Vikash said:
Former Diameter affects area and heat transfer - thus power handling.
Wire gauge affects power handling, minimum impedance, Bl product (Qes).
Cones are stiffer than flat diaphragms
The air gap should be as tight as possible to maximize field strength and minimize fringing. You have to be able to build it, though, and allow for expansion/contraction, mfg. tolerances etc...
I've never taken apart a modern midrange, but the woofers I have reconed have gaps of ~0.5 mm on either side of the coil and former.
Phase plugs minimize the problems associated with dustcap cavity resonances and can help heat transfer if made from metal.
VC layers is a decision based mostly on inductance, for tweeters and midranges, usually only two (perhaps one) layers. For subs, four layers are common.
VC/former bonding is a materials problem that took years to perfect - at least for high power speakers. Bonding would depend on the materials. Epoxy might work, but has a tendency to peel.. other glues are probably more suitable.
try www.thistothat.com
BTW, on the diyspeakers (http://www.snippets.org/mailman/listinfo/diyspeakers) mailing list, there have been a few people who are employed in the industry and might be able to give some better hints than me.
Does my spreadsheet at least give you some idea of the complexities and how changing one or more of the factors effects the T/S parameters?
Hi Vikash,
Here's a recent thread from another intrepid tweaker who hacked a set of Coral Beta 10s. He uses a string suspension. It's interesting to note that he found that fuzzy string makes less self-noise than fishing line, etc.
Ron's covered your questions pretty well. I would add the following:
1.) Former diameter has a very complex relationship with cone behavior and plays a key role in how the cone launches waves. For example, a 4" cone driven from a single central point (via a conical-tipped former) will transmit all mechanical waves radially outward while the same cone driven from its perimeter (by a 4" former) will transmit all waves toward its center. How these two versions couple to the air load above their first breakup mode is obviously going to be quite different. A typical 4" cone with a 1" ring former will display a combination of both behaviors across its cone and dustcap.
Also, you generally want to minimize former/VC mass since it contributes no radiating area. Generally speaking, the heavier the former/VC, the lower the efficiency and the sooner the HF rolloff.
4.) I believe a phase plug also also provides a loading surface for the higher frequencies generated close to the former connection, increasing the acoustic impedance of the nearby air load and boosting HF efficiency. It also widens HF dispersion by keeping HF waves from crossing the center of the cone and creating interference patterns.
Here's a recent thread from another intrepid tweaker who hacked a set of Coral Beta 10s. He uses a string suspension. It's interesting to note that he found that fuzzy string makes less self-noise than fishing line, etc.
Ron's covered your questions pretty well. I would add the following:
1.) Former diameter has a very complex relationship with cone behavior and plays a key role in how the cone launches waves. For example, a 4" cone driven from a single central point (via a conical-tipped former) will transmit all mechanical waves radially outward while the same cone driven from its perimeter (by a 4" former) will transmit all waves toward its center. How these two versions couple to the air load above their first breakup mode is obviously going to be quite different. A typical 4" cone with a 1" ring former will display a combination of both behaviors across its cone and dustcap.
Also, you generally want to minimize former/VC mass since it contributes no radiating area. Generally speaking, the heavier the former/VC, the lower the efficiency and the sooner the HF rolloff.
4.) I believe a phase plug also also provides a loading surface for the higher frequencies generated close to the former connection, increasing the acoustic impedance of the nearby air load and boosting HF efficiency. It also widens HF dispersion by keeping HF waves from crossing the center of the cone and creating interference patterns.
Ron, thanks for putting me out of my misery. The spreadsheet proves your point and is very useful. Only comment is it would be nice to work in mm instead of just gauge (don't know what gauge 0.265mm wire falls into 31.5?).
Blather:
Bill, The Beta 10's thread was certainly interesting. In particular the comment about the spider's contribution to the sound (a second diaphram in essence). This has always been the feeling deep down, but how to overcome? His lines (also mentined by Otherwise) looks very interesting - and seems like a good way around the spider's additional output problem, but...
...my rubber suspension makes a twanging sound when plucked. This perceptually shows a longer decaying motion which does not stop the former as quickly as say, a corrogated paper spider. I get the same feeling when considering springs, or lines. Can I conclude that a spider at least fulfils one of its criteria well - that of resetting the former as quickly as possible.
On to the second criterion then: To add as little as possible to the cone output. In this case our spidery friend doesn't appear to do so well.
Are there any other considertions in choosing a suspension system (aside from keeping the former aligned)?
Blather:
Bill, The Beta 10's thread was certainly interesting. In particular the comment about the spider's contribution to the sound (a second diaphram in essence). This has always been the feeling deep down, but how to overcome? His lines (also mentined by Otherwise) looks very interesting - and seems like a good way around the spider's additional output problem, but...
...my rubber suspension makes a twanging sound when plucked. This perceptually shows a longer decaying motion which does not stop the former as quickly as say, a corrogated paper spider. I get the same feeling when considering springs, or lines. Can I conclude that a spider at least fulfils one of its criteria well - that of resetting the former as quickly as possible.
On to the second criterion then: To add as little as possible to the cone output. In this case our spidery friend doesn't appear to do so well.
Are there any other considertions in choosing a suspension system (aside from keeping the former aligned)?
The best spider will always be no spider at all. 
Back in the 50s (?) Hartley had a novel idea for a means of magnetic suspension. I believe he incorporated a thin iron ring into the former under the voice coil so the gap field provided centering force along the Z axis. However, he still used a spider for lateral positioning. A string/magnetic suspension could be interesting...
The downsides to wrapping your VC around iron are increased inductance, necessarily wider gap spacing, and hysteresis.
Bill
Back in the 50s (?) Hartley had a novel idea for a means of magnetic suspension. I believe he incorporated a thin iron ring into the former under the voice coil so the gap field provided centering force along the Z axis. However, he still used a spider for lateral positioning. A string/magnetic suspension could be interesting...
The downsides to wrapping your VC around iron are increased inductance, necessarily wider gap spacing, and hysteresis.
Bill
Dangnabit! Now you got me thinking about this! 
Here's an idea that could be one-up on Hartley's: electromagnetic suspension (see pic).
The green winding is a normal voice coil, and the two sets of red windings would carry opposed DC, pushing the voice coil toward the zero-excursion point.
Advantages would include no addional Le or hysteresis, and perhaps less mass.
Adjust the DC voltage to dial in a target compliance!
Obviously, you'd still need a means of lateral positioning...still thinking...
Bill
Here's an idea that could be one-up on Hartley's: electromagnetic suspension (see pic).
The green winding is a normal voice coil, and the two sets of red windings would carry opposed DC, pushing the voice coil toward the zero-excursion point.
Advantages would include no addional Le or hysteresis, and perhaps less mass.
Adjust the DC voltage to dial in a target compliance!
Obviously, you'd still need a means of lateral positioning...still thinking...
Bill
Attachments
I was browsing thru my favourites and found this:
http://www.quadesl.com/minimite.shtml
If you view the Quicktime movie, it shows you how to build a 'NXT' variant with what appears to be plastic film (food clingfilm?). You can't argue with the f response!
http://www.quadesl.com/minimite.shtml
If you view the Quicktime movie, it shows you how to build a 'NXT' variant with what appears to be plastic film (food clingfilm?). You can't argue with the f response!
I think you've hit a good idea there Bill. Quick, I'm running down to the patents office.
So now you'd have power requirements for the suspension as well. The potential for variable compliance sounds excellent. Imagine tuning your driver somwhat per application.
Rather than increasing the the air gap to fit another set of voice coils inside, perhaps it might be possible to raise the whole suspension part of it. Let's see of I can knock up a drawing...
Would the AC current going through the outer coil be affected in anyway by the two DC coils underneath?
So now you'd have power requirements for the suspension as well. The potential for variable compliance sounds excellent. Imagine tuning your driver somwhat per application.
Rather than increasing the the air gap to fit another set of voice coils inside, perhaps it might be possible to raise the whole suspension part of it. Let's see of I can knock up a drawing...
Would the AC current going through the outer coil be affected in anyway by the two DC coils underneath?
Stop! Thief!!
Yeah, if you're thinking what I think you're thinking, I already thought of that ...
The trouble with moving the suspension mechanism away from the VC is that you'll need a second magnetic gap and a longer former which equals complexity and added mass. I just thought of a way to keep the gap narrow: stay tuned...
Bill
For those like you and I in pursuit of performance--hang the expense and damn the torpedos--an underhung motor's advantages greatly outweigh the drawbacks, IMHO.
The core sweetness of underhanging is that it reduces distortion by flattening and smoothing the BL curve within excursion limits. No more nonlinear, modulating fringe fields to worry about.
You could also explore an XBL topology, an interesting hybrid of underhung and overhung (I sometimes call it innerhung--see pic). The public isn't really supposed to know what XBL is yet, but I believe I've unmasked it in this thread.
Go with God, my son.
Pat graciously accepted--Thank you.
But you're not exactly rid suspensions yet--you still have to constrain your VC laterally within the gap. May I suggest a simple, low-friction sliding bearing, like a straw through a washer? If you want to get fancy, you could position the former between a trio of guide wheels. Etc, etc.Bill
something important about voice coils
First making a voice coil former out of paper is a bad idea,because when the voice coil gets hot it will burn the paper.
So a simple solution wrap aluminium foil[kitchen foil] around the paper former inside and out and glue it to the paper coil former[bobbin].Kitchen foil is used in the oven and in barbuques as that doesn't burn under those temperature extremes,it must be fine for voice formers.
First making a voice coil former out of paper is a bad idea,because when the voice coil gets hot it will burn the paper.
So a simple solution wrap aluminium foil[kitchen foil] around the paper former inside and out and glue it to the paper coil former[bobbin].Kitchen foil is used in the oven and in barbuques as that doesn't burn under those temperature extremes,it must be fine for voice formers.
Wow this is getting too interesting.. 
Hmm, now because of this talk about spider problems I thought I should try this flat diaphragm type. I was aware of that it might be too flexible, but in fact it's more rigid (size was ~4" x 4" and vc ~20mm) than any of the cones I have.
I built a prototype with this cardboard which has those "waves" one one side (I don't know what it's called exactly) and glued two of 'em like the pic shows. The "surround" was stiff enough that it didn't need any spiders..
Hope you can see the pic.. Otherwise I'll try again when I get home.
Hmm, now because of this talk about spider problems I thought I should try this flat diaphragm type. I was aware of that it might be too flexible, but in fact it's more rigid (size was ~4" x 4" and vc ~20mm) than any of the cones I have.
I built a prototype with this cardboard which has those "waves" one one side (I don't know what it's called exactly) and glued two of 'em like the pic shows. The "surround" was stiff enough that it didn't need any spiders..
Hope you can see the pic.. Otherwise I'll try again when I get home.
Attachments
Hi Vikash
Have you already got a solution for your basket ?
Since you are skilled in metalworking you might try to build something like Audiotechnology uses for their modular baskets.
See at the lower part of the "specification form" under
http://www.flexunits.com/
for an idea how it looks like.
Regards
Charles
Have you already got a solution for your basket ?
Since you are skilled in metalworking you might try to build something like Audiotechnology uses for their modular baskets.
See at the lower part of the "specification form" under
http://www.flexunits.com/
for an idea how it looks like.
Regards
Charles
Waved cardboard = Corrugation.
My dad and I have experimented with this, and have found that you can make a very sturdy and pro-looking building material using corrugated 20-pound laser printer paper but that's probably irrelevant to this post, except maybe basket construction.
The surrounds and spiders seem to be the hardest part for people to get right. I made some surrounds of silicone but finally gave up on it--the silicone doesn't have enough Q factor and if you move the cone too much it splits--the industry standard seems to be cloth but I haven't the foggiest idea how to make cloth form those shapes!
The reason they go with cloth though, is that (contrary to popular belief) you can get greater Xmax using accordion surrounds, plus they don't break apart after 10 years of use. I would *love* to be able to make my own but I can't figure out how.
The magnetic suspension idea is intruiging, but I suspect it's also impossible. It seems like there's a fundamental theorem out there someplace about only being able to make magnetic stability on 2 axes, which is why magnetic bearings still have to have friction, and skateboarders have given up on the idea of a passive magnetic bearing.
Maybe there's a way around it? Who knows. I'm wondering if you could put a piece of lightweight magnetic material on the outside edge of the cone and magnets on the basket and hold everything in place... I'm just babbling now though.
This is a fun thread. Let's keep hacking at it.
My dad and I have experimented with this, and have found that you can make a very sturdy and pro-looking building material using corrugated 20-pound laser printer paper
but that's probably irrelevant to this post, except maybe basket construction.The surrounds and spiders seem to be the hardest part for people to get right. I made some surrounds of silicone but finally gave up on it--the silicone doesn't have enough Q factor and if you move the cone too much it splits--the industry standard seems to be cloth but I haven't the foggiest idea how to make cloth form those shapes!
The reason they go with cloth though, is that (contrary to popular belief) you can get greater Xmax using accordion surrounds, plus they don't break apart after 10 years of use. I would *love* to be able to make my own but I can't figure out how.
The magnetic suspension idea is intruiging, but I suspect it's also impossible. It seems like there's a fundamental theorem out there someplace about only being able to make magnetic stability on 2 axes, which is why magnetic bearings still have to have friction, and skateboarders have given up on the idea of a passive magnetic bearing.
Maybe there's a way around it? Who knows. I'm wondering if you could put a piece of lightweight magnetic material on the outside edge of the cone and magnets on the basket and hold everything in place... I'm just babbling now though.
This is a fun thread. Let's keep hacking at it.
Agreed. But it's a great material to test with. Plus I haven't noticed any burning from the card (even when I've got smoke coming out - hehe). Maybe the varnish is helping. Anyway, I've got about 20 Black Kraft formers to test with now, and some Kapton which I'll save until I know what i'm doing
I'm aiming for a flat diaphram too
I found that the whole cone/former assembley held at just one point isn't enough. i.e a surround and a spider is needed to position the assembley laterally (as Bill puts it). Was your former kept perfectly in the centre of the gap with just a stiff surround?
I wish. I just give the output of my 3D studio drawing to my uncle and it comes back as I want it. A handy tool he is.
I haven't decided on a basket yet. I think I'm just going to use two rings to sandwhich the surround inbetween. Perhaps another smaller two for a spider (incase I ever try it). Not sure how I'll end up holding it all in place. The method used by AT looks interesting. I do know I want as minimum intrusions under the cone as possible. If the magnetic suspension works, noise from the spider has been removed effectively. Great, but I also want minimum reflections and maximum breathing space inside the basket as well - so maybe just the two brackets. I haven't thought about this much yet.
Not sure if this is what you mean, but the idea is that the magnet suspension only replaces one of the spider's jobs. And keeping the former perfectly aligned in the gap is still something that requires attention. I've been thinking how to do this using magnets as well, but no joy...yet. Perhaps I'm going too far.
Vikash said:I'm aiming for a flat diaphram too
I found that the whole cone/former assembley held at just one point isn't enough. i.e a surround and a spider is needed to position the assembley laterally (as Bill puts it). Was your former kept perfectly in the centre of the gap with just a stiff surround?
Quite perfectly.. The cardboard is very stiff. But of course there's the thing that when the VC moves, it (I think
) tries to go also to other directions than just up or down... So some kind of spider would be good. But we have forgotten the ferrofluid! I don't remember what was the brand, but in local HiFi-magazine there was a test of few speakers and one woofer had no spider - just ferrofluid. I tried to put oil in that "flat-diaphragm" driver to see what it does, but it would really need much of very fine iron dust to keep all that oil between the pole pieces. Hmm. No spider, increased magnetic flux, better heat transfer, better impedance characteristics..
Agreed. Ferrofluid makes a lot of sense if it can be adapted to your project. Just look at the trophy case of benefits in Otherwise's link.
I was especially interested to note the radial centering force ferrofluid provides--didn't know about that. Combined with an EM suspension, I wonder if it could provide enough centering force to let you skip mechanical positioning (spider, strings, sliders, etc.) altogether. As the centering force is related to B-field density and gap height, an underhung topolgy with a go-for-broke motor (1.5-2T+ across the gap) sounds promising...and heavy!
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