Sewn spiral dynamic mic

[Chrisfromthepast]

Has anyone out there in the mic building world tried a dynamic mic using the approach in this blog post?

https://www.kobakant.at/DIY/?p=4008
Essentially, a spiral of conductive thread is sewn into a pillow or other fabric substrate, and a permanent magnet is placed very nearby to create a speaker.
Im curious how this approach would sound inside a kick drum or, woven into a guitar grille cloth. Probably not amazing, but Im curious all the same.

 

[gyraf]

my guesses are 1) extremely inefficient and 2) extremely imprecise / nonlinear

..not to mention that there's no shielding really possible - neither magnetic nor electrostatic..

..But if you feel the urge, go ahead and make sure to let us know how it turns out..

/Jakob E.

 

[Chrisfromthepast]

Yes, even after some revision and optimization this is going to sound pretty bad. But, here we go anyway.
Adafruit sells a conductive thread that comes pre-spooled onto a shape that will fit in the bobbin housing of my pfaff 545.

I inserted it in “backwards” with less tension on the bobbin tensioner.

With the help of an embroidery form, I was able to get a spiral onto a lightweight but tightly woven poly blend.

 

[Chrisfromthepast]

Sitting this contraption on top of the back side of a 15” speaker, I could see signal on my mobile interface (gain cranked up all the way). Soldering leads to the steel wire is tricky.
I had to stop short of a headphone test because I had to leave unexpectedly.

Perhaps thats good because a woofer magnet isn’t practical anyway.
I think its time to talk about the orientation of the field lines and which type of magnet mounted in which manner will give this thing the best chance of a useful result.
My hunch is a speaker magnet is not ideal unless the spiral can be in the center of the hole of the donut.
My spiral would require a 21”subwoofer.

With two donut shaped magnets, the diaphragm could be sandwiched between the two, and possibly get a strong enough field in the center.
This seems difficult to build the spiral to have enough turns a very tight area, or you have a HUGE magnet on the outside of your mic.
This may be the approach with the most predictable magnetic field, so there is hope of finding a mathematical approximation.
I’ll have to build a tighter spiral to test this method.

When I sat the spiral on top of the speaker magnet, the field lines were mostly curving around and sometimes perpendicular with the movement of the diaphragm, so the efficiency was very very low.

This first spiral is large and flat, so to get it inside of a uniform magnetic field would be unweildy.
What if the magnet was a small dipole placed perpendicular to the diaphragm in the center, so that the field lines are roughly parallel to the direction of travel again. Like a tiny bar magnet pushing up against the diaphragm with one pole.

 

[Chrisfromthepast]

The thin poly material didn’t really work so I tried again with some run of the mill vinyl I had scraps of.

And I mounted in between 2 magnets and made the spirals tighter.

I was not too hopeful on the REW level check. The preamp on this focusrite 3rd gen scarlett was maxed out.
Also, to get enough level, the genelec coaxial speaker is just under clipping. The “mic” is 2 inches from the center of the cone.
The outcome is very damped and not linear on a waterfall. [graphics on the way, I botched the capture bandwidth]
The voice test sounds like this:
I was rolling around in an office chair, and some guys were talking 40 feet away during this recording
View attachment 1 inch 2 magnet vinyl test.wav

 

[Chrisfromthepast]

I noticed by tugging on the material outside of the magnet, I could adjust the resonant frequency.
Should I go for lowest possible pitch?
I just kind of aimed for as tight as I could go without clamping.
For more level can I stack spirals?
If I stack and reverse the spiral direction and then swap the polarity of the 2nd coil output is this a humbucking coil?
I assume humbuckers are wired in series for more amplitude or in parallel for lower impedance?

 

[Chrisfromthepast]

Faraday’s law is given below:

ε=−NΔϕ/Δt

Where ε is the electromotive force, Φ is the magnetic flux, and N is the number of turns.

So for a stronger signal I need more turns and or a stronger magnet. Thats easy to understand.
In the denominator, the delta time is hiding the materials of the diaphragm.
To achieve the lowest delta T, Id guess that the diaphragm should be very stiff and large, but very compliant at the boundaries so that movement isn’t resisted.
Mylar reminds me of sail cloth. I might have some scraps to try. Would that help?

 

[cyrano]

Reminds me of Yamaha's "Elephant Ear" speaker:

https://www.diyaudio.com/community/...ar-with-alnico-magnet-in-sa-australia.385343/
Of course, the Yamaha still uses a classical magnet and coil.

In the seventies, there was a UK brand that made Hifi speakers out of styrofoam. Flat speakers, didn't sound awful. Don't remember the brand name, as they didn't last long and were wiped out by the big overstock sale in the eighties.

 

[Chrisfromthepast]

Holy cow the magnet is so small and the radiating surface is so large!
Thanks for this piece of history @cyrano.

 

[Chrisfromthepast]

This is REW’s sine sweep test from 2” away gain all the way up.
Diaphragm held in place by the magnets alone, when you tap it, the resonance is in the 800 hz range.

 

[soliloqueen]

it sounds kind of nice honestly. the roll-off is very pleasant and "naturally muffled", rather than being resonant or weird in any way.

 

[soapfoot]

In the seventies, there was a UK brand that made Hifi speakers out of styrofoam. Flat speakers, didn't sound awful. Don't remember the brand name, as they didn't last long and were wiped out by the big overstock sale in the eighties.

Bertagni

http://www.hifi-classic.net/review/bes-sm300-289.html
Fender also experimented with styrofoam speakers in the Bantam Bass

 

[cyrano]

Yep. Bertagni, by the looks of it. Didn't know about Fender. There are probably others.

 

[Chrisfromthepast]

it sounds kind of nice honestly. the roll-off is very pleasant and "naturally muffled", rather than being resonant or weird in any way.

Thanks for your encouragement! This second try with a vinyl diaphragm definitely has a distinctive sound. Is it the right tone for a kick drum pillow? Probably not.

Trying to force more turns onto the surface of the diaphragm will add weight and I will probably not be able to use this vinyl in the end.
Im currently toying with the idea of stiff sail cloth in the center and sewn to a stretchy material around the perimeter.
Is this approach naive? It seems closest to the loudspeaker/moving coil mics having a rubber surround.

 

[Chrisfromthepast]

Condenser tangent:
I bought a yard of golden colored stiff woven material that claims to be made of 72%metal.
Backplate material of copper clad FR4 Is easy to design modify and share, so Im leaning this direction.
After I choose a diameter, how do I wrap my head around how to deal with damping and the target volume of the blind holes?

 

[rock soderstrom]

Im curious how this approach would sound inside a kick drum or, woven into a guitar grille cloth. Probably not amazing, but Im curious all the same.

I appreciate your attitude. Curiosity and the urge to try things out in reality are deeply human characteristics.

 

[cyrano]

I finally had the opportunity to listen to your sample and that's far, far better than some mics I concocted in the past. Pretty amazing, really!

 

[k brown]

First came Ribbon Mics - now Nauga-Mics . . .

Outstanding adventure - carry on!


Love the marketing names GM gave upholstery that was simply Naugahyde:
- Cordaveen
- Morocceen
- Morrokide

 

[Chrisfromthepast]

I finally had the opportunity to listen to your sample and that's far, far better than some mics I concocted in the past. Pretty amazing, really!

Thanks! I had to triple check the right mic was plugged in at first. I was expecting more noise, more distortion, and tone more like a carbon mic or something.
I still haven't had any quiet time to properly test it, but once I put on headphones I knew I had to make a recording just to hear it again on a larger system, and make sure my memories weren't fooling me.
Getting the level up without ruining the tone will be a challenge.

 

[soapfoot]

I wonder whether a very thin or sheer fabric (chiffon?) might improve bandwidth by being lighter weight, or perhaps even level (by allowing more excursion)?