Yes, the second option would be good. I myself made the magnets extend all the way to the end of the pole plate. But experiments can be useful. In my opinion, the most important thing with the Rubanoid is to prevent reflections at the end of the membrane. This hasn't been discussed enough.
out of interest, was the logic of extending the magnets across the entire pole-plate so you could squeeze in more magnets and therefore a stronger magnetic field, or is there a relationship between the plate and the magnets that means keeping them consistent gives you the best use of your magnets
thinking as i type, assuming the magnets are all the same size and strength, then 10 x 25mm magnets in 250mm plates must be a more concentrated mag field than the same magnets in a 500mm plate, for example, right?
Sorry if that seems a simplistic statement to the guys who understand magnetism - but assuming its true, its definitely one thing to add into the mix , and from what i see in previous postings, its not something many others are doing
Awesome - only 1 day into this thread, and i am already having new perspectives and options to try out 🙂
thinking as i type, assuming the magnets are all the same size and strength, then 10 x 25mm magnets in 250mm plates must be a more concentrated mag field than the same magnets in a 500mm plate, for example, right?
Sorry if that seems a simplistic statement to the guys who understand magnetism - but assuming its true, its definitely one thing to add into the mix , and from what i see in previous postings, its not something many others are doing
Awesome - only 1 day into this thread, and i am already having new perspectives and options to try out 🙂
? well the coil should be as small as possible compared tot the magnet stack (slightly longer) (so use as litle as metal compared to the length of the magnets and let the coil peep out just above the magnets). ... clearly else halve the coil would be in a weaker field if they would be 30 cm and the magnets only 20cm.
What i thought when i read the post was, a smaller ruba sounds better in the top end then a longer one, and there is also something to say about. since if they are long they act like a line source. and a 30cm line source... does not sound nice. also the top end does not add upp after 6-10cm everything ebyond 10 cm length adds more and more to the lower end. could be nice but could also make it heavy on everything under 3 khz etc
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What i thought when i read the post was, a smaller ruba sounds better in the top end then a longer one, and there is also something to say about. since if they are long they act like a line source. and a 30cm line source... does not sound nice. also the top end does not add upp after 6-10cm everything ebyond 10 cm length adds more and more to the lower end. could be nice but could also make it heavy on everything under 3 khz etc
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Hi WrineX - very excited to be discussing this with you. there are 2 main points you raise
On the first, you are certainly supporting to the view that the coil needs to be HIGHER than the magnet stack, and I assume this is for the earlier stated reason that the curve of the coil is outside the mag field, and yes, it follows that the height should be as little as possible outside the magnet stack - except that the measures i took kinda support the opposite, which was the best measures were from a 140 coil (including curves) within a 275 magnet stack, so SHORTER - the one i did where the coil was higher than the magnets (but still shorter than the pole plates) gave the poorest measures of them all, . I can only think that other factors were influencing or masking this, possibly including your line source comment
A little aside at this point - just after covid i did a 3-month consultancy with a Belgian company who made drive units for upmarket cars - they were having some inconsistency with the end-of-line tests, and i laid out a series of design experiments and analysis to pinpoint possible root-causes and the impact of each factor on the final results - the point of sharing is to let you folks know that I am very much aware of interactions between different factors combining in surprisingly different ways, and my intent with this rubenoide project is to treat it the same as i would a professional contract and run, as much as possible, design experiments to understand the correlation and regression across all the factors, and hopefully use it to optimise the design
which is a nice way 🙂 of saying: just because my measurements (so far) don't quite align with what you and Ruba1 are saying is not any indication that your advice or wrong, but more a reflection of what i regularly see in my work - an initial result which deals with only a single factor can often provide results which are misleading - if a result does not align with the logical expected result, its a mistake to immediately conclude that this disproves the logical expected result - its more likely that an important factor was excluded from the experiment
ok - back on your post again, yes, the line source is a good point- my original sample was smaller and i thought sounded better in some ways (but not all) than the current larger ones. the current design (i promise to share more info soon) is actually a 2-way, with the intention that the bottom "unit" is larger and the upper is smaller - as my current poles are 360mm this is a little tricky to mock up so will need some shorter steel, but its great that you are sharing some thoughts around an optimum size of 6-10cm
my trouble is that my brain is a box of frogs on things like this, and i have too many ideas at the same time - for example, i really dont like the idea of elastics to centre, for example, and i am thinking of having a central "spine" where the coil sits, made of thin carbon of similar, and to have it "resting" between 2 rows of adjustable ceramic ball bearings, at the top and the bottom, so the coil can be is perfectly centered and the movement of the membrane is controlled to only be forwards/backwards
i know - probably a crazy idea, but sounds like a fun side-project, right?
thanks again
On the first, you are certainly supporting to the view that the coil needs to be HIGHER than the magnet stack, and I assume this is for the earlier stated reason that the curve of the coil is outside the mag field, and yes, it follows that the height should be as little as possible outside the magnet stack - except that the measures i took kinda support the opposite, which was the best measures were from a 140 coil (including curves) within a 275 magnet stack, so SHORTER - the one i did where the coil was higher than the magnets (but still shorter than the pole plates) gave the poorest measures of them all, . I can only think that other factors were influencing or masking this, possibly including your line source comment
A little aside at this point - just after covid i did a 3-month consultancy with a Belgian company who made drive units for upmarket cars - they were having some inconsistency with the end-of-line tests, and i laid out a series of design experiments and analysis to pinpoint possible root-causes and the impact of each factor on the final results - the point of sharing is to let you folks know that I am very much aware of interactions between different factors combining in surprisingly different ways, and my intent with this rubenoide project is to treat it the same as i would a professional contract and run, as much as possible, design experiments to understand the correlation and regression across all the factors, and hopefully use it to optimise the design
which is a nice way 🙂 of saying: just because my measurements (so far) don't quite align with what you and Ruba1 are saying is not any indication that your advice or wrong, but more a reflection of what i regularly see in my work - an initial result which deals with only a single factor can often provide results which are misleading - if a result does not align with the logical expected result, its a mistake to immediately conclude that this disproves the logical expected result - its more likely that an important factor was excluded from the experiment
ok - back on your post again, yes, the line source is a good point- my original sample was smaller and i thought sounded better in some ways (but not all) than the current larger ones. the current design (i promise to share more info soon) is actually a 2-way, with the intention that the bottom "unit" is larger and the upper is smaller - as my current poles are 360mm this is a little tricky to mock up so will need some shorter steel, but its great that you are sharing some thoughts around an optimum size of 6-10cm
my trouble is that my brain is a box of frogs on things like this, and i have too many ideas at the same time - for example, i really dont like the idea of elastics to centre, for example, and i am thinking of having a central "spine" where the coil sits, made of thin carbon of similar, and to have it "resting" between 2 rows of adjustable ceramic ball bearings, at the top and the bottom, so the coil can be is perfectly centered and the movement of the membrane is controlled to only be forwards/backwards
i know - probably a crazy idea, but sounds like a fun side-project, right?
thanks again
is the impendance the same ? if you used the same coil but longer/smaller it would change 🙂 also its hard to use femm for instance to see exactly what happens in the middle. it might be a tiny bit stronger then using all of the field in combination with lower impedance. (at the ends there is more steel less magnets, i mean the metal extends further then the magnets) due to the way these motors are contructed. when sing fem you cant model those 🙁 sicne it is 2D
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Yeh, impedance was around the same level - its not identical across a the different sizes, but i calculated the turns needed at each size to result in similar resistance - the 140mmm one had a couple of turns too many and was the highest at 8.7ohm (it was calculated at 40 turns, and i may have miscounted.... ) , but all the others were 8.0 or higher
I am certainly not ruling out that the differences were influenced by the resistance
and yes, i was kinda wondering if the field is constant or concentrates out from the centre - that might also influence why a smaller membrane of 6-10 is preferred for high frequencies?
as I said above, i suspect there are many factors interacting, and for sure the impedance could be one of them - after easter i can create a series of membranes at 140mm with different ohm and see what effect it has, beyond sensitivity - but i would like to spend a little effort in doing a proper Design of Experiment (DoE), working out the factors i want to test and running different combinations of them together, as that is the only way that the interactions can be spotted - unfortunately i no longer have the statistical software thar helps with this, so i will need to calculate it myself, so need a nice clear day
I am house and dog-sitting for my old mentor over easter, and he still has the software, so may be able to get some access to help me do the calc.......
I am certainly not ruling out that the differences were influenced by the resistance
and yes, i was kinda wondering if the field is constant or concentrates out from the centre - that might also influence why a smaller membrane of 6-10 is preferred for high frequencies?
as I said above, i suspect there are many factors interacting, and for sure the impedance could be one of them - after easter i can create a series of membranes at 140mm with different ohm and see what effect it has, beyond sensitivity - but i would like to spend a little effort in doing a proper Design of Experiment (DoE), working out the factors i want to test and running different combinations of them together, as that is the only way that the interactions can be spotted - unfortunately i no longer have the statistical software thar helps with this, so i will need to calculate it myself, so need a nice clear day
I am house and dog-sitting for my old mentor over easter, and he still has the software, so may be able to get some access to help me do the calc.......
Hey all — just wanted to share a bit of what I’ve been working on recently, inspired by a lot of the insight from this thread.
I’ve been developing a Python-based calculator specifically for racetrack planar voice coils. It was built to address the complexity of designing with enameled wire, where factors like geometry, wire gauge, material, and target resistance are all tightly linked — and not easy to solve manually.
The tool lets me input things like:
It then cycles through wire gauges (using a resistivity reference CSV), checks how many turns can physically fit, and calculates total length, resistance, and estimated stack height. It also handles multi-coil setups (e.g. 2, 4, or 8 coils) with both series and parallel options, depending on the resistance goal.
Now I know WrineX doesn’t use wire at all — he etches his voice coils, which I fully respect. But I personally prefer using wire, since it gives me more freedom to tune the resistance and try different materials. That’s especially useful when I’m doing iterative builds or prototyping.
It's funny that finally people are starting to realise that the the VC shouldn't be 100% within the magnetic motor, I think this was something I pointed out privately some time ago. I became apparent when modelling COMSOL:
That’s one of those things that’s easy to overlook in basic field plots, but very obvious once you get into 2D/3D FEM simulation.
Anyway, if anyone is interested in the Python tool or wants to adapt it for etched coil designs instead of wound wire, I’m happy to share. Always appreciate the depth of this thread — especially WrineX’s contributions on field shaping and geometry logic.
I’ve been developing a Python-based calculator specifically for racetrack planar voice coils. It was built to address the complexity of designing with enameled wire, where factors like geometry, wire gauge, material, and target resistance are all tightly linked — and not easy to solve manually.
The tool lets me input things like:
- Slot length
- Inner diameter
- Voice coil height
- Target resistance
- Material type (Copper, CCA, Silver, etc.)
It then cycles through wire gauges (using a resistivity reference CSV), checks how many turns can physically fit, and calculates total length, resistance, and estimated stack height. It also handles multi-coil setups (e.g. 2, 4, or 8 coils) with both series and parallel options, depending on the resistance goal.
Now I know WrineX doesn’t use wire at all — he etches his voice coils, which I fully respect. But I personally prefer using wire, since it gives me more freedom to tune the resistance and try different materials. That’s especially useful when I’m doing iterative builds or prototyping.
It's funny that finally people are starting to realise that the the VC shouldn't be 100% within the magnetic motor, I think this was something I pointed out privately some time ago. I became apparent when modelling COMSOL:
That’s one of those things that’s easy to overlook in basic field plots, but very obvious once you get into 2D/3D FEM simulation.
Anyway, if anyone is interested in the Python tool or wants to adapt it for etched coil designs instead of wound wire, I’m happy to share. Always appreciate the depth of this thread — especially WrineX’s contributions on field shaping and geometry logic.
heeej that i aint fair 🙂
hehe well i always advocate for wires if you want to get excursion/efficiency. the only thing why foil might be nice is top end, it works better up high, easier to cross. but i did a test wire vs foil, and foil wins a little in distortion. reason is . wire to wire distance is to big compared to wider foil to foil. i myself would say using 10 wires in the magnet gap will rock 3 traces of foil in terms of lows/drive force. there is a video where i try to laydown nice coils (called pancake coils). but i cant make the angles i need for smaller magnets (like in a planar) i am sure i can use the same thing as how wire esl are made. to get more density . but still it wont get near the density as my pancake coil method 🙁 if ur interested to look both for something that could ? please let me know (i would love to work together to try make something that might work). since i am thinking about this for years. for lows i think adding as much coil in the best field makes a huge difference !! and could be king !
WIRE VS FOIL same panel
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I'm playing around with flat CCA flat wire, however I'm thinking of going big... 1000/1200mm in length 😱 I'm at the moment modelling impedance vs inductance trying to work out what the sweat spot is, as well the ratio of MMS to the HF range.
As you said -
Low inductance is your friend!
I will ask you to jump in with a project I am working on, however at the moment I have to wait for a vender to produce the diaphragm to my specifications, (which requires tooling to be made).
..and thanks to Sparrowlegs for the update on what you've been up to.... 👍
Me? Still awash with Walsh tweaks.... 🙂
Me? Still awash with Walsh tweaks.... 🙂