Category Archives: Main La Folia Site

The Woofer – The Membrane Conductor Design

This is the fun part and also the part that requires most patience and dexterous hands, but using the recommendations in this La Folia Project the results can be easily attained.

The woofer conductor design must fulfill the general layout as shown in the diagrams.

This explains also shows you how and why the  speaker actually works. See how the conductors between each magnet polarity have the current flowing in the same direction?

You then use the general three finger rule and figure out the force exerted  on the membrane, change the polarity and the force is 180 degrees shifted, thus the speaker membrane will move in and out as the current changes flow, as would be expected…

More on that at a later moment…

And the tweeter design?

Check out the crossover design

The Loudspeaker Frame (MDF or plywood)

The ribbon loudspeaker dimensions

Figure 1

The wood I used was a MDF fibreboard, 30 mm. thick, a great width which gives a very good resistence to vibration and a very sturdy heavy frame for the membranes (together with the perforated metal plate), helping considerably to make the bass frequencies all the more attractive, something that can be considered the soft point of the project.

That’s is, at least, the theory, I now think it’s overkill and a thinner wood frame is the way to go.

Having such a thick piece of wood, makes the cutting a real adventure, MDF boards create an incredible amount of wood dust, so beware!

It also creates a problem not considered in the original project (they were smarter), how do you get the magnets close enough to the membrane?

It is much heavier than the original project frame, the loudspeaker foot has to be much stronger than they recommended, obviously!

I think it is best to have the wood cut by an expert, here in Chile that is very inexpensive, that may not be the case in your country.

If you plan to paint your loudspeakers do it after it’s been cut and before you start doing anything else.

How to cut the MDF plate for the speakers

Figure 2

Here are the original Danish project dimensions:

a=30 cm
b=22 cm
c=95 cm
d=4,5 cm
e=95 cm
f=14,5 cm
g=11,5 cm
h=75 cm
i=130 cm

If you want a bigger loudspeaker, use the following proportion relationship:

4 : 3 : 13

This also changes other dimensions, so be careful and check everything out before doing anything irreversible to your material.

The list of materials

The construction recipe

Contact: Magnetostatic Loudspeakers

LaFolia Magnetostatic Ribbon Loudspeaker Frequently Asked Questions (FAQs)

What are the membrane-conductor material dimensions?

  • Membrane material: Mylar Film 12 micrometers (0.012 mm)
  • Alu-foil: aprox. 12 micrometers (0.012 mm)
  • Total width: 0.024mm

Did you use a different material for the mid-tweeter membrane?

I used the same combination on both the woofer and mid+tweeter.

Do you corrugate the bass membrane?

By using the thin aluminium material I had no reason to worry about rattling which is the reason why there was no need to corrugate in any way. Thicker aluminium foil may have problems but I can not be sure.

I have found another material than mylar for the membrane base, what should I take care about?

Do not forget to test the base plastic (mylar, kapton, kevlar, etc) foil for humidity variations in the tension. Take a piece, extend it over a cup or anything like that, stretch it and then check it when you go into a sauna (or some place humid) with it. If the tension is maintained its alright, if not change the material.

Can’t find the right aluminium foil, not wide enough…

Have you tried going to the packaging companies in the US. I went to a chocolate – candy factory and got a meter and a half wide roll of chocolate alum on waxpaper material.

Based on what you say on your site, the Woofer ribbon is probably the most tricky one to make, why?

More than tricky it requires a lot of work and is a very delicate job if something goes wrong you can destroy a lot of work…

How are Apogee- Magnepan – Martin Logan – Dali – etc. loudspeaker compared to yours? Have you ever heard Apogee- Magnepan – Martin Logan – Dali – etc speakers before?

Just indirectly, never been in front of a pair of factory built magnetostatic nor electrostatic loudspeakers. I left Denmark a year before even trying LaFolia and in Chile there are not enough hifi freaks to justify anybody bringing Apogee speakers to the country, now I’m in Mexico but I would have to travel to the capital or the US, one day I may sit in front of a pair and give my opinion. 🙂 (Update: That did not happen, though I did enjoy a pair of Martin Logan’s – not the same thing though).

How can you tell on a magnet which is North pole and which is South pole?

Look for the polar bears! That’s the North pole! 🙂 hehehe… well I don’t know the exact way (a magnetometer?), just define one pole as North and the other as South and go on based on that asumption, it really doesn’t change anything. In my case I took one magnet wrote N on one side and S on the other and I used that reference magnet to coordinate all the others.

How did you corrugate the mid-tweeter membrane?

I used a lego plate and a stick. Others have created weird high-tech machines to do this job, I don’t have the money for that 🙂

Have you had any experience using electrical coils instead of magnets?

Nope…

Useful resources on Ribbon Loudspeakers

Analysis Audio, La Folia taken to a higher level – review of the Epsilon

Dahlberg Audio Design (Swedish)

DIY Radial Loudspeaker Project, by Steve Deckert

HiFiHeaven, good starting point for all DIY’ers

How to make an electrostatic loudspeaker

Illustrated Guide to Loudspeaker Health Care

Magnepan Loudspeakers

Magnetostatic speakers (Wikipedia)

Magnetostatics, some options

Paper thin magnetostatic loudspeaker (Youtube)

Silverline LaFolia Speaker

Contact: Magnetostatic Loudspeakers

Obviously I can’t assure you all links work, if links are dead please  inform me

The Aluminium L-Profiles (plate support)

You yourself will have to figure out the L-Profiles you’ll need and their specs.

Remember to include the aluminium profile’s width when you are figuring out the hole for your perforated plate.

Also take into account that this is the solution I found to the design problem I encountered when I chose to use a very thick wood plate as the frame for the loudspeaker.

Your requirements may be completely different, this is part of the fun. You design and adapt the project to your conditions.

I would now recommend going for a thinner wood plate, more like the original design. The air moved by the thin membrane and its own intrinsic weight does not justify using such a thick piece of wood, plus the plate itself adds to the weight and sturdiness of the frame that supports the membranes.

Once you have the magnets and membrane, choose the wood thickness so you can sandwich the magnets between the membrane and iron plate directly on the wooden frame.

Contact: Magnetostatic Loudspeakers

The Magnets

Magnet polarity

Figure 1

This, I feel, is the hardest part of the project (ON YOUR POCKET :-)), invest a large amount of money on a bunch of ceramic rectangles, for a project you still aren’t completely
sure will work…

On top of it all, the project gives you very little help with respect to the real life specifications they need to have. I had the “luck” that a Physics Instrumentalist friend of mine casually had 500 magnets of more or less the specs I needed lying around unused… after having callibrated some special astronomical CCD camera and was willing to give them to me for free…!

That they were a bit weak in magnetic force was no problem for me. I just planned to put the membrane closer to the magnets, 2mm instead of the project’s original 5 mm distance. I had two different types of magnets, one type was plastic, sort of the type you use for your refrigerator, 5.0 x 1.2 x 0.6 cm, of those I used 38 per loudspeaker (76 in total) for the tweeter section. And for the woofer section I used aprox. 150 magnets per speaker, of a ceramic type, 2.5 x 1.9 x 0.4 cm, of a much lower magnetic strength than the others. I recommend you find long and thin rectangular magnets, that follow the diagrammes specs with respect to the plane of magnetization (Fig. 1).

Figure 2

I have not been able to find magnet distributors in Chile…. but I have not tried too hard either. I hope you do find them where you live. The original project recommends rare earth magnets due to their strength and due to the fact that their magnetic fields are not strongly affected by the presence of other magnets close by. I will in the near future include more info on this subject.

Fig. 2 is a diagramme showing how to place the magnets on the perforated metal plate, notice the fact that the first row on one plate is the contrary of the other plate, this relates to the tweeter magnets too.

A diagram showing the magnets with respect to the membrane conductors

A list of materials needed.

The construction recipe

Contact: Magnetostatic Loudspeakers

Construction materials for the loudspeakers

Tools:

  • Hammer
  • Screwdrivers
  • Drill
  • Ruler
  • Cardboard Knives
  • Multimeter
  • Pliers

Materials:

Crossover:

  • Condensator 105 microF.
  • Coil, 0.95 mH.

Optional:

  • Circular Saw
  • Oscilloscope
  • Tone Generator
  • Magnetic Field Meter
  • High Power Amplifier

Contact: Magnetostatic Loudspeakers

The Ribbon Speaker Membranes

woofer-bThis is the crucial part of the project, the “bass section” and “tweeter section’s membranes. I put them between quotation marks because they are neither, the cutting frequency is 1000 Hz (or is it 500 Hz…?), and the filter is of a very low slope, so they overlap a considerable amount with respect to the frequencies.

The membrane is at the moment the most time requiring part of the project, as it is designed. I will give, at the end, a series of possibilities as to how this could be shortened. It is
also the part that requires most care, patience and “abilities”, but these will develop during the building and design.

serpentiThe membrane consists of three parts, aluminium foil, a plastic support membrane and adhesive glue, that binds the two.

The plastic support in the original project was an oven polyester bag slit along the edges, giving you a strong, very much, non-stretchable film at a very low cost. The negative sides were the limited size, the crease at the middle, that can even have a discontinuity cut and the relative thickness of the film (and some oven bags are humidity sensitive to my horror!).

I have chosen Mylar Film, something I was not able to find in Chile, but as I casually was in Buenos Aires, Argentina, I bought in the most unlikely shop, in a very adequate width of 12 micrometers (micra), 1 m. width and 15 m length, material for a big amount of loudspeakers… Some builders do not get to stuck up on the width or weight of the materials used on the membrane, with the argument that “the weight of the air moved by the membrane is so high that a few grams more or less on the membrane will be of no effect, I have no way of supporting or counterargument this, I chose the lighter and thinner materials, that give you a very flexible and non rattling membrane, a problem that arises when you have thick aluminium and a thick polyester material.

Here in Chile, and after I ripped the second membrane I was making (!!!%$$~@, booo hoooo), I was forced to find some more membrane material for the first full size prototype, so I looked
hard and found what can be a very good solution for those that can’t find Mylar film, stickers…. Stickers, in Chile at least, are mostly made of Mylar film with adhesive gum on the backside.

I found they sold the prime material for stickers in 1 x 1 meter sheets, just the right side for the project’s loudspeaker membrane, and then you need not worry about the sticking of the aluminium, the mylar already comes with it.

What about the aluminium foil on the polyester membrane? How do we get that and how do we stick it on? What width must it have? How do we cut it?, etc.

I’ll start with the glue. In the original project the aluminium foil they use is a foil used as thief alarm conductors for windows, a 30 micra wide foil, that included glue on it’s backside.

In my design, I was required to stick the aluminium foil on the Mylar, as some other designs do too. I recommend you use a 3M, Super 77 Spray adhesive, this permits a thin glue layer that is strong and durable. You can use other glues but remember to dilute them well, so you can spread them on easily, and thinly.

With respect to the foil, some have chosen to use aluminium foil, cut in long bands. I have chosen an easier route, I have used a special paper that includes a thin aluminium foil on waxed paper, here in Chile, this paper is used to pack chocolates here in Chile…. I do not know if this is universal… This paper has the characteristic that the foil comes off the paper if you
heat it and liquify the wax. Thus you can leave a very thin foil (aprox. 12 micra) on the polyester and eliminate the heavy paper.

How to place the membrane on the frame

This bi-layer permits an easier cutting of the membrane design. You cut the design directly on the foil, avoiding cutting through the base paper, thus the design is held together by it.

What about the cutting, you have to leave a 1 mm. space between each conductor! You may then say, “How are we to cut so precisely!”

Easy: You use a double sharp bladed “knife”, we call them in Chile, cardboard knives, sharp blades, wide and easy to handle, you put two together with a coin between, or something 1 mm. wide, scotch them together and start cutting! With care, obviously, but deep enough to cut well through the foil, once you do so it is easy to pull off the thin 1 mm. wide aluminum foil string from the waxed paper, you must be careful doing this, if the foil isn’t completely cut you may pull a piece off the conductor too, slowly, slowly, that’s the way.

In my opinion, this is the easiest way.

Specs:

  • Membrane material: Mylar Film 12 micrometers (0.012 mm)
  • Alu-foil: approx. 12 micrometers (0.012 mm) Total width: 0.024mm
  • Membrane is not corrugated, just stretched out as is.