I thought it could be worth to try to optimize the distances between the magnets, both over the membrane gap and between the magnets on either side.
Membrane gap 5 mm along the membrane (tangential flux density B,t) and the peripendular to the membrane (normal flux density B,n).
The latter only plotted 2 mm, as much as the membrane is suppose to travel:
So a 3 mm gap gives too much variation. 4 and 5 mm is more or less the same apart from a lower flux density for 5 mm.
Real life tests will show the clearance I need; 4 or 5 mm.
Different spacing between the magnets, 4 to 11 mm:
4 and 5 mm looks good, 6 mm and above not so good.
If the foil is placed 0.5 mm in on 4 or 5 mm it looks really good.
5 mm will give a wider membrane in total so 5 mm it is.
Membrane gap 5 mm along the membrane (tangential flux density B,t) and the peripendular to the membrane (normal flux density B,n).
The latter only plotted 2 mm, as much as the membrane is suppose to travel:
So a 3 mm gap gives too much variation. 4 and 5 mm is more or less the same apart from a lower flux density for 5 mm.
Real life tests will show the clearance I need; 4 or 5 mm.
Different spacing between the magnets, 4 to 11 mm:
4 and 5 mm looks good, 6 mm and above not so good.
If the foil is placed 0.5 mm in on 4 or 5 mm it looks really good.
5 mm will give a wider membrane in total so 5 mm it is.
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Nice one !! i would go for 4 or 5 mm then 
another option if you do not need insane output is adjusting the trace width on the edge of the magnets to a wider trace... (so you could use a wider distance between magnets, if you wanted) i know a can of worms... but you could mitigate some of the spikes in output of the magnets near there edge of the magnets. then it does not have to be completely flat. but will make the design more complicated
another option if you do not need insane output is adjusting the trace width on the edge of the magnets to a wider trace... (so you could use a wider distance between magnets, if you wanted) i know a can of worms... but you could mitigate some of the spikes in output of the magnets near there edge of the magnets. then it does not have to be completely flat. but will make the design more complicated
WrineX triggered me to do some more simulations; how does the magnetic field density vary over 5 mm spacing between magnets vs 10 mm spacing?
So I plotted the magnetic field density from flat to 1,00 mm.
5 mm:
10 mm:
The vertical scale is the same for both chart, 0,15 T to 0,5 T.
For the 5 mm, the magnetic field density increases compaired to flat for the first millimeter on both sides and it is fairly constant in between.
But only 3 mm can then be used.
For the 10 mm, magnetic field density increases compaired to flat for the first millimeter on both sides and decreases inbetween.
Only 8 mm can then be used only if the width of the aluminium strips varies.
So which is most effective?
Looking at the integral of the two alternatives, we have for the 5 mm spacing 3 mm with a medium magnetic field density of 0,42 T and
for the 10 mm spacing 8 mm with a medium magnetic field density of 0,25 T.
So that's 1,26 Tm for 5 mm spacing and 2,0 Tm for the 10 mm spacing.
But can the aluminium strips for the 10 mm spacing compensate for the varying magnetic field density between the magnets.
What's your opinion?
So I plotted the magnetic field density from flat to 1,00 mm.
5 mm:
10 mm:
The vertical scale is the same for both chart, 0,15 T to 0,5 T.
For the 5 mm, the magnetic field density increases compaired to flat for the first millimeter on both sides and it is fairly constant in between.
But only 3 mm can then be used.
For the 10 mm, magnetic field density increases compaired to flat for the first millimeter on both sides and decreases inbetween.
Only 8 mm can then be used only if the width of the aluminium strips varies.
So which is most effective?
Looking at the integral of the two alternatives, we have for the 5 mm spacing 3 mm with a medium magnetic field density of 0,42 T and
for the 10 mm spacing 8 mm with a medium magnetic field density of 0,25 T.
So that's 1,26 Tm for 5 mm spacing and 2,0 Tm for the 10 mm spacing.
But can the aluminium strips for the 10 mm spacing compensate for the varying magnetic field density between the magnets.
What's your opinion?
hmm yeah thats the major thing, how much decrease in magnetic field from the coil when they are lets say twice as wide. i actually dont know.
most efficient will remain smaller gap less coil, im pretty sure. but i guess it will be a trade off the 10 mm gap has more coverage ? maybe more evenly driven (more coil vs blank mylar) but less intense over the foil.
by the way i myself always do use that hump on the edge of the magnet (should make the trace wider there). what i noticed with my tweeters is having coil almost everywhere on the foil even if that means some are in a less ideal fields gives a smoother response then purely put them in the flatest/intense field only. so my tweeters waste good coil on less then ideal magnetic field. i even only leave like 1 1.5 mm on top of the magnet free from any coil (aligning the membrane to the magnets does get harder, if you go over the middle of the magnet/misalignment, the magnetic field swaps and a part is playing out of phase then).
Most have a much wider part without any aluminium on the center of the magnet where the field flips, i had tweeters where that made a difference in the upper range (maybe because there is a part lagging behind the driven parts bit like breakup) maybe having more tension on the foil could have fixed that to ,not sure.
most efficient will remain smaller gap less coil, im pretty sure. but i guess it will be a trade off the 10 mm gap has more coverage ? maybe more evenly driven (more coil vs blank mylar) but less intense over the foil.
by the way i myself always do use that hump on the edge of the magnet (should make the trace wider there). what i noticed with my tweeters is having coil almost everywhere on the foil even if that means some are in a less ideal fields gives a smoother response then purely put them in the flatest/intense field only. so my tweeters waste good coil on less then ideal magnetic field. i even only leave like 1 1.5 mm on top of the magnet free from any coil (aligning the membrane to the magnets does get harder, if you go over the middle of the magnet/misalignment, the magnetic field swaps and a part is playing out of phase then).
Most have a much wider part without any aluminium on the center of the magnet where the field flips, i had tweeters where that made a difference in the upper range (maybe because there is a part lagging behind the driven parts bit like breakup) maybe having more tension on the foil could have fixed that to ,not sure.
When I made some ribbons I etched the membranes to a fashion that made the thinner att the edge nearer to the magnets and thicker in the middle in order to have a uniform current density over the membrane so to drive them with an equal F(=BIL) force independent of the location on the foil... ;-)
What do you think?
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What do you think?
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well the wider trace version works too, since its still fed by the small trace. so current will be the same , but less dense so i dont know if your method worked , the wider trace lower resistance sounds like the opposite of what you want but since its fed by the lower resistance one. it can never achieve higher magnetic field. and is spread over a wider distance.. making it less ideal, witch in this case was the purpose
so i dont know if your method worked , the wider trace lower resistance sounds like the opposite of what you want but since its fed by the lower resistance one. it can never achieve higher magnetic field. and is spread over a wider distance.. making it less ideal, witch in this case was the purpose
I should have said that this was a true ribbon i.e. one single piece of aluminum foil - not a planar with traces.well the wider trace version works too, since its still fed by the small trace. so current will be the same , but less dense
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So instead of looking at home made integrals of the magnetic field density over the width of the foil in that field,
one can ealisy let FEMM integrate the cross product of JxB and get the scalar F on the whole foil.
Big thanks to I-or on the Swedish forum faktiskt.io (faktiskt translates to actually, an often used word on that forum).
Here's my thread on SMAPPP on faktiskt.io.
This is how to do it.
Set the desired depth in the problem definition, I have used 2200 mm for SMAPP:
Set up a circuit, give it an appropiate name, I've used the name 7 um foil.
Set the current. I thought 1 W over 8 ohm could be a good value:
Create an rectangle for the membrane. I made it 7 um thick:
Make it Aluminium 1100 and assign it to the circuit:
Simulate:
Chose the membrane area:
FEMM calculates the current density J in this area based on the current you have assigned above.
Integrate JxB:
May the force be with you:
So that's equivalent to what the pull gravity has on a 32 gram weight.
SMAPPS has eight strips in total, so that's
Not that changing the area will change the current density, so you cannot compare the force if the area changes.
I my case I will only move the membrane up and see the force change with the distance from flat.
It now be faily easy to get the forces on a membrane that in the gap has several foil strips with different widths; the goal is to have the same force from all the strips. You'll need to have one circuit for each foil with the same current.
It should also be possible in the true ribbon case above, to get the forces from each segment of the foil, each segment have different thickness.
I think that all the segments in that case much be the same circuit but I'm not sure if FEMM distributes the current density over the segments.
Please test the latter and tell us if it works.
one can ealisy let FEMM integrate the cross product of JxB and get the scalar F on the whole foil.
Big thanks to I-or on the Swedish forum faktiskt.io (faktiskt translates to actually, an often used word on that forum).
Here's my thread on SMAPPP on faktiskt.io.
This is how to do it.
Set the desired depth in the problem definition, I have used 2200 mm for SMAPP:
Set up a circuit, give it an appropiate name, I've used the name 7 um foil.
Set the current. I thought 1 W over 8 ohm could be a good value:
Create an rectangle for the membrane. I made it 7 um thick:
Make it Aluminium 1100 and assign it to the circuit:
Simulate:
Chose the membrane area:
FEMM calculates the current density J in this area based on the current you have assigned above.
Integrate JxB:
May the force be with you:
So that's equivalent to what the pull gravity has on a 32 gram weight.
SMAPPS has eight strips in total, so that's
Not that changing the area will change the current density, so you cannot compare the force if the area changes.
I my case I will only move the membrane up and see the force change with the distance from flat.
It now be faily easy to get the forces on a membrane that in the gap has several foil strips with different widths; the goal is to have the same force from all the strips. You'll need to have one circuit for each foil with the same current.
It should also be possible in the true ribbon case above, to get the forces from each segment of the foil, each segment have different thickness.
I think that all the segments in that case much be the same circuit but I'm not sure if FEMM distributes the current density over the segments.
Please test the latter and tell us if it works.
In the post above I simulated with a foil that was 5 mm wide in the 5 mm distance between the magnets.
I have also simulated a 3 mm wide foil:
For the first 5 mm foil I got: 0.318 N flat, 0.320 N 0.5 mm above flat and 0.329 N 1.0 mm above flat. That's a 3.5 %difference.
For the 3 mm foil I got: 0.3190 N flat, 0.3196 N 0.5 mm above flat and 0.3192 N 1.0 mm above flat. That's a 0.157 % difference.
If these number would manifest themselves as distortion, the 5 mm foil will have a -29 dB distortion level and the 3 mm foil -56 dB.
I have also simulated a 3 mm wide foil:
For the first 5 mm foil I got: 0.318 N flat, 0.320 N 0.5 mm above flat and 0.329 N 1.0 mm above flat. That's a 3.5 %difference.
For the 3 mm foil I got: 0.3190 N flat, 0.3196 N 0.5 mm above flat and 0.3192 N 1.0 mm above flat. That's a 0.157 % difference.
If these number would manifest themselves as distortion, the 5 mm foil will have a -29 dB distortion level and the 3 mm foil -56 dB.