W-Frame or H-Frame dipoles? performace issues?

[JinMTVT]

nice nice Rudolf,

i hear you!

what about making the sides not paralell ?
will that have any benefit on resonnances of the upper region ?

since it will be used with manger, i could see them going up to 200-250hz at max
probably crossed at 150hz where i'll feel that the manger starts going down

or heck, i'll just ghet the manger, make the baffle
and measure it before i construct the woofer enclosures...


So rounding off the edge is only good in baffle or box with HF operating range, for dispertion right ?

Then i can see from your drawing that you are using it with the 2 drivers on top of each other?

is that a good way to use it ?

then from your graph, ur system doesn't go down as soon as i would've thought ..
still going strong past 300hz nah ?

i am not sure i understand how to measure the wavelength effective for the enclosure of the H and W

do we measure the tour around the enclosure side sarting from the driver or we only use the measure of the width of the enclosure?

thanks agian for your time

[Retsel]

I have two 18 inch drivers (Ascendent Audio Avalanche drivers)in "W" baffle (phoenix style with one driver mounted in line with the other, not offset at 90 degrees like your diagram shows) and two 15 inch drivers on "U" baffles (mounted on the same baffle as the midrange drivers which in my case are Lowther DX4s).

The commenters are right that the W baffles tend toward resonances (mounting them at angles that you show would help to increase the frequency that the resonances occur). When I low pass them at 100 hz with a 24 db/octave filter, I avoid most (all?) of these resonances. But I tried one configuration where I crossed them over using a 12 db/octave filter at 250 hz and these resonances were clearly audible.

I don't know that the U baffles had similar resonances, but certainly they were less than the W baffles at the frequencies that they occurred.

I had significant problems with the U baffles shaking significantly, despite the bracing I was using (they are made out of 3/4 inch conventional plywood) when they were the sole drivers I was using. When I construct a final baffle and if I continue to mount thes 15 inch drivers in this fashion, I will at least double and maybe triple up the thickness of 3/4 inch material that I use for this baffle. This is one of the reasons I why picked up the pair of 18 inch drivers and mounted them in the separate boxes.

The W baffle is constructed out of 13 ply birch plywood. The baffles that the drivers are mounted on are 1 1/2 inch thick while all other faces are 3/4 inch ply. This thing is about 21 inches high, 24 inches deep and 40 inches wide (I suppose I could have made it narrower, but I did not want to increase the Q, I was hoping to avoid resonances, and I wanted to create a TV stand out of it). Playing very dynamic CDs (i.e., drums), any lighweight items sitting on the box can shake off it which demonstrates that the panels can still vibrate despite the cancellation benefits it enjoys. It weighs 160 pounds (80 pounds for the box and 80 pounds for the drivers). If I keep it for the long term, I will double up on all the 3/4 inch panels which will reduce its vibrations significantly.

I am planning to make a rectangular box that I will bolt onto the back side which has the common vent for the two drivers. The sounds being emitted from the drivers off that side of the box will have a lot longer distance to go before the sound can cancel. Once I have this in place, it will increase the equivalent baffle width from 24 inches to 60 inches (it will create a hybrid W-U baffle), thus providing significantly deeper bass extension.

Retsel

[moray james]

http://www.tr.ca/2006/english.htm. some nice looking drivers. Regards Moray James.

[johninCR]

Retzel,

I believe you're definitely on the right track adding the extension to the back. Not only do you pick up the extra rear wave travel distance on a 2 for 1 basis, but dipole bass is extension limited by the room's fundamental resonance, which the cardioid characteristic resulting from the U hybrid helps avoid. I've had great results with my W/U baffle hybrid test boxes using cheap drivers. I look forward to doing something similar with quality drivers.


Jin & Rudolph,

Once you get that kind of splay in your panels, I don't believe 1/4 wave resonances can exist. GM told me anything greater than 1" per 6" of depth should avoid resonance. The parallel top and bottom panels are a problem though. Something that will help avoid resonance (mentioned by DJK for boxes with manifolds) is to fill in the deep corners both front and rear with angled wood or rigid fiberglass.

As Retzel mentioned, you need a lot of mass to avoid vibration. Also, even though the box isn't closed you've got to address panel vibrations. Closed box alignments don't have panel vibrations due to a change in absolute pressure changes in the box, it's due to the pressure waves striking them with significant force. The angled alignments pictured will help, but the forces are still significant.

In a pure dipole alignment like Rudolph's pic, the distance differential is only the depth of the cab. The part you can't really quantify is the floor benefit, which is more than just boundary loading. The difference to measure is the travel distance of the rear wave from the cone to your listening position vs the travel distance from the front of the cone to your ears. That's why a U-baffle is +6db at the bottom over a same dimension H-baffle, because the "D" is doubled, which moves the F-equal point down a full octave.

BTW, as far as bass is concerned, nearfield measurement means little for any OB alignment because it greatly discounts the effect of the rear wave both in terms of spl vs distance as well as the front/rear wave travel differential.

[JinMTVT]

john...
nice comments

For the measurements of differential
u say that it is the difference in distnace between the listener's ear and the front VS rear cone ?
does that account for going "around" the baffle
or straight hypotetically through the enclosure and difrectly to the cone ??


The non paralell edges would help ?
but by doing what exactly ?

[moray james]

JohninCR: would be interested to know what specific drivers you have had sucess with. Chops has used inexpensive Pyle drivers and another source of inexpensive yet suitable drivers was posted here http://www.mach5audio.com/product_in...425a028ed2259.
Inexpensive driver will make experimentation with dipoles of all kinds more accessable to builders who might take the plunge and start building if the price was right.

I did want to ask you about your comments on regarding small amounts of angle being introduced to normally parallel drivers especially the parallel top and bottom of "W" style dipoles. First am I reading correctly that you are suggesting lengths of small 45 degree filler blocks the depth of the cabinet be used to break up resonances? So all around the inside cavity where the two drivers fire into ("W" style cabinet)? I wonder if resonances in this band are not more a result of the basket which is inserted into the cavity than the cavity itself, just a thought. I have not read anything about such resonances being an issue with Ripole versions. Please note though that most of the posted information (so far) on Ripoles is in German and I have to depend upon Google tramslations so the text is not a wholey reliable translation.
As you start to add angle between the two drivers then you start along the path which will lead you to a flat baffle with a driver on each side. So one extreem of that line is a "W" dipole and the other a "H" dipole. It would seem that for a "W" dipole to work then the fundamental cavity resonance needs to be preserved since in that design you have a short 1/4 wave duct on each face of the driver. The benefit of the driver cones firing at each other generates compression which increases the resistive load on the drivers which also happens when you angle the drivers in a "V" orientation in a "H" dipole though to a lesser degree. This results in the decrease of the drivers Fs when mounted in a "W" or Ripole cabinet. (Question: are similar reductions in driver Fs also observed in "H" dipoles?). From what I have read so far (perhaps Rudolf or Calvin will comment on this) it would seem that the sub's polar patern can be manipulated into a cardioid or perhaps more correctly into a more cardioid polar patern than purley dipole one.
Your comments on these thoughts will be most welcome. Regards Moray James.

[johninCR]

Originally posted by JinMTVT
For the measurements of differential
u say that it is the difference in distnace between the listener's ear and the front VS rear cone ?
does that account for going "around" the baffle
or straight hypotetically through the enclosure and difrectly to the cone ??

The non paralell edges would help ?
but by doing what exactly ?

Jin,

The rear wave can only go around/over the baffle. You have to keep in mind that Linkwitz's 6db/oct and starting point is free space. Once you get in room it's a whole different ball game. You've got the benefit of the floor, which is significant. You also have placement from the rear wall, which with judicial placement can increase bass from the predicted free space response. Use Linkwitz's SPL max spreadsheet as your starting point, then like any good engineering design you have plenty of cushion.

Re: non-parallel panels, I'm talking about all of the vertical panels in the drawings. If they aren't parallel, 1/4 wave &/or 1/2 wave resonances are going to be hard to stimulate. Panel resonances would be a much greater concern, because bracing is difficult but necessary.


Moray,

I bought a 20 Nippon 12" woofers that I found locally for $12/ea. I'd have trouble recommending them because I found them only once online but for over $20 and they aren't worth it especially plus shipping (too easily banged against the back plate). I built 2 cabs of four 12's each in a W/U hybrid using the Ripole approach of small pathways. They sound great with my only problems for below 100hz use being panel vibrations and output limitation due to Xmax.

Especially because OB's are open, parallel surfaces need consideration (more important than with boxes). Then if you have a manifold, you have to worry about depth. If surfaces are far from parallel, as I understand, 1/4 wave resonances can't be a problem, however, depending upon placement, reflections off of the rear wall can create cavity resonances.

Yes on the filler blocks, but only in those deepest corners to break up the rectangle shape of the cavity. Also, it seems intuitively better to have that air forced out of the pathway instead of compressing in that otherwise dead corner.

I have 4 Hawthorne Augies for my next attempt at high output OB bass augmentation needing only subwoofer assistance for HT. I want to construct a pair of units that will also be a solid base for whatever OB solution I want to put on top. Mechanical cancellation is a must. Push/pull with equal time alignment is another given. I also want to be able to use them up to 200hz or higher, to give me flexibility on drivers to use above that. Although I plan to use a slightly angled W alignment, for minimum width and lowering Fs reasons I want to try to get as close to a double ripole as possible. Plus I want to keep the splay angle of the drivers to a minimum for mechanical cancellation reasons. For upper end extension, I'll concede some added height to completely avoid parallel panels within the cavities, and utilize the resulting top and bottom voids to add mass and act as bracing and damping. I believe I can get 100db+ at 30hz potential fitting the pair of 15's in a cab 21" tall, 20" deep (including a U extension in the back), and close to 12" wide, while hitting all of my targets. My biggest problem, other than the construction complexity that angles create, is making sure I can mount the drivers once they're built.

[moray james]

http://www.pvconsultants.com/audio/boxmodel/uhframe.htm

[Rudolf]

Originally posted by johninCR
Once you get that kind of splay in your panels, I don't believe 1/4 wave resonances can exist. GM told me anything greater than 1" per 6" of depth should avoid resonance.

John,
the initial impulse to build my W-frames was the hope to avoid that lambda/4 resonance - exactly on the reasons you have stated. But – with all respect for GM – it doesn´t work like a switch you can turn at certain angle values and get rid of that resonance.

Compared to the same drivers in ripoles the resonance is “smeared” somewhat, but in hindsight I would have liked it more to be small and well defined so it could be “notch filtered” better. If I had to do it again I certainly would do one of the purer principles: either a conventional W-frame / ripole or a conventional H-frame.

The 300-400 Hz resonance peak of my W-frames surely results from the depth of the frame and NOT from the height or width, since the depth is building the only cavities to speak of in the design, that can be correlated to 300-400 Hz.

So changing the left and right panels in the way JinMTVT has illustrated in fig. 1b will change almost nothing compared to fig 1a. He will have to make the outermost angles almost 90° to make a difference.

BTW, as far as bass is concerned, nearfield measurement means little for any OB alignment because it greatly discounts the effect of the rear wave both in terms of spl vs distance as well as the front/rear wave travel differential. [/B]

That´s true for sure. My nearfield measurement was only meant to illustrate the resonance.

Rudolf

[johninCR]

When it comes to 1/4 wave resonances I can only go by what a TL expert like GM would tell me, since I don't really understand how they work.

In looking at your latest pic with the vertical dimension being the longest, my gut reaction is that the resonances in question are panel vibrations in those side panels. Just look at the size of those unbraced and undamped panels. I bet they vibrate quite audibly.

A complete lack of bracing is pervasive in virtually all folded OB designs I've seen, and it's based on the widespread misconception that since the cab is open there is no pressure exerted on the panels. This is simply not the case. In fact, because the front and rear edges of OB's typically don't have the structural support of a corner like boxes do, one could make the case that bracing is more important with folded OB's than with boxes. You do have to be more careful in how you orient your bracing than with a box, because you want to ensure that the pressure flow is equal on both sides of any braces.

I learned the hard way when I built a pyramid shaped U-baffle. With no bracing the large panels vibrated quite audibly. Then I added relatively small rectangular panels as cross braces. It cleared up the side panel vibrations, but the cross braces sang like guitar strings because the pressure flow wasn't equal over the top and bottom of the panel braces. Rod type braces with damping is probably the way to go with odd shaped cabs.