Thoughts on Ribbon Microphone; 'CORRUGATION' of aluminum ribbon theory, effects &design?

[RoadrunnerOZ]

You need to look at the ribbon as a generator - the back damping effect of the current passing through the ribbon is relative to the impedance/loading of the transformer and ribbon combined as a closed circuit. The intensity of the pole-piece magnetic field is many thousands (or even hundreds of thousands) of times the field generated by the ribbon - the ribbon is going to have virtually zero effect on the field through which it passes. The braking effect of the magnetic field on the ribbon is jointly the amount of eddy current from the actual ribbon itself plus the reactive effect of the field generated by the current flow induced in the ribbon by motion - as there is no externally supplied current flowing through the ribbon, only the current induced in the ribbon by motion which in turn creates its own braking effect.

 

[s wave]

You need to look at the ribbon as a generator - the back damping effect of the current passing through the ribbon is relative to the impedance/loading of the transformer and ribbon combined as a closed circuit. The intensity of the pole-piece magnetic field is many thousands (or even hundreds of thousands) of times the field generated by the ribbon - the ribbon is going to have virtually zero effect on the field through which it passes. The braking effect of the magnetic field on the ribbon is jointly the amount of eddy current from the actual ribbon itself plus the reactive effect of the field generated by the current flow induced in the ribbon by motion - as there is no externally supplied current flowing through the ribbon, only the current induced in the ribbon by motion which in turn creates its own braking effect.

I see your point in that it has almost 'no-effect' but I would still think using paramagnetic wires/conduits in an audio chain must have some effect. What if it was an aluminum ribbon specifically designed to have a large paramagnetic foot print? (I guess I add that to my '24 project list!) Appreciate it you gave me another project. (does this 'braking effect' yield any sonic /differences or effects other than voltage?) I will try to find out. Thx dearly...

 

[RoadrunnerOZ]

The paramagnetic effect will be directly proportional to the number of aluminium molecules in line with the magnetic field - the thicker the ribbon, the more effect will be realised. The wider the ribbon the greater the effect but then it’s reduced by the increase in pole gap thus reducing field intensity between the magnetic poles. A more deeply corrugated ribbon, thus longer with more aluminium present between the pole area would exhibit a larger effect as well. As this effect is inherently part and parcel of the “sound” of an aluminium ribbon the only way to minimise this is to reduce the amount of material between the poles of the magnet - as ribbons are in the region of 2 microns or less, realistically the only other way to reduce this is to use electrostatically deposited super thin metal film on a strong, light formable plastic ribbon - as is present on microphone capsules.
A typical layer thickness is around 400 Angstroms of gold on a diaphragm. This equals a thickness of 0.00000156 inch. The Angstrom unit is one ten-billionth of a meter; or 0.0000000039 inch. However using gold (also copper, zinc, silver) is maybe not a good idea as it’s diamagnetic (acts perpendicular to magnetic field by repulsion) and this may tend to twist the ribbon.
A ribbon in a microphone is around 20,000 Angstroms - 10,000 Angstroms to the micron.
Aluminium is the best choice of materials as it is the most workable metal having strength and low mass and is the lowest in paramagnetic effect beside magnesium which is not really useful here. You can test yourself how much force is applied to ribbon by cutting a thin strip of aluminium foil and holding it between the poles of a magnet at right angles to the field direction an see how much it twists into line with the field - compare this with blowing lightly on the strip when it’s between the poles and see how much holding effect there is due to paramagnetism.
Point to note - oxygen is also paramagnetic, however nitrogen is not - does this create turbulence in the airflow between magnetic poles when sound waves pass through?

 

[RoadrunnerOZ]

Another point to note - once you start alloying or plating aluminium with other metals you will introduce the paramagnetic effects of those metals which may have even more adverse effects as aluminium is about the lowest usable metal on the paramagnetic scale. We’re like talking only nano-Newtons or pico-Newtons of force here anyway. The force is a constant and does not vary with motion as long as the element is between the magnetic poles so would not have a varying response effect on a moving ribbon - it would be equivalent to very slight addition of stiffness in the membrane. It only tends to stop the membrane from twisting and likely has no impedance to movement across the lines of magnetic flux - the impedance to movement would come from the effect of the eddy currents and the current flow generated in the ribbon going through the load.

 

[s wave]

Last couple of posts are gold! Well said and explained. Do narrower magnets (vs wider/thicker/larger) increase or decrease 'twisting'? Is there any advantage to concave faced magnets? Great discussion. ty

 

[RoadrunnerOZ]

There are ribbon mics using convex magnet poles to modify the airflow surrounding the ribbon - I read through one of the patents on this:
https://patents.google.com/patent/US8433090B1/en

The idea being to minimise the effects of reflected sound waves between pole pieces causing aberration of the original sound wave - this more noticeable off-axis than on-axis.
I would doubt a concave magnet face would be an advantage in any way as the curvature at the centre of travel away from the ribbon would reduce the magnetic field strength where you would want it strongest and also create massive turbulence in the airflow causing the ribbon to distort in its travel and twist.
The wider the ribbon the less twisting will occur due to the mechanical restraint, a narrow ribbon can twist more easily. The paramagnetic influence on this would be increased by an increase in mass of ribbon between poles but then at the same time reduced if the same strength of magnet were used with a wider pole gap. A thicker ribbon would exhibit more stiffness similar to a wider ribbon and as well, have more molecules between the poles contributing to a larger paramagnetic attraction.
Air shock is a potential destructive mechanism for ribbon mics as the ribbon is fragile so frequency response vs SPL handling ability is always a trade-off. However I use ribbon mics as overheads for recording drums - did a session 2 weeks ago with a pair of Coles 4038’s and no problems.

 

[abbey road d enfer]

My understanding is the paramagnetic effect magnetizes the ribbon in a direction that is colinear with the flux, i.e. laterally.
I don't see how it would affect the damping of the ribbon in the normal plane.

 

[RoadrunnerOZ]

As far as I can understand it doesn’t - only when the excursion carries it (the ribbon) outside of the pole pieces (if that occurs the ribbon is likely going to be damaged anyway) or it attempts to twist out of plane.

 

[s wave]

Thx for Patent link Roadrunner OZ; interesting. abbey road d enfer; do spiralities come into effect at all? This link may be slightly OT but worth a look: https://www.science.org/doi/10.1126/sciadv.aau6386
Alignment of the 'spots' that may be paramagnetic, are they forever magnetized in a colinear fashion? The paramagnetic spots may not be perfectly aligned with the larger grain/magnetic field of the whole. Adding that; these spots may be also affected by the increase/decrease of the varying self contained electrical circuit? I will delve deeper. ty

 

[abbey road d enfer]

Alignment of the 'spots' that may be paramagnetic, are they forever magnetized in a colinear fashion?

Paramagnetism happens only in the presence of a magnetic field; it's not like hysteresis. So it is always colinear.

 

[s wave]

(Side question) Is it possible to 'squeeze' the magnetic field in ribbon motor by introducing 2 new parallel magnetic pole pieces adjacent between magnets in the motor; where these pole pieces line up as north to north and south to south? Effects?

 

[RoadrunnerOZ]

(Side question) Is it possible to 'squeeze' the magnetic field in ribbon motor by introducing 2 new parallel magnetic pole pieces adjacent between magnets in the motor; where these pole pieces line up as north to north and south to south? Effects?

Not sure what you envisage here - drawing? The gap between ribbon and magnetic poles is fraction of a millimetre - no room for anything between that.
Basic single magnet motor (1930’s)

Basic dual magnet motor:

 

[s wave]

Not in the gap between ribbon/magnet. I did not explain it well. On the images you show... it has 2 magnets in the truss/motor. If you added 2 magnets (in same alignment) paralleling the flat side of the ribbon. BELOW is an image looking down from the TOP of the mic. a and b is the typical magnet array with the ribbon between them. Magnets 1 and 2 are added. Hopefully this is clear enough?

 

[RoadrunnerOZ]

The magnets would get in the way of the airflow causing turbulence distortion as well as reducing output.
Normally in a ribbon mic there’re not N/S - S/N magnets either side but each pole is a singular N or S
The pole piece magnets either side of the ribbon should be sole N and S without a S or N behind as this would change the shape and intensity of the ribbon field as the rear pole would bend the line of the field away from the pole opposite - that’s why a single pole of a magnet to each side of the ribbon is normally used. The front and rear magnets you’ve shown however in your idea would need to be double pole to get the relationship you’re talking about. These would introduce angled field lines which would generate less current and possibly unwanted back resistance to motion as the ribbon approaches these. See magnetic braking. The idea to get the most out of a ribbon is to not impede the velocity of travel hence using light ribbon material and having a free airway - velocity ribbons have normal air resistance back pressure - pressure ribbons use a tuned chamber behind the ribbon to tailor the frequency response, but the entry portal to the ribbon is clear of obstruction for obvious reasons. .
You would get some current generated as the ribbon approaches the front and rear magnets due to the wrap-around magnetic field between the N & S of each of the face magnets but the overall field shape would be distorted from crossed over fields between the two opposing dual pole magnets and they would contribute nothing to the performance of a mic, rather destroying the smooth velocity of air pressure waves into the face of the ribbon and strange interaction with the complexity of field lines between magnetic poles. Try putting a horseshoe magnet pole faces up and two bar magnets (with N &S either side of the bar) at opposing corners and put some iron filings on a piece of paper over these and have a look at the field lines - the field strength from N to S of the horseshoe with the ribbon between those poles would be reduced by the interaction.

 

[s wave]

"Normally in a ribbon mic there’re not N/S - S/N magnets either side but each pole is a singular N or S" yes agreed. I am trying to make a microphone that is 'not normal'. Magnet 1 & 2 are 'added just to squeeze the magnetic field'. Magnets a & b are the normal mic polarity set-up. I also understand that magnet 1 & 2 would restrict the airflow as well as drastically change the magnetics and strength of the motor. I guess I will build one to see what transpires. Magnets 1 & 2 can be slotted-diamond shaped-round or other as well as be adjustable. I will work on airflow-distances-slotting and possibly using 4 magnets; whereas magnets "1 & 2" are shortened and 1a and 2a are added - to allow airflow to the center of the ribbon assembly. And the original assembly magnets steps are increased to a N52; while Magnets 1, 1a, 2, 2a are reduced to N32s.

I am trying to introduce distortion and color to a normal style ribbon microphone chassis. I have absolutely no clue as to the effects. But I am experimenting to 'create' and since it seems like it will distort; i will just see where it leads. OK; I got a new N52 magnet/motor chassis made. I am thinking I will try to find some shorter thinner cylinder N30ish magnets... thx appreciate your insight and advice on this. BELOW is a diagram of VIEW of FRONT of what the new assembly will look like.

 

[RoadrunnerOZ]

The overall strength of the field between a and b magnets would have a lot of the field intensity wrapping around to the back of these bipolar magnets and curving the field lines outward that would run, in a singular N & S pole scenario straight along the line of the ribbon surface giving maximum output, the ribbon then moving at an angle to the lines of flux. thus probably negating the “squeeze” you’re attempting to obtain to increase field strength. Any motion in the direction of the lines of flux generates nothing, only when motion is at right angles cutting the lines do you get output. The use of opposing N/S - S/N a & b magnets either side of the ribbon would tend towards having the pole to pole field more onion shaped than straight line, inefficient and probably just a weaker output without actually changing the sound of the ribbon as it would stop generating so much as it passes into the weaker field at the apogee of travel, rather just worsening the signal to noise ratio. Also as the ribbon travels through either side and outwards of pole to pole centre line the edges of the ribbon just tend to become baggage as the crosswidth centre is doing most of the work - the edges almost travelling in line with the field as it wraps around to the rear of your bipolar magnet.
I think it’s worth looking at the flux lines using iron filings on paper to see what you’re dealing with and what the net field shapes will be at both engagement scenarios - with and without the 1 & 2 magnets as in the centre section of the ribbon the 1 & 2 will be absent in your layout.

 

[s wave]

Thanks RoadrunnerOZ - much to chew on... I will also do your filings test to maybe gain greater insight. Much appreciated, time to digest it!