- MDF Frame (or Plywood)
- Iron Plate
- Ribbon Speaker Membranes
- Aluminium L-Profiles
First of all, get a hold on all the material necessary to begin. You’ll need new material along the way. Depending on the design you choose and the material ($) at your disposition, you’ll need more or less stuff and one or another type.
I will describe my development process to build the speakers:
Here you use any magnet that has the characteristic of being “anisotropic”, this means that it’s magnetized through it’s largest dimension, these can be either ceramic ferrite/strontium magnets. The strontium ones are stronger and are better considering the fact that they can be set closer to one another without affecting their magnetic field as much as with other types. The Danish project used 102 magnets (42mm x 7,7mm x 9mm) (length, width, thickness) for the woofer and 60 magnets (50mm x 19 x 5mm) for the tweeter section, thus twice the amount for a pair of speakers.
Considering the fact that magnets are cheaper if bought in large quantities, you can consider buying them together with other builders.
2- I started experimenting with common household material, kitchen plastic foil, chocolate paper as foil and cardboard as a frame, then later wood as the frame material and bigger membranes.
3- Once the principle and concept was proven to work, I started getting the material for the full scale prototype.
4- Found the best place to buy wood, if you can buy the pieces cut out while you purchase, all the better, if they can also make the inner cuts, fantastic. Remember to get a piece cut to use as a transitory foot for your loudspeaker, in my case it’s the definitive foot. It is here you make your first decisions, will you build the frame out of one solid piece, or a series of thinner plates stuck together, maybe mixing wood with some other stronger material.
5- At this moment you’ll have to decide if you want to paint the design, I didn’t with the prototype.
6- Go searching for the perforated iron plate and decide what thickness you want, depending on your possibilities to cut it, your budget and the sound considerations. Try to have a well perforated plate, with at least 50% air with respect to the iron. Thus you’ll avoid making the speaker even less effective.
7- Find a place where they can cut the plate to you specifications, remember to get the plate cut to the size you made the holes in your wood plate, but even BETTER, get the plate cut before to your specified sizes and cut the wood plate correspondingly. Or even better, have the thickness of the wood plate adapt to the size of the magnets so you don’t need to adapt to the perforated plate that way the iron plate will be screwed directly on to the plate, so its size will only need to be a bit smaller than the MDF frame size.
The woofer hole should be cut based on the size of the membrane you’ll be using for the woofer part of the flat speakers, just follow the following proportions:
This gives you a trapezium form for the membrane, which helps to propagate the vibrations and eliminates standing waves on the membrane, etc.
How big the dimensions really are depends a lot on the design of your project, the width of your MDF plate, the thickness of your magnets, etc. In my project the magnets were not too wide,
the MDF plate was very thick and the distance I wanted to place the magnets, due to their low magnetic field strength, made me put the magnets 2mm from the membrane, that meant the perforated iron plate had to be cut to fit the woofer membrane hole.
The original Danish design, due to a thin plate, thick magnets and thanks to the strength of the magnets, could have the perforated plate screwed directly to the back of the MDF wood plate, this made the design easier and faster to build, less “engineering” and precision involved.
8- Find the membrane material, a strong, thin, resistent plastic, unstretchable, not affected by atmospheric conditions, humidity, etc. I recommend Mylar, you may find something else, tell me about it.
9- Find the foil you’ll use. I had a very hard time finding the chocolate foil paper here in Chile. I will not tell you where I found it, you won’t have such a hard time. In Denmark it is sold in the Supermarket. You could go to a paper store.
10- Make experiments with your foil material, you need to determine it’s resistence. Thus you’ll be able to determine the length of foil you’ll need to fulfill the 3-4 Ohm resistence the membrane must have. I will talk about that later.
11- The glue is a 3M product, there may be a number of competitors where you live, here in Chile it was my only option at that moment. Practice using it, it’s effectivity, how much should
you use, the wrist movement, etc.
12- Get the required amounts of L aluminium profiles, these you’ll use to hold the metal plate with the magnets close to the membrane,
if you chose a 30 mm MDF fibreboard, like I did, the magnets must be placed with the plate, closer to the membrane than those 30 mm, I wanted them to be 2 mm. from it, so the plate was placed approx. 20 mm into the MDF board. The L-profiles let you hold the distance constant.
The L- Profiles are also used to stretch the membrane.
Here I will now include some tips and tricks I forgot to place on the original docs here on the site. I have felt compelled to do so due to a few mails that I have received from friends around the world. Here we go:
Aluminium conductor foil
I know this has been one of the very hard parts of this project, how the h… do I cut a perfect strip of aluminium foil that, to make things harder, is very fragile and then stick it on the membrane in an easy way?
Well, this is not my own original idea but I used it very successfully while building my model. In Chile there is a special type of paper that is used for chocolates, it is a waxed paper that on one side has a very thin aluminium foil, much thinner than the foils used for food. The only real issues was where to go in Chile to buy a sheet large enough to include the complete design in one piece.
I had to travel to Santiago and then go to a packaging industry, there in the trash (they didn’t sell any of that material, no surprise), I found some 10-15 meters of the stuff, enough and even some extra material I could use for future projects. The guy there could not believe I would use it to build a loudspeaker, he gave me his address so I could send him a picture… never did… but if he has access to Internet he might find this site one day… if he cares 🙂
In Denmark this paper is used to package sandwiches (well, the Danish style sandwiches smørrebrød) and can be bought in the right width at a normal supermarket (lucky them). If you live elsewhere you must see where you can find it – good luck – but it is very much worth the effort.
Once you have it, this is what you do:
You design your membrane by drawing it on the foil, then you use two very sharp blades stuck together, separated by a small coin or equivalent, be careful to align them well. These blades you will use to cut the covering aluminium foil without cutting the underlying paper, so be careful and control the pressure as precisely as possible (don’t do this after a long night or party). This way you will obtain the design by removing, once again, very very carefully, the thin strip created by the cuts between the blades. With some sweat and patience, you will have the complete design of the membrane on supporting wax paper, with the strips much closer and perfect than you could ever do by sticking aluminium strips onto the plastic membrane.
Now you stretch the plastic membrane on a table, attach the edges with tape and spray the adhesive onto the surfaces. Stick the aluminium side on the plastic membrane – be very careful when you flatten and remove any wrinkles on the membrane, the supporting paper is humid with the spray glue liquids, when the paper is wet it can easily tear, a horrible thing that happened to me killing one membrane…
Now it’s a matter of taking off the paper without taking the aluminium off… and that’s where the wax comes in handy, you use a hairdryer in the hot setting and heat the paper until the wax melts and you are able to pull it off from the aluminium! This is delicate too, but quite a bit of fun too.
The membrane material
Now another interesting subject is the membrane material. In the first model I used a rather thick Mylar membrane my brother had for windsurf sails around 50 um. (micrometers) and later I used a sticker material with golden colour of half the width.
In the second model I used what the original design recommended, two slit oven bags. I did not test them out before and the surprise was that the speakers became gigantic hygrometers, I was able to predict bad weather by looking at my speakers, they would go limp and stretch out on sunny and dry days… 🙁 This my recommendation is that you test the membrane material’s response
to humidity before you use it. Take a piece of the material, stretch it out over a frame and go into the bathroom and turn on the hot water and see if it goes limp. If it does change the membrane material.
I have since bought a roll of 12 um. (micrometers) Mylar for the next model, Mylar is a very resistant material and considering the good results I obtained in the first model I think this will help considerably with the next model.
The adhesive glue
Here is an interesting subject that can be easily overcome by using a very good, effective and easy to use glue made by the 3M company, free advertising I am doing for them, it’s a spray glue that comes in a black can, Super 77. This is the solution, follow the instructions given and try to use it sparingly and uniformly, practice on useless surfaces before the real stuff.
Once you have designed the membrane you’ll notice there are sections that connect the different portions of the conductors that are between the magnet poles, these are not contributing to the membrane movement, and therefore do not need to be between the magnetic poles, a way to have them do no harm by adding to the membrane dead weight is to place them outside the stretched portion of the membrane, that is on the MDF plate. This way the real portion that is doing the work is suspended between the magnets and the dead weight is lowered help in the efficiency of the system, at least a bit.
OK, that seems to be all for now, if I remember anything more I will add it. Hope this can come in handy for you all.
The glue I recommend using.
Aluminium conductor foil
The foil material I recommend
An important element in my construction design
A list of materials needed linked to the corresponding pages
How to build the simple crossover needed for the project
Magnetostatic Loudspeaker frequently asked questions
Related sites and interesting links
The magnets and their characteristics
MDF Plate or the Frame
The wooden frame that’ll hold the whole thing up
The most delicate element in the design
Here you need to do your home work well
How about receiving a bit of advise, you might avoid some mistakes I did
The material I recommend you use and how to test it
Ashort account of the personal experiences in this project
My building recipe
My personal recipe to Hi-fi heaven
Original building recipe
The original Danish recipe
Perforated Iron plate
This part will hold the magnets behind the membrane
Some very useful and practical tips on the LaFolia subject
The high frecuency elements in the LaFolia speakers
The magnets are held on to the tweeter hole in the MDF frame by their own magnetic force, and the fact that you have long strips of metal plates covering the sides
of the hole (don’t you….???!!), thus the north pole of the
magnets are on one side and the other side holds the south
This is done so the top row of magnets on the woofer section of that speaker must have the same polarity as the row of magnets in the tweeter section closest to the woofer section (sounds wierd but it’s simple, don’t worry on what exactly is the N and the S polarity, just assume that one is the N and the other is the S and work based upon that assumption.
When this is done, you are thus sure
that the speakers are working in fase.The magnets are arranged as shown in the figure. In my case, as in the Danish original project, two
magnets are placed on top of each other to generate a stronger magnetic field. The distance between the magnets on the same side is obvious, because they will not stay any closer than they want to (their magnetic field wants to).
The personal dimensions involved will depend on the material you use, just make the distances such as to have a 1 mm. distance between the tweeter membrane and the magnets, on each side.
This diagramme shows the returning conductors made out of the aluminium material , I chose to use wires and make the tweeter conductors not as wide as the original project. You experiment and find out.
Contact: Magnetostatic Loudspeakers
The magnets do not float in mid-air, they are held by their own magnetic force on an iron plate, as the loudspeaker works as a dipole, emitting sound pressure in both directions, the plate has to be perforated, thus permitting the sound production and avoiding destructive interference.
The plate is cut into pieces, one big piece for the bass baffle, in my case 93 cm. in height, 29 cm in the top part and 34 cm. in the widest part, see diagramme. You also need a piece for the back and sides of the tweeter hole, thus a long 93 x 5.5 cm piece, with long pieces to cover the sides of the tweeter, also permitting the adherence of the tweeter magnets, 2 pieces of 93 by the width of the MDF plate
you buy per loudspeaker.
A thicker plate makes the construction more robust, with less propensity to vibrations, makes the loudspeaker all the more sturdy and would theoretically help to achieve a better overall sound.
The wider the plate, the tougher it is to find a place they can cut it. I
had a terrible hassle finding a pneumatic guillotine that could
cut the plate, on top of it all, the plate weighs a lot, a car is
a must, or taxis if they are affordable in your country. They normally
sell the plates as one piece, 2 x 1 meter, at least in metric countries,
you can make four back pieces and the rest of the magnet supports
with one plate, so get together and buy the plates as a pair.
I would recommend from my own experience, to give
the plate an antioxidant treatment, paint or varnish, so your loudspeaker has a nicer appearance and lasts longer. It is also good to get the edges polished after they are cut by the guillotine, they can be very sharp and dangerous to work with, if they are not dulled
|Contact: Magnetostatic Loudspeakers|
which she promptly did, thank you Helle.
|Contact: Magnetostatic Loudspeakers|
In its original design, the loudspeakers are very
ineffective (70-72 dB SPL).
The most complicated part of the whole project is to get the materials and also depends on how much time you use experimenting with magnets,
the membranes, the MDF
wood frame, the membrane design, etc.
The basic idea behind the project is a concept used by other very famous speakers in the Hi-Fi World: Stratshearn, Fostex,
Magnepan & mostly Apogee. It is a simplified version, in many aspects, of Apogee’s two-way speakers. Carver together with a friend of the original Danish writer of the project, Ole Thofte, have produced a speaker with the same principles as in the “La Folia” project.
Step by Step
1- Cut the wood pieces, one the mirror image of the other.
2- Resonance damping of the woofer hole of the MDF plates, just the top and bottom side of the hole, don’t choose a very thick sponge, it’ll be very hard to stretch the membrane if its to thick, and wrinkles will be an unavoidable part of your design, not so nice…
Cut the tweeter hole taking into consideration the magnet’s dimensions and the metal plates that cover the inner side of the hole, and so on.
3- Prepare the metal plate design, cut a piece either to fit the woofer hole or to be screwed directly to the back of your MDF Plate, you find out what is best for your particular case. The tweeter hole needs a piece on the back side too.
4- Put the magnets on in the desired design, all with due consideration to the conductor design you’ve chosen, the length of conductor you need, the resistance, the impedance (something I never considered, I just measured the Ohm resistance, and calculated it to be 3 ohm). If you’re not as unlucky as I to have very weak magnets, there should be no need to stick them on to the plate, their magnetic strength should be more than enough.
5- Design and create the membranes, more about that in the membrane section, and the possible improvements of the design. SWEAT, SWEAT, this is the tough part, so I feel…
6- The row of magnets in the tweeter section closest to the woofer must have the same polarity as the top row of magnets in the woofer section (important!!). The tweeter magnets are double, boosting
the magnetic field strength.
In the case of the woofer conductor, it’s necessary to have an unbroken conductor, that covers the membrane, this way it moves as a single flat surface.
This design has two conductor strips, placed in series. The way the conductors transmits the current is always in the same direction and this creates a force that is either inward or outward, depending on the current direction with respect to the magnetic field. (In my project I used approx. 68 windings of the conductor). The total length of the aluminium conductor in the original project was 18 meters x 9 mm. (30 micrometers in width) and had a 3 ohm resistance.
The tweeter conductor was originally built with four conductor coils, this makes the resistance larger, the conductor is 9 meters long and half the width, 4.5 mm, thus 3 ohm resistance.
The membrane should be corrugated, (now that I think about it I don’t know why…) thus the length shrinks aprox. 10%. Corrugating it will certainly avoid rattling and it tends to move too when the woofer is reproducing lots of bass.
8- Crossover and speaker connections: The woofer and tweeter are connected in parallel. Serially connected to the woofer you put a coil (0.95 mH), and serially connected to the tweeter a capacitor, 105 microFarad. The speakers impedance is approx. 3-4 ohms, very frequency linear.
9- The speaker feet, build them either out of the same wood, held by strong shelve angles or build a more sophisticated design.
10- Check if things are alright. Check out if there should be any short circuits with the multimeter, check if there are any missing connections too. the resistance measured at the speaker connections to the amplifier should be approx. 3,5 ohm. Now
connect the amp… See what can happen, if all is well
I assure you you’ll have a hard time erasing the wide and shiny
smile you’ll have on your face, once you’ve constructed such a beauty out of such lowly materials, you’ll need to do a number of adjustments to the sound balance, I had no spectrum analyzer, I just used my ears, the best instruments I have at my disposition, adjust the balance between the woofer and the tweeter by putting more or less resistance on the tweeter, this will crave either more or less current from the amp, thus having more or less treble, balancing the sound image accordingly, I know this is quite makeshift, but so is the
whole loudspeaker project!! :-).
11- HAVE LOTS OF FUN, I DID.
This 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.
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 (!!!%[email protected], 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.
- 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.
- Cardboard Knives
- MDF Fibreboard
- Perforated iron plate
- Perforated iron strips
- Mylar or Polyester Membrane
- Aluminium foil
- Spray glue (3M recommended)
- Epoxy glue
- Aluminium “L” profiles
- Isolated conductor wires
- Screws (wood screws of different sizes, lengths, etc.)
- Screws and bolts. – Cables. – Tin solder (is this the name…?)
- Long wood pieces (100 x 3 cm)
- Sponge, 1 cm width, medium density
- Condensator 105 microF.
- Coil, 0.95 mH.
- Circular Saw
- Tone Generator
- Magnetic Field Meter
- High Power Amplifier
Contact: Magnetostatic Loudspeakers