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Old 8th February 2009, 06:27 AM   #11
des1a is offline des1a  United States
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Default Installment #1

I’ll start with some information taken from Charles Pike’s measurements. He shows a nominal impedance of 5 ohms with a free-air resonance of 40-41Hz! His impedance curve is very smooth, approximately constant at 5 ohms, rising to 10 ohms at 20kHz. He and I both measured the DC resistance of our Dynapleats to be about 4 ohms (3.7 - 4.4 ohms for all my samples). For that reason I used series-parallel wiring to try to keep the system impedance to around 4 ohms.

There’s not much to see from a photo of the rear of my dipole panel. It looks just like the front minus the tweeter. The Dynapleats are mounted back-to-back in pairs, making the total thickness of the panel about 3 inches, given that I put a 1/4-inch spacer (a thick plastic washer on each mounting screw) behind each driver. In that 1/4-inch space I put a square of damping foam -- nothing special. I’m guessing it was a thin piece of packing foam I had handy.

The drivers are mounted on a piece of 1/4-inch thick perforated masonite (I call it pegboard). It is available in 1/8-inch and 1/4-inch thickness and I chose the thicker material. The masonite panels were eventually mounted in a framework and that entire panel was trimmed on the sides and bottom with pieces of solid oak, I believe, and stained. I’m not known for my woodworking, so I’m pleased the trim ended up looking acceptable to my wife.

I’ll provide this link to a drawing I made which shows the basic layout. Note that I inverted the panel in the final installation (don’t remember why).

http://www.la-tierra.net/abq8/speakers/dynapanel.jpg

I’ll give you the info on the wiring and crossover in the next posting.

regards.
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Old 8th February 2009, 02:45 PM   #12
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Default Dynapleats

Here's a few questions.

How many drivers did you use? your drawing shows 2 Dynapleats and one tweeter.

And why did you choose to use "perforated" masonite? I always thoutht that any dipole driver had its bass response defined by the size of the panel wich holds the drivers, but if it is perforated, the back waves of the drivers are allowed to go through, therefore, it is as if, acoustically, your drivers were hanging in free air, isn't it? My reasoning is that if your masonite was "not" perforated, and much wider and taller, you would get a more extended bass response. Just a few thoughts. I might be wrong, of course.

Sylvain
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Old 8th February 2009, 11:56 PM   #13
des1a is offline des1a  United States
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Default more details

In theory, low-frequency roll-off is affected by the panel size with dipole radiators. The small holes in the panel do not enter in because they are so small with respect to the wavelength at lower frequencies. A larger mounting panel may provide more extended bass response but I found the panel size I used (12”x24” with trim) provides excellent mid-bass performance and my subwoofer covers the lower bass. In practice, I think that positioning, walls, furniture, etc. will have a larger effect than modest differences in panel size.

I used 4 Dynapleats per channel. I wired each pair of Dynapleats in series (for about 10 ohms impedance in the crossover region) making sure that the back drivers would be opposite in polarity to the front drivers (connected minus to minus). This causes the pair to operate as a dipole when they are installed back-to-back. However, you must be sure that when you mount the second pair on the panel that polarity is preserved between the 2 pairs. I used long mounting screws through the assembly -- 1st driver, spacer (and foam), panel, spacer (and foam), 2nd driver.

The crossover elements were all first-order. However, I used different inductors for each pair of drivers to roll the system off very gradually. One pair had a .6mH inductor in series for a corner frequency of about 2.5kHz and the other pair had a .3mH inductor in series for a corner frequency of about 5kHz. Recall, however, that the impedance is rising smoothly with frequency, so the roll off is more gradual than 6db per octave.

The Fostex spiral ribbon tweeter is a 6 ohm tweeter. I padded it back a bit with a 2.5 ohm resistor and a 2µF capacitor in series for a crossover point of about 9kHz (really a supertweeter).

The 3 drivers (treating each pair of dynapleats as one driver) were all wired in parallel and that’s it. Note that if you choose to wire 3 pair of dynapleats in parallel, your system impedance will be lower.

I said earlier that my subwoofer was crossed over at 55Hz. I checked and it’s actually 70Hz. It’s a compound woofer (similar to the Linn Isobarik if that’s familiar to you) made up of 2 KEF 8” woofers mounted front to front in a 1.5 cu. ft. ducted port reflex. The left and right bass channels are summed and the subwoofer is driven separately from the 2 panels. I’m sure I’ve lost the details of the subwoofer design, but there are so many interesting powered subwoofers available these days that I consider that to be a very manageable part of the design.

The blending of the drivers sounds excellent to me on a wide range of music. There is a sweet spot for listening but, at that location, the imaging of voices and instruments is remarkable. There is a large sense of space around the music (the dipole pattern I assume). My speakers are about 18 inches from the walls behind and 4 inches from adjacent furniture.

I hope I addressed your questions for now. I'll keep an eye on the thread. It is an easy project so I don't think you'll have much trouble.
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Old 9th February 2009, 01:20 AM   #14
des1a is offline des1a  United States
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Default another sketch

As I re-read my various posts, I can see my descriptions might be confusing. In an attempt to either clear up the confusion (or add to it), here's another sketch which shows a side view of the mounting panel with the 5 drivers (2 pairs of Dynapleats and the tweeter). It illustrates how the two pairs of Dynapleats are mounted back-to-back with the tweeter in the center of the panel. Each of Dynapleat pairs is wired together in series to act as one coupled dipole radiator (each pair will then measure about 8 ohms in resistance).

http://www.la-tierra.net/abq8/speakers/sideview.jpg
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Old 9th February 2009, 01:26 AM   #15
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Default Dynapleat

Interesting..

I had not figured out that you were using 4 drivers per side, using them back to back, in oposite phase.

The problem I have is that having a dozen of them, it would make 6 per side. Difficult to match, impedance-wise. No matter what the number of drivers, I would have thought of using them all on the front side of the panel, with a opening behind them, to let them breath.
As well, I noticed that the "pleats" are placed "vertically" on your design. I was wondering if there is a up/down position with them. I would have placed the pleats horizontally, without even knowing why.. Glad I saw your design too.

As for a sub, funny you mention the isobarik, since it was in my plans to build a small speaker (like a LS3/5a) with Isobarik bass.. Too bad your plans are not available, since the different designs I have looked at where all going in different directions ..

I'll at least make some trials/errors, and let you know what happens.
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Old 9th February 2009, 03:21 AM   #16
des1a is offline des1a  United States
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I was initiallly considering mounting all 4 drivers in a vertical array on the front of the panel for each channel, but decided on the more compact version for a variety of reasons. I have to say that the apparent size of the soundstage is just as large as it is with my small Magneplanars which are larger in size.

You are right -- if you try to use 6 per channel, you will have to come up with a different way of controlling the impedance through a different series/parallel hookup. Maybe wire 3 in series and then connect the two sets of 3 in parallel But I suggest you test your drivers first for output and for simple resistance. I bought 10 and rejected 2 in testing that measured different from the others. You may find that you don’t want to use all 12.

I don’t think it makes a bit of difference whether the pleats are vertical or horizontal.

I’m very pleased with my Isobarik-type subwoofer and I have been able to use it with both the Magneplanars and now the Dynapleats. It doesn’t go much below 30Hz based on my in-room measurements, but provides a really clean and solid foundation for all types of music.
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Old 9th February 2009, 11:11 AM   #17
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Ok, I have one piece only and I don`t know what to do with it.
Some measurements under raw conditions (click to enlarge):

Click the image to open in full size.

Click the image to open in full size.

regards

miro



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Old 10th February 2009, 08:06 PM   #18
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Default A few more questions

Hi there,

About matching your Dynapleat elements to a sub.. Do you use a "high pass" filter to remove work from the Dynas? I imagine that by doing so, one would relieve lots of stress from these drivers and allow extra dynamics within their work-able range.
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Old 10th February 2009, 11:18 PM   #19
des1a is offline des1a  United States
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I strongly recommend that you don't use a high-pass filter on the low end. They are full-range speakers with unusually high mechanical damping (Pike measured a mechanical Q of 0.8 -- overdamped). If you mount them as I did (no opening in the panel at the back), I believe the perforated board and the second coupled driver introduce additional control. I have never heard any evidence of distress at the low end. I was listening at length to them today and was pleasantly surprised (all over again) at the transparency and detail throughout the range they cover. They have just the right amount of warmth that makes a singer sound like he or she is in the room. I don't mean to overstate the case, but I think you'll be very pleased.

By the way, it occurred to me that there is a good analogy as to why the perforations don't contribute to the low frequency dipole cancellation. It is the same as the small perforations in the window screen of a microwave oven -- the holes are very small compared to the RF wavelength and the screen "appears" to be opaque to the microwaves. In the case of the dipole radiator, the overall size of the panel is the important thing.
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Old 10th February 2009, 11:35 PM   #20
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Default Dynapleats

Since perforated masonite is inexpensive, it should be quite easy to try "perforated", plain, or with a hole behind the driver to let them breath and see what happens, as well as having drivers back to back, or all at the front. Sounds like a fun project anyway.
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