Dynamic displacement vs resistance vs temperature measurement for ribbon transducer

[GusK]

United States Patent Application: 0080152186

is the patent that everybody is talking about on the prodigy-pro board

Gus

[Steve Dunlap]

In my "Direct Drive amplifier for DIY ribbon" thread, you suggested the Agilent AT-34420A as a suitable tool for measuring the resistance of the membrane.

Electro Rent Corporation - Product Detail Page

For transients, I assume the temperature will change very rapidly. Since resistance is linearly related to temperature, a high speed measurement device may be required to capture the resistance shift.

^Would this be able to measure dynamic resistance?

That will measure the resistance change with temperature, but not the way you probably have in mind. The signal source will affect the readings. You will most likely need to generate an AC frequency well away from the test frequency, very low in amplitude so it doesn't contribute to heating. Use a filter to remove the test signal, and calculate resistance change from the amplitude change in the AC signal.

I'm having difficulty calculating the natural frequencies of the membrane.

I can't help you with that as I have no background in that field.

Would it be possible to calculate the minimum tension applied to the membrane which does not result in sagging without FEM? My laptop runs OSX and I do not currently have enough funds for a linux workstation.

The magnetic field will have no effect on the sag of the membrane when no signal is present.

[1audio]

Before you get too deep into the theory of operation of a ribbon transducer you should get one or two to play with. I learned a lot from reworking and modifying a Decca ribbon.

First, there are real ribbon transducers (Like the Decca) and planar magnetic transducers often called ribbons. We are discussing the genuine version here. In the examples I have seen (including the Apogee and the Magneplanar tweeter) the ribbon is unstressed, in fact loose. If it has a resonance it is well below its operating frequency. Many examples use tricks to keep the ribbon in the magnetic field (the Decca had some grease at strategic locations). Most have corrugated ribbons to provide the elasticity.

A large part of the performance comes from the very light "diaphragm" reacting against air. In many cases the air load is higher in mass than the diaphragm.

The resistance of the ribbon isn't that big a deal. In many cases the matching transformer has a higher internal resistance than the diaphragm. Making an amplifier that will work directly into a ribbon is difficult and very inefficient so transformers are usually used.

Be foil is a great idea, however the stuff is extremely toxic and should be handled very carefully. BW should be able to give you guidelines. Its also astonishingly expensive.

Measuring the resistance of the ribbon in operation is a difficult task. You can't use DC with the low resistance and thermals everywhere. I have the earlier version of the HP meter you mentioned and it works pretty well. But it may be possible to build a version that could go inline with a ribbon, but difficult. The heating may be pretty small and the effect on the DCR of the ribbon will be small. Its usually only an issue on commercial sound reinforcement drivers. In normal audio the average power is rarely even a watt when playing loud.

I'm really interested in this and would like to support your efforts. Let me know how.

[thadman]

Originally Posted by 1audio

Before you get too deep into the theory of operation of a ribbon transducer you should get one or two to play with. I learned a lot from reworking and modifying a Decca ribbon.

I have built a pre-prototype, which includes an apparatus for tensioning the membrane. I used standard aluminum foil. The aforementioned system is VERY fragile.

Some thoughts:

The fragility of the element is significant. Mechanical shielding is probably required to prevent foreign objects from entering the gap. However, placing obstructions in front of the element may lead to turbulence at higher sound pressure levels and energy storage (similar to the effects of a phase plug in a compression driver). I have some ideas, however CFD will be required for resolution.

Originally Posted by 1audio

First, there are real ribbon transducers (Like the Decca) and planar magnetic transducers often called ribbons. We are discussing the genuine version here. In the examples I have seen (including the Apogee and the Magneplanar tweeter) the ribbon is unstressed, in fact loose. If it has a resonance it is well below its operating frequency. Many examples use tricks to keep the ribbon in the magnetic field (the Decca had some grease at strategic locations). Most have corrugated ribbons to provide the elasticity.

I'd like to stay away from non-newtonian fluids (grease?), as I believe they may be a source of inherent non-linearity.

Corrugating the element appears interesting. However, the solution space is incredibly large. I'm not sure how to quantify the length of time the membrane will hold a particular shape. I'll have to schedule further meetings with faculty in the Materials Science department for resolution.

Originally Posted by 1audio

A large part of the performance comes from the very light "diaphragm" reacting against air. In many cases the air load is higher in mass than the diaphragm.

Yes However, the membrane-fluid-duct interaction is EXTREMELY complex. I believe an optimized membrane may approach functioning as a massless, oscillating barrier between the fluids.

Originally Posted by 1audio

Be foil is a great idea, however the stuff is extremely toxic and should be handled very carefully. BW should be able to give you guidelines. Its also astonishingly expensive.

This is causing me significant trouble. Brush Wellman has expressed interest in providing a quantity of foil for my research. With regards to Aluminum, I may simply purchase an extremely large quantity. However, I'm not sure how much Beryllium to request due to the incredible cost.

I'd like 25 samples for each experiment to achieve sufficient statistical correlation. The material will be incredibly fragile and I expect a percentage of the material may be destroyed during the research process (tensioning/handling/testing). How much extra should I request to account for this? I don't think there is any way to avoid this problem.

Originally Posted by 1audio

Measuring the resistance of the ribbon in operation is a difficult task. You can't use DC with the low resistance and thermals everywhere. I have the earlier version of the HP meter you mentioned and it works pretty well. But it may be possible to build a version that could go inline with a ribbon, but difficult. The heating may be pretty small and the effect on the DCR of the ribbon will be small. Its usually only an issue on commercial sound reinforcement drivers. In normal audio the average power is rarely even a watt when playing loud.

I agree. Unless a user was interested in acquiring Tinnitus, <.1 watts should be sufficient for normal playback levels. However, operating the transducer at its limits should offer opportunity for significant contrast.

Thank you for your interest in this project thus far With a little bit of effort, I believe some very interesting conclusions could be reached.

[1audio]

I had to replace a diaphragm for the Decca once. I used the foil separated from its paper backing from some chewing gum. It worked very well.

BW has some alloys that may also be useful and a lot cheaper. Both a magnesium-aluminum and a BE-aluminum that have good properties are available from BW.

I suspect the sensitivity of the tensioned ribbon will be lower but I may be wrong.

Most of the existing samples I know of have corrugated ribbons, The MG has a 5' corrugated ribbon. They seems to be quite stable for a long time.

The grease is used to keep the ribbon in the gap at rest. There is 4 or 6 small blobs in critical locations. However Ferrofluid could be used in some effective way as well. That would need some thought.

[thadman]

Originally Posted by 1audio

BW has some alloys that may also be useful and a lot cheaper. Both a magnesium-aluminum and a BE-aluminum that have good properties are available from BW.

Does Brush Wellman produce these alloys as foils?

Originally Posted by 1audio

I suspect the sensitivity of the tensioned ribbon will be lower but I may be wrong.

I would tend to agree. The reactive force (tension) will counter the input force. A tensioned membrane may also store more potential energy (ie higher mode energy).

However, I believe a tensioning (whether minimum or maximum) mechanism may be required for repeatable experimental results as it may reduce human assembly error.

[1audio]

I believe BW makes all of them as foils and others to explore as well. I recently discussed making domes for tweeters with them. They do know their materials.

While playing with a FLIR would be fun you could also start by modifying a Raytek IR thermometer by adding an output and coupling it to a scope to see how fast it responds. If its quick enough you could use it and see how the different parts of the ribbon respond. To keep the ribbon in a consistent place you would need an AC excitation, I would use 100 KHz to keep the motion small.

If I had to do this with the limited budget and resources of a commercial enterprise this would be my approach:
First measure the resistance change with heat by heating the foil indirectly. Check to see if the results are consistent with published tables.
Then set up the transducer and drive it with a bridge that would show the effective impedance and see what happens as the drive level changes. It should be pretty straightforward (Ohm's Law), some scopes can do the necessary math. The ribbon should be pretty resistive in impedance above its resonance (or so I remember) so the change should be related to the temperature of the ribbon. A really thin ribbon should have a very short time-constant so it would be pretty clear if the resistance changed with signal and how much.