There is a tiny dusting of ReMount sticking to the actual membrane left over from the cutting mat. So technically, I don't need to fold over the top and bottom edges.
And 160 might work, this is probably easier if your oven has a hot air fan to help circulation but you can slightly open / close the oven door to regulate the temperature. It is what I did to regulate it to 150 +- 1 degree instead of 150 +- 10 degrees.
The shaded membrane is starting to take shape:
This is a much shorter membrane than the real one, 1/6 of the total lenght but otherwise the proportion should be correct. It is shorter to make it easier to view because otherwise it is too long to fit in my monitor 😆
The red numbers are, in theory, the amount of current that, in theory, should flow in each section.
The idea behind them is to split the trace into a parallel resistor to attenuate the signal. But unlike a parallel resistor, since the parallel trace has current flowing in the same direction as the rest of the coil, it will still generate useful force. Because of that, the overall membrane will be shaded, and loose max power output, but the per watt efficiency losses should be very small.
What the code generating the above membrane is trying to do is to model the resistance of the traces incrementally. So in this example, my model estimates that when measuring form red to red, then the resistance of the blue trace should be exactly the same as the resistance of the green trace.
So right now the target resistance ration is 0.5, 0.5 but I can set other ratios if I want, it is all parametric.
In practice, what it does is that I run multiple calculations. The first calculation is where the initial widths are the same, such that the trace width of the blue and the initial green part is the same. Then I look at the the comparative resistances and look at the error. If the green resistance is too big, then I halve ratio for the next iteration and try with 0.25, 0.75 widths. This will also be wrong, but for each step I will get a little bit closer.
This method of course already has a name, it is binary search where fore each iteration, the delta error relative to our target is halved. I run 20 iterations which gets very close to the ideal width.
But this is all very nice in theory, I still do not know how well my theroretical model actually matches with reality. So before I start making the real membranes I need to cut some smaller tester pieces and then progressively cut the membrane into parts and measure the resistance of each part with my 1 mOhm multimeter. If all my calculations are correct (which they probably aren't) then the theoretical resistance will match the actual resistances, but if not then I can use it to find where the errors are and fix them and then try again.
This is a much shorter membrane than the real one, 1/6 of the total lenght but otherwise the proportion should be correct. It is shorter to make it easier to view because otherwise it is too long to fit in my monitor 😆
The red numbers are, in theory, the amount of current that, in theory, should flow in each section.
The idea behind them is to split the trace into a parallel resistor to attenuate the signal. But unlike a parallel resistor, since the parallel trace has current flowing in the same direction as the rest of the coil, it will still generate useful force. Because of that, the overall membrane will be shaded, and loose max power output, but the per watt efficiency losses should be very small.
What the code generating the above membrane is trying to do is to model the resistance of the traces incrementally. So in this example, my model estimates that when measuring form red to red, then the resistance of the blue trace should be exactly the same as the resistance of the green trace.
So right now the target resistance ration is 0.5, 0.5 but I can set other ratios if I want, it is all parametric.
In practice, what it does is that I run multiple calculations. The first calculation is where the initial widths are the same, such that the trace width of the blue and the initial green part is the same. Then I look at the the comparative resistances and look at the error. If the green resistance is too big, then I halve ratio for the next iteration and try with 0.25, 0.75 widths. This will also be wrong, but for each step I will get a little bit closer.
This method of course already has a name, it is binary search where fore each iteration, the delta error relative to our target is halved. I run 20 iterations which gets very close to the ideal width.
But this is all very nice in theory, I still do not know how well my theroretical model actually matches with reality. So before I start making the real membranes I need to cut some smaller tester pieces and then progressively cut the membrane into parts and measure the resistance of each part with my 1 mOhm multimeter. If all my calculations are correct (which they probably aren't) then the theoretical resistance will match the actual resistances, but if not then I can use it to find where the errors are and fix them and then try again.
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I cut a test membrane, fetched my 1 mOhm precision ohmmeter and then started measuring and progressively cutting away the membrane to see if the expected resistances match my theory.
The goal is that, at every junction where two traces are in parallel, that the resistance of each side should be the same. Hence half the current should flow to each trace. I can adjust the ratio with a parameter in my python code but for now it is 50% for each side.
First I measured the resistance with both traces in parallel, then I cut into the membrane and measured the inner and outer trace individually.
And here is the measurements inserted into a diagram of the membrane with error percentages. And it looks to be pretty spot on so I am very pleased!
It makes sense that the error is larger at the teeny tiny resistances, but at such a high attenuation it isn't as important and it is still pretty close. And the errors seem pretty random, not biased towards the inner or outer so I can probably leave it as it is. All in all this is definently usable! But with these errors in mind I think it is a good pragmatic choice to stay at -3 dB steps instead of going down to -1.5 dB steps.
I will probably redo this with a full length membrane and see if the errors are still low enough or if they need adjustments. My guess is that the errors will be lower than this small scale experiment but I still want to know 🙂.
So, shading network is pretty much done for now. Next is to finalize the attachment on the bottom where I will glue the membrane to a PCB and solder the terminals. Then it is time to start scaling up to full size!
The goal is that, at every junction where two traces are in parallel, that the resistance of each side should be the same. Hence half the current should flow to each trace. I can adjust the ratio with a parameter in my python code but for now it is 50% for each side.
First I measured the resistance with both traces in parallel, then I cut into the membrane and measured the inner and outer trace individually.
And here is the measurements inserted into a diagram of the membrane with error percentages. And it looks to be pretty spot on so I am very pleased!
It makes sense that the error is larger at the teeny tiny resistances, but at such a high attenuation it isn't as important and it is still pretty close. And the errors seem pretty random, not biased towards the inner or outer so I can probably leave it as it is. All in all this is definently usable! But with these errors in mind I think it is a good pragmatic choice to stay at -3 dB steps instead of going down to -1.5 dB steps.
I will probably redo this with a full length membrane and see if the errors are still low enough or if they need adjustments. My guess is that the errors will be lower than this small scale experiment but I still want to know 🙂.
So, shading network is pretty much done for now. Next is to finalize the attachment on the bottom where I will glue the membrane to a PCB and solder the terminals. Then it is time to start scaling up to full size!
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I think I am pretty much done with the membrane, at least for now.
I plan to cut it in 2 layers, which is neccesary with the combination of coaxially driven and shaded, they overlap if done on a single side.
First is the top layer
Next is the bottom layer
Here is both at the same time
And the traces on the left that stick out of the rest I plan to cut and fold back onto itself. This the rear will fold to the front, and the front will fold to the rear:
After folding, and with a PCB it will look something like this
The membrane will be glued to the top of the PCB, but folder such the red pads will wrap the fold and connect to the red pads on the PCB.
Then the speaker wires will connect to the rear of the PCB. Thus there is much less risk of future ripped traces since I have a conventional PCB as a middle layer in between. And since it is a normal copper PCB it is much easier to solder.
I haven't yet decided if I should solder wires to the back of the PCB or add a terminal. I am leaning towards wires since it will be inside the speaker anyway.
Next up is to finalize the PCB so I can order some. And then finalize the 3d printed parts that for example the PCB will be attached to. And also to finalize the other 3d printed parts of the speaker. And then it is time for full length testing!
I plan to cut it in 2 layers, which is neccesary with the combination of coaxially driven and shaded, they overlap if done on a single side.
First is the top layer
Next is the bottom layer
Here is both at the same time
And the traces on the left that stick out of the rest I plan to cut and fold back onto itself. This the rear will fold to the front, and the front will fold to the rear:
After folding, and with a PCB it will look something like this
The membrane will be glued to the top of the PCB, but folder such the red pads will wrap the fold and connect to the red pads on the PCB.
Then the speaker wires will connect to the rear of the PCB. Thus there is much less risk of future ripped traces since I have a conventional PCB as a middle layer in between. And since it is a normal copper PCB it is much easier to solder.
I haven't yet decided if I should solder wires to the back of the PCB or add a terminal. I am leaning towards wires since it will be inside the speaker anyway.
Next up is to finalize the PCB so I can order some. And then finalize the 3d printed parts that for example the PCB will be attached to. And also to finalize the other 3d printed parts of the speaker. And then it is time for full length testing!
I have some experience on measuring low resistances. Down to u Ohm. Not easy!! But this is how it is done:
Connect a current source @ 1.0000 Amps, Start with the Current output set to 0A, Measure uV with your probes with zero current. Null the uV voltmeter when the probes is attached (This is the important part, since you create a battery with the different plating of the probes and the measured object, there is also a thermo electric effect). Start the current, and read the uV voltage directly before current has heated up the resistor.
Connect a current source @ 1.0000 Amps, Start with the Current output set to 0A, Measure uV with your probes with zero current. Null the uV voltmeter when the probes is attached (This is the important part, since you create a battery with the different plating of the probes and the measured object, there is also a thermo electric effect). Start the current, and read the uV voltage directly before current has heated up the resistor.
Cool! Although in my case the 4-wire ohmmeter I have seems to be precise enough. It is rated at 1 mOhm precision and based on how consistent enough it measures I believe that.
On the topic on my project, right now it is the calm before the storm (full scale testing)...
I have a 700 mm wide x 35 m roll of 23 um mylar on the way, and I am also waiting for the PCBs. All packages will arrive by May 9:th so by then I plan to start cutting full length membranes.
In preparation I am fixing some mechanical oversights in my current 3d printed parts.
One of the issues I have fixed is that the lengthwise sections can shift ~ 0.5 mm to the side. Since I want the membrane to be exactly in the middle, I wanted to fix this:
My fix was to add some indexing knobs:
Works great so far.
Also while adding the knobs I finalized the top and bottom parts, they will be 25 mm high on the top and bottom and the PCB will be 12 mm wide and 70 mm long.
The next problem I have fixed is that in my final speaker, the front and rear steel plates will overlap in a staggered way. Thus when all parts are screwed together, it will be very strong but when only the rear is assembled, it is pretty weak and is held together by threaded inserts in 3d printed plastic parts.
To solve that I have made an arc I can place the bottom plates on with the screws facing upwards such that I can first screw all the bottom plates to each other, then mount the membrane to the edges and then progressively add the front parts one by one.
It is not a beauty, but it will do the job just fine.
And while at it, I wanted to validate my corrugated vs uncorrugated lengths such that the final membrane length is correct.
So to do that I took one of my baked membranes and rolled it flat, only to realize that it instantly re-corrugated itself. The baking truly works!
Corrugated, the length was 27 cm. Stretched flat and rolled with a roller, the length was 30.5 cm. And then afterwards, it re-corrugated itself to a length of 28.5 cm!
So yeah, baking the mylar is amazing at making it hold corrugations!
In fact, the corrugations hold so well that when I previously used the EPDM rubber suspension, which previously crushed the corrugations flat under the rubber, the baked 23 um mylar was strong enough to resist this crushing force and kept the corrugations. It also reverted to the same shape after removing the EPDM rubber where the kapton + alu membranes had permamently crushed edges.
I will still use the soft foam though, since it measures better, but it is cool that the baked mylar is so elastic while also holding the corrugations so well!
So to do that I took one of my baked membranes and rolled it flat, only to realize that it instantly re-corrugated itself. The baking truly works!
Corrugated, the length was 27 cm. Stretched flat and rolled with a roller, the length was 30.5 cm. And then afterwards, it re-corrugated itself to a length of 28.5 cm!
So yeah, baking the mylar is amazing at making it hold corrugations!
In fact, the corrugations hold so well that when I previously used the EPDM rubber suspension, which previously crushed the corrugations flat under the rubber, the baked 23 um mylar was strong enough to resist this crushing force and kept the corrugations. It also reverted to the same shape after removing the EPDM rubber where the kapton + alu membranes had permamently crushed edges.
I will still use the soft foam though, since it measures better, but it is cool that the baked mylar is so elastic while also holding the corrugations so well!