel, and moving the vent to the top back of the rear baffle, to keep the same line length. If it hasn't been already, ensure that it's modelled in Martin's MathCad sheets to get the dimensions spot on. Could be worth pursuing though -I'd certainly be very interested to see one and hear your views.
Greets!

Acoustically, not enough difference to matter.

Considering its stepped shape for the driver area, how do you sim it with MJK's WS? I mean 'ballooning' it out in the middle of an expansion creates a low pass filter like a simple muffler so its response may look similar to a modest length BLH, with the 'neck' down at the end acting as a huge vent to fill in/lower Fb.

Anyway, it has plenty of Vb and interesting enough that if I were still actively building, I'd have to try one out of curiosity and see what, if any, changes were required.

GM

rjb 28th October 2005 02:29 AM

Interesting. One of the issues that can arise with FR TL designs is the reflection of higher frequencies back through the cone from the close rear wall.How important this is depends on a number of factors, but I did once build the same basic T line with two driver positions, one front mounted with a divider close behind, and the other with the driver end mounted, with no wall close. I prefered the end mount version, atributing this to no mid frequency reflection. But there was not much in it.
It seems to me this design attempts to absorb higher freq. back reflections too.
You may like to read up about the DaLine as well, (use google).

s_zimble 28th October 2005 05:02 AM

I built a folded TQWT using the Rutter sheet as a guide for interier dimnetions. The position of the driver is at the fold, half in and half out, if you know what I mean. I have also thought that the top end of the tube might not be functioning as intended unless the driver is positioned further in the top end of the tube. As I have no equipment to take any comparitive measurments, does anyone here have a comment or advise on this idea? Also, I want to make a BLH using a single, small, full range driver and heard that BLH can be quite directional. Would designing an enclosure with a floor facing mouth eliminate some of the directionality?? would a folded TQWT act similarly to a BLH?

Thanks for your comments.
Stuart

Scottmoose 28th October 2005 08:20 AM

Well, as Martin and others have shown, a TQWT (and most other TL & BR types) are actually specialised vartiations on the horn theme, so in certain respects, some things look a little similar. Yes, it probably would eliminate some directionality. However, if the mouth is rearward facing, and designed to be loaded by a rear wall or corners, a la the Buschorn, that should also eliminate much of the directionality issues sometimes seen / heard with horns. I'm not a horn type person myself -I haven't heard any I've liked so far, but this is supposed to be one of the better ones.

It's an interesting point, that driver positioning question. If I was designing a folded pipe, all I can say is I'd position the driver and begin the internal baffle in the same way, at the half-way point of the driver, and I couldn't advise shifting it. I don't claim to have knowledge even approaching GM's Martin King's, Terry Cain's or Bob Brines -few do. However, my assumption is that the baffle and driver are positioned this way to ensure the correct loading in the throat, and I doubt I'd deviate from that particular feature.

However, if you wanted to improve things, I suspect that the best way would be to either increase the area of the throat choke so that its area equals So, if it doesn't already, and possibly / probably mass-loading the thing. Plug the basic dimensions into Martin's ML TQWT worksheet and see what gives -what driver are you using? I'm not knocking John and his sheet, it's a neat little tool I still frequently use, usually as a fast way of establishing and checking line-lengths etc. However, the Weems calculations it's based upon are now a bit outmoded. While still useful, I'd go with Martin's MathCad worksheets as a design tool any day of the week -I'm very firmly in that camp. The only exception I've heard so far equalling the performance of a cabinet designed this way is Terry Cain's monster Bigger Is Better TQWT -sheer artistry and creative thinking still have a place! I actually used parts of the Rutter spreadsheet & Weems calculations when trying to figure some of this cabinet out.

Ropie 28th October 2005 08:28 AM

Thanks for the interesting replies. I have had a look at Martin J King's Mathcad sheets before but they didn't make much sense to me. Anyway, the general concensus seems to be that these will be worth trying, with some reservations. If the top firing port seems to create problems I can easily move it the the rear.

rjb: I almost built a Daline before I built my current speakers (some Buschhorns) but I want a more compact speaker this time.

As for materials, the designer recommends 16mm chipboard but I think I might try MDF. Bob Brines gives a good case for using it in TQWPs and it's easier to work with than chipboard.

Quote:

Originally posted by Scottmoose However, if you wanted to improve things, I suspect that the best way would be to either increase the area of the throat choke so that its area equals So, if it doesn't already, and possibly / probably mass-loading the thing.
Scott: Do you mean I could use extra stuffing in the throat around the driver?

Scottmoose 28th October 2005 10:33 AM

Not exactly. This might help with the MathCad sheets too.

So is the closed end of the (or 'a') pipe. In the case of a TQWT it's the narrow end.

Sm is the open end of a pipe (or at least, it has some form of opening nearby).

As a general guideline for a TQWT, it's best not to use a surface area for So smaller than the surface area or Sd of the driver. There are exceptions to this rule, but it's valid 99.9% of the time. If you use a smaller surface area, what tends to happen is a viscious ripple in the response curve often called comb filtering (whether it is actually comb filtering, which as I understand requires two radiating sources is a moot point -it looks like it anyway!). So top tip -make sure the pointy[ish] bit directly below the driver has a bottom surface area equal to that of the Sd or surface area of the driver, if it isn't already. It won't damage the response, and might improve it.

Mass-loading refers to restricting the area of the open end with some form of port or vent, as seen on, say, a BR design. That doesn't mean it's no longer a Quarter Wave Resonator and has become a bass-reflex box by the way! What happens is the air mass in the constricted area of the port provides an additional load on the quarter waves a TL cabinet is designed to activate and utilise. This can do a few things -provide a much lower cut-off point, and also damp the response down a bit, reducing ripple. But it depends on the circumstances -driver, cabinet geometry, line length, vent size etc. as to whether it's appropriate or not.

MDF should be fine. Chipboard's Ok -I've used it in the past, and I believe Terry Cain of all people now uses it for the rear panel of his much respected Abby (though I could be wrong there) -you use the best material for the job. I usually stick with MDF generally myself, though that says more about me, my budget and wookworking skills or lack thereof than anything about natural materials. Perhaps a combination of the two would be good, or a decent grade of ply -you can't really predict this though -this is where the suck it and see art form comes into speaker design.

Best
Scott

Bjohannesen 28th October 2005 10:54 AM

Hi Ropie, until a year ago I had two of those, built as the original drawing mounted with Fostex FE164.

I was very happy with these spekaers. I mounted them on the wall with a small distance to the wall and used back ground light.

It looked as if the speakers where floating in the air
:)

This construction was very popular in Scandinavia during the 70-ties and the 80-ties, and was called the "minihorn".

I just used a piece of carpet behind the driver and at the bottom of the cabinet. No further stuffing.

Nowadays I am more into mass loaded quarter wave with the use of Martin´s work.


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