DIY capsule Q to Bob, Harvey or anybody who cares

[Harvey Gerst]

<No secret. After the usual 3 hour phone discussion with him, Stephen basically said (in his usual kindly manner), "Look dummy, the diaphragm has to be able to move." "Ohhh", I said. End of discussion.
I really miss Stephen; he would have really enjoyed this thread. (He would have made everybody here miserable, but he would have been able to contribute so much.)>

Thank you Harvey. So, as I understand, by 'freestanding' you mean--'not tensioned', is that right?
As I wrote earlier, Altec once made a glass diaphragm capsule commercially (I have to dig the model#). I am wondering why it was the only attempt.... I would think it is pretty fragile.

Yeah, "freestanding" was an unfortunate choice of words on my part. I reasoned that the 1 micron piece of glass woudn't need tensioning, just glueing. When I was discussing this with Stephen, it was just a possible idea, nothing more.

This would have been around 1987 or '88, when they announced that a 1" by 1" piece of 1 micron thick glass had been developed and was commercially available. I just thought, "Hmmm, 1 micron, no tensioning, and the resonance was probably above the audio range".

 

[guitarlover]

<Guitarlover,

While you are in the library, try to find old (I mean OLD) books on acoustics. Also, check if they have 'Microphone Engineering Handbook" edited by Michael Gayford, ISNB 0 7506 1199 5.
None of these books, however, will give you any practical information about capsules. Also, try to search for patents. In my opinion the best resource on the whole net for DIY traditional (Neumann style) is a thread which is still going on TT. You will find there a lot of links, practical designes, tips, etc. Have a look:

http://www.recording.org/postt1597.html

BINGO!! I found the book and I have the articles coming over from another library in the state (I'm glad I voted for the last tax levy!) I am going to email a good freind of mine who is a law professor and a patent attoney. When I resolve the issue of copywrites, I will make this article available, in PDF format, for anyone who is interested. I believe the copywrite has run out but I want to be sure so I don't get my butt in the wringer. Assuming I do get the OK, put your reading glasses on! The print is very small.

Thanks for getting me started and I will look for the book you suggest.

BTW, what is TT? (Never Mind, I figured it out)

Regards, Steve

 

[crazydoc]

Thanks Harvey, Marik and Bob. Your posts and links were quite helpful.
I think most of my confusion stemmed from thoughts of a tensioned membrane, not really realizing that a ribbon is just flopping in the wind, more or less. So the way I am understanding it now (in my simple fashion) is that a bunch of air molecules slam against the ribbon to move it (the molecules moving in response to the sound pressure wave) back and forth. So the less tension on the ribbon, the better, as tension would limit its excursion.

Let me know if this is more or less correct, or just FOS.

Thanks again.

 

[Marik]

<So the way I am understanding it now (in my simple fashion) is that a bunch of air molecules slam against the ribbon to move it (the molecules moving in response to the sound pressure wave) back and forth. So the less tension on the ribbon, the better, as tension would limit its excursion.>

Sorry Crazydoc, the last is little messed up.
Think about the ribbon as a woofer, but other way around. If you tune woofer's enclosure too high you compromize LF response, if too low, it will just inefficiently flip around. Since, unlike the woofer, the ribbon is not a stiff system, you will also get unwanted resonances, so it should be tuned above its fundamental resonance and its frequency will depend (don't remember the formula from top of my head) on ribbon size, mass, and compliance.
If I remember correctly, Coles 4038 is tuned to 20Hz, RCA44 to 44Hz, and Reslo to 80Hz.

 

[crazydoc]

Thanks, Marik.

How do you tune the ribbon? By eye, ear, or test equipment?

What is the approximate maximum excursion of the center of the ribbon (at highest SPL and resonant frequency I assume)? Are we talking tens or hundreds of microns?

Do the corrugations serve only to increase the effective length of the ribbon inside the magnetic field, or are they important in the resonance of the ribbon? (I would imagine they are.)

How do the number and depth of the corrugations affect this?

It would seem that there are so many difficult to control variables just in the ribbon itself that it would be difficult to make these mics with consistent results.

Sorry for all the questions, but I appreciate the info you all have given.

 

[Marik]

<When I resolve the issue of copywrites, I will make this article available, in PDF format, for anyone who is interested. I believe the copywrite has run out but I want to be sure so I don't get my butt in the wringer.>

Guitarlover,

I would be EXTREMELY interested in seeing this article, as I believe, it will help tremendously for designing this capsule. If there are any problems with copywrites, could you send me a copy, please. (I won't tell anybody ) Please PM me.
Meantime, I will look for a big magnifying glass

<How do you tune the ribbon? By eye, ear, or test equipment?
What is the approximate maximum excursion of the center of the ribbon (at highest SPL and resonant frequency I assume)? Are we talking tens or hundreds of microns?
Do the corrugations serve only to increase the effective length of the ribbon inside the magnetic field, or are they important in the resonance of the ribbon? (I would imagine they are.)
How do the number and depth of the corrugations affect this?
It would seem that there are so many difficult to control variables just in the ribbon itself that it would be difficult to make these mics with consistent results.>

Crazydoc,

The corrugation helps to make ribbon tension smooth and constant, and also, gives the ribbon freedom to vibrate in the manner of a string. For thicker and heavier ribbons however, in order to avoid resonances, there is another arrangement, where corrugation is applyed to the ends of the ribbon only and the center part is stiffened by means of lateral curve. Then, the whole system works in a piston-like mode.

The tuning of the ribbon is very similar to method used for measure of the free air resonance of a speaker.
In order to do it you will need sweep generator, AC voltmeter, preamp, and 100 ohm resistor.

Actually, to get consistent results is not that hard, as long as you use the same material for a ribbon, same size, same corrugator, and tune it to the same frequency.

 

[crazydoc]

Again, many thanks, Marik.

Do you or anyone have an idea of the excursion of the ribbon ?

 

[Marik]

<Do you or anyone have an idea of the excursion of the ribbon?>

Sorry, I never thought about it and never seen this spec. Probably Harvey or Bob might know...

 

[Harvey Gerst]

Again, many thanks, Marik.

Do you or anyone have an idea of the excursion of the ribbon ?

Probably about 1/8", but that would be pushing it hard, I would imagine. There's a point at which the velocity would cause the ribbon to stretch beyond repair. That's why the max SPL decreases with frequency. In the Coles manual, about 3/4 of the manual is devoted to what to watch out for; no sudden mic movements, avoid drafts, don't close doors fast, close the box slowly, don't blow into it, don't take it outside, don't swing it on a boom arm, etc.

 

[crazydoc]

Thanks, Harvey. I had no idea it would be that much. No wonder they're so easily damaged.

 

[arcanemethods]

How do you tune the ribbon? By eye, ear, or test equipment?

While there isn't a whole lot you can tune, it would be done by measurement of response.

What is the approximate maximum excursion of the center of the ribbon (at highest SPL and resonant frequency I assume)? Are we talking tens or hundreds of microns?

No idea.

Do the corrugations serve only to increase the effective length of the ribbon inside the magnetic field, or are they important in the resonance of the ribbon? (I would imagine they are.)

How do the number and depth of the corrugations affect this?

The length has no effect other than on increasing mass. Counter to intuition in this kind of transducer the mass decreases overall sensitivity rather than having a frequency dependant effect.

The corrugations are to increase the compliance of the ribbon which lowers the resonance.

It would seem that there are so many difficult to control variables just in the ribbon itself that it would be difficult to make these mics with consistent results.

It would seem that way to me too but perhaps it is less sensitive to the things that you can't control to high precision.


Bob

 

[arcanemethods]

The corrugations are to increase the compliance of the ribbon which lowers the resonance.

And to linearize it over a larger range of excursion.


Bob

 

[Marik]

<The length has no effect other than on increasing mass. Counter to intuition in this kind of transducer the mass decreases overall sensitivity rather than having a frequency dependant effect.>

Bob,

While it is right that the mass of the ribbon does not effect frequency response, against your intuition, the length of the ribbon actually increases sensitivity. There is, however, another aspect of increasing ribbons length--it is worse vertical directional response, but in such low output system, designers have to do this compromize.

<While there isn't a whole lot you can tune, it would be done by measurement of response.>

Why? As I said before--the easiest (and how they do it) is to measure ribbon's impedance.

And BTW, check your PM box.

 

[arcanemethods]

<The length has no effect other than on increasing mass. Counter to intuition in this kind of transducer the mass decreases overall sensitivity rather than having a frequency dependant effect.>

Bob,

While it is right that the mass of the ribbon does not effect frequency response, against your intuition, the length of the ribbon actually increases sensitivity.

Sorry, thanks for the correction. It's given by E = L*B*dx/dt.

Wait a minute, the L in that is only the L normal to the directon of motion so increasing length by corrugation won't change sensitivity. Right?

Funny, I think that's what my intuitor did in the background that made me answer that length didn't matter. It just didn't bother to share it with me correctly.

<While there isn't a whole lot you can tune, it would be done by measurement of response.>

Why? As I said before--the easiest (and how they do it) is to measure ribbon's impedance.

Right again. Time to go to bed (after I check my PM.)


Bob

 

[guitarlover]

<Guitarlover,

I would be EXTREMELY interested in seeing this article, as I believe, it will help tremendously for designing this capsule. If there are any problems with copywrites, could you send me a copy, please. (I won't tell anybody ) Please PM me.
Meantime, I will look for a big magnifying glass

Hi Marik, My attorney friend had a couple of questions and wanted more info so I await his input. The basic rule of thumb though is 26 years for a copywrite and a 26 year renewal, if used (I doubt it in this case)

I hope you won't be dissappointed though. The capule size suggested in the article is of 2" diameter. Remember, this mic was designed for normal speaking voices going over a narrow band transmission format (amatuer radio). I have my licence and used to be quite active so when I say narrow, I mean narrow. I'm not an audio engineer, per se, but the requirements of ham radio telephony were and are quite different than those for recordists. It's slow reading but I'm finding much more useful information in the articles referenced on the other forum. I do have copies of these articles on order from another library and hopefully the copies will be more legible than the scans I downloaded.

Hope the info helps, Steve

P.S. you asked me to PM you. What does that mean?