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Old 21st October 2014, 04:21 AM   #1
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Default Question About Aluminum Ribbon Material

Few questions on Ribbons -

Is Aluminum the primary choice as a Ribbon because of its low mass? Does the amount of magnetic field force generated when current is run through Aluminum matter, or is the magnetic gap induction based only on the Neodymium magnets?

Are there any other materials being used as Ribbons, and why?
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Old 21st October 2014, 04:45 AM   #2
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I'm a ribbon microphone expert, not a ribbon speaker expert particularly, but I will address your question from my perspective.
Aluminum is one of the very few metals that has the three key qualities of low mass, durable flexibility, and completely non-magnetic. To my knowledge, all ribbon/quasi-ribbon speaker drivers use, for these simple reasons, pure aluminum or aluminised plastic. I've never seen it done in speakers, a very few ribbon microphone makers, starting with Western Electric and followed by Reslo and Bang & Olufsen, used an alloy called duralum or duralumin, which is mostly aluminum, but had two other metals mixed in, to allegedly increase long term freedom from oxidation, which actually proved untrue.
In theory, it's possible to make ribbons with other non-magnetic, conductive material, but aluminum works so extremely well, that any commercial attempts at using other materials has not been economically successful, at least in microphones.
How much a ribbon moves per watt of input electrical energy, i.e., how efficient a ribbon driver is, is determined almost entirely by the magnetic flux density of the magnets used, and much less by how light & flexible the ribbon is and the surface area of that ribbon.
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Old 21st October 2014, 04:52 AM   #3
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Thanks for the response. I'm specifically referring to Ribbon Drivers in a 2-way or 3-way speaker system, which many people say are among the best sounding drivers.

Can you elaborate further on the non magnetic part? Ribbons operate by the varying audio signal current inducing a varying magnetic field that interacts with the permanent magnet correct (pushing/pulling the Aluminum Ribbon)?

I'm just wondering how important the material being non-magnetic to start out with is / the amount of magnetic field that has to be induced etc

Quote:
Originally Posted by stephensank View Post
I'm a ribbon microphone expert, not a ribbon speaker expert particularly, but I will address your question from my perspective.
Aluminum is one of the very few metals that has the three key qualities of low mass, durable flexibility, and completely non-magnetic. To my knowledge, all ribbon/quasi-ribbon speaker drivers use, for these simple reasons, pure aluminum or aluminised plastic. I've never seen it done in speakers, a very few ribbon microphone makers, starting with Western Electric and followed by Reslo and Bang & Olufsen, used an alloy called duralum or duralumin, which is mostly aluminum, but had two other metals mixed in, to allegedly increase long term freedom from oxidation, which actually proved untrue.
In theory, it's possible to make ribbons with other non-magnetic, conductive material, but aluminum works so extremely well, that any commercial attempts at using other materials has not been economically successful, at least in microphones.
How much a ribbon moves per watt of input electrical energy, i.e., how efficient a ribbon driver is, is determined almost entirely by the magnetic flux density of the magnets used, and much less by how light & flexible the ribbon is and the surface area of that ribbon.
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Old 21st October 2014, 05:24 AM   #4
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If the ribbon material is magnetic, AT ALL, it will get stuck to the magnets. Motion is induced by electron flow within the magnetic field(of the magnets), not by magnetic repulsion. The moving electrons only *incidentally* generate a magnetic field of their own, which is a fairly weak field. But this field is not the motive force.
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Old 23rd October 2014, 04:52 PM   #5
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Aluminium is used because the it has the best ratio between density and conductivity, thus maximizing efficiency. This is the case regardless of whether it is used as a speaker or a microphone. Additionally it is low cost, and malleable and so relatively durable.

The next best material is beryllium, and this has been used by TAD and Pioneer in ribbon tweeters, but not to my knowledge in microphones.

The ribbon must be non-magnetic. As stephensank said, it would just collapse to the nearest pole if it was magnetic.

Andrew
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Old 24th October 2014, 07:41 AM   #6
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Quote:
Originally Posted by AndrewJ View Post
The next best material is beryllium, and this has been used by TAD and Pioneer in ribbon tweeters, but not to my knowledge in microphones.
The drawback for beryllium is that it is highly toxic and the dust causes nasty lung diseases including cancer. I don't know what happens when a beryllium tweeter overheats, but I suspect you don't want to be breathing in the same room (or at anytime thereafter) when it blows.

Many of the beryllium tweeters actually use copper-beryllium alloys which turn out to be nearly all copper. Fraud is everywhere.

A fascinating, and readable treatise, is here:
"The Whole Truth About Beryllium Diaphragms"
Steve Mowry
http://www.audioheritage.org/vbullet...chmentid=40060

For a given geometry the first bending (break-up) frequency is proportional to the material speed of sound. However, the density and thus the mass of the diaphragm must also be considered. Then the ratio of the Speed of Sound to the Mass Density can be used as the materials’ acoustic figure of merit. Table 1 lists the material properties of audio transducer diaphragm materials. A look at the material properties for typical diaphragm materials listed in table 1 show clearly that Beryllium is all around the “best” material for high frequency audio transducer diaphragm applications.
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Old 26th October 2014, 12:54 AM   #7
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Pure Aluminum is non-magnetic, light weight, low resistance, and low cost. It can be rolled to under 2micron thickness, does not rust or tarnish.

A few non-magnetic metals:

Pure Al is 2.65 [/10-8 ohm m; or micro_ohm cm]:
2700 kg/m3

5056 alloy Aluminum is 5.6 [/10-8 ohm m; or micro_ohm cm]:
2786 kg/m3
NOTE: 2x higher resistance, and more durable than pure Aluminum
NOTE: work hardens after corrugation with greater tearing at bent seams

40 alloy Ti is 3.52 [/10-8 ohm m; or micro_ohm cm]:
4507 kg/m3

302 alloy stainless steel 72 [/10-8 ohm m; or micro_ohm cm]:
7963 kg/m3

Household aluminum foil is 1mil = 25 microns thick. It makes alot of NOISE when you shake it.
--5micron - 7micron thick aluminum foil is QUIET when you shake. Probably the ideal thickness for a speaker ribbon.
--2-3micron aluminum foil is commonly used in ribbon microphones where only modest air pressure pushes on the foil.
--Magnapan uses a 2.5micron aluminum foil on ribbon tweeter linesource. Frequent ribbon breaks are reported

Ribbon construction technology includes:
1) To create a reasonable piston motion instead of just twisting from magnetic and air forces, fragile 6u aluminum foil requires either horizontal corrugation or 2D patterned emobossing.
"Gear tooth" type corrugations is being replaced with diamond shape pattern embossing plus some extra top+bottom suspension
2) Aluminum bonded to plastic foil:
--a) aluminum sputtered onto plastic film as used in capacitors;
--b) a few narrow strips of aluminum foil bonded to plastic and wired in series for amp-friendly resistance;
--c) a few narrow strips of plastic film bonded to one aluminum foil to increase strength and horizontal regidity.

Kaladex 1450 kg/m3 polyethylene naphthalate (PEN) films start to melt at 160C
Mylar 1360 kg/m3 widely available. 2 micon and 5 micron films available. starts to melt at 100C
Kapton 1535 kg/m3 a durable but "noisy" plastic film with 400C melting point
Kapton tape with 0.5 mil of silicone adheasive 2047kg/m3
Attached Images
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File Type: jpg emboss.jpg (36.4 KB, 52 views)
File Type: jpg options.jpg (206.6 KB, 53 views)
File Type: jpg End_suspension.jpg (142.4 KB, 54 views)
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Old 27th October 2014, 05:00 PM   #8
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It is not Beryllium that is toxic, it is beryllium oxide, in dust form.
This happens when beryllium is machined.
The beryllium that TAD uses is vapour-deposited in a vacuum, so the processing is safe.
In use in a diaphragm, there is nothing to be concerned about.

Any diaphragm that is fake will not be beryllium copper. This is used for springs, not diaphragms. Diaphragms use either an alloy of aluminium and beryllium, which is actually a legitimate material though not as good as beryllium, or the fake ones at best use a very light coating of beryllium over a plastic or metal diaphragm: enough to clainm the use of beryllium, but not enough to make a difference to performance or cost of the base material.

AJ
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Old 27th October 2014, 05:12 PM   #9
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Beryllium copper is used in non magnetic screwdrivers used in radar. Interesting material, very pretty alloy, very hard/stiff with a very light weight. Oddly soft feeling to the touch and insanely expensive.
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