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The DIY Ribbon Velocity Microphone, a home project


I'm not a magnet Guru, so please don't hit me too hard if something is wrong!!!
Update:  Lately, I have been getting my magnets from www.magnet4less.com.  Man, they are cheap these days.  They are even stronger than the ones I used to get, by a good bit.  This, of course, help raise the output of the microphone. 

Concerns of the magnet type.

Just like every industry, the magnet industry has its own terminology.  I'll go through some of these with you to help you select the best magnet possible.  Now please understand that I am not an expert in magnets either (so I may be a little off in my explaination), but everything I have read and studied indicates that I am at least on the right track.
Basically, what you are looking for the strongest magnet you can find.  But it has to be in the right shape and size, and has to have a uniform magnetization pattern(is anisotropic).  We'll start with how to measure strength.
Magnet strength is usually rated in units of Gauss, but MGO can be used as well.. 
MGO stands for MegaGauss Oersteds and can be expressed in a "grade" number that represents the end of an equation that takes into account coercitivity, flux, retentivity, and all the magnetic-related goodies.  It is the maximum energy product of a magnet, and it doesn't really tell the true strength of the magnet.  What it does tell is the quality of the materials the magnet is made out of.  So it is a good indicator or the strength of a magnet.
Gauss is a number that represents the magnetic flux density.  This is a set number that represents the magnet's strength in relation to a standard.
You know what?   I'm going to use this from the website credited below.
  • Magnet Strength Measurements (B)--The units for measuring the field strength (flux density) of a magnet are Gauss or Tesla. 1 Tesla = 10,000 Gauss. The Earth's magnetic field is on the order of 1 Gauss. There are different ways to classify and measure field strength:

  • B (flux density): This is the measurement (in Gauss or Tesla) you get when you use a gaussmeter at the surface of a magnet. The reading is completely dependant on the distance from the surface, the shape of the magnet, the exact location measured, the thickness of the probe and of the magnet's plating. Steel behind a magnet will increase the measured 'B' significantly. Not a very good way to compare magnets, since B varies so much depending on measurement techniques.

  • Br (residual flux density): The maximum flux a magnet can produce, measured only in a closed magnetic circuit. Our figures for each magnet are provided to us by the magnet manufacturer. They are a good way to compare magnet strength...but keep in mind that a magnet in a closed magnetic circuit is not doing any good for anything except test measurements.

  • B-H Curve: Also called a "hysteresis loop," this graph shows how a magnetic material performs as it is brought to saturation, demagnetized, saturated in the opposite direction, then demagnetized again by an external field. The second quadrant of the graph is the most important in actual use--the point where the curve crosses the B axis is Br, and the point where it crosses the H axis is Hc (see below). The product of Br and Hc is BHmax.

  • The general rule is that the higher number is much better, in any case.  At all of the websites I have visited, the strength was expressed in a MGO grade or in a gauss number, so I hope this helps you.  Sometimes it's in a B, but usually in a gauss or MGO.  I will put examples of MGO numbers in with the magnet types so you can compare from one type to another.

    Now for the types.  As far as we are concerned (for this project), there are 4 major magnet types.  There is the ceramic magnet, the alnico magnet, the samarium cobalt magnet, and the neodymium. 
          I lifted these from a magnet site, mainly because I don't have the time to be typing this stuff out myself.  The site also has some other good information, so the address is http://www.wondermagnet.com/magfaq.html#q9
  • NdFeB (Neodymium-Iron-Boron) -- MGO of 30-45, gauss of 10000 to 14000...The most powerful 'rare-earth' permanent magnet composition known to mankind. This formulation is relatively modern, and first became commercially available in 1984. NdFeB magnets have the highest B, Br, and BHmax of any magnet formula, and also have very high Hc.  They are however very brittle, hard to machine, and sensitive to corrosion and high temperatures. Useful in the home, workshop, pickup truck, laboratory, wind turbine, starship and more. NdFeB are the best choice for incredible strength and coercivity at a reasonable price! In power generation applications, NdFeB magnets can be expected to give 4-5 times the power output of ceramic magnets.

  • Ferrite (Ceramic) -- MGO of 1-5, gauss of 2000-4000...Also known as 'hard ceramic' magnets, this material is made from Strontium or Barium Ferrite. It was developed in the 1960s as a low-cost and more powerful alternative to AlNiCo and steel magnets. Less expensive than NdFeB magnets, but still very powerful and resistant to demagnetization. Useful everywhere.  Ferrite magnets are lower in power (B, Br, BHmax) compared to other formulations, and are very brittle. However, they have very high Hc and good Tc (see below), and are quite corrosion-resistant. A very cost-effective choice.

  • AlNiCo (Aluminum-Nickel-Cobalt)   MGO of 5-8, gauss of 5000-8000...for medium strength and excellent machinability. Developed in the 1940s and still in use today. They perform much better than plain steel, but are much weaker in strength (lower B, Br and BHmax) and must be carefully stored since they are prone to demagnetization. Contact with a NdFeB magnet can easily reverse or destroy the field of an AlNiCo magnet.

  • SmCo (Samarium Cobalt)-- MGO of 18-26, gauss of 8700-11000...for high power and resistance to high temperatures and corrosion. Developed in the 1970s, these were the first so-called 'rare earth' magnets. They are almost as powerful as NdFeB magnets, and far more powerful than all the others (high B and Br). They are the most expensive magnet formulation, and usually only used where resistance to high temperatures (high Tc) and corrosion are needed. Also very brittle and hard to machine.

  • All the MGO and Gauss measurement numbers are general references.  They are not exact.  In fact, there are some ceramics that have a gauss of 8000.  That's a good bit better than the top of the average.  I suspect the ceramics I used for this project are in the 7000 range, as they are the strongest ones I have ever run across,.  Just so you know, the type and the gauss number can be quite separate in some situations..

    Magnet orientation
    There are two ways to describe the orientation of a magnet's magnetization. 
    Isotropic is used to describe the material that has no coherent magnetic pattern because it is not subjected to a magnetic field when it is made at the factory. 
    Anisotropic is the one that we are concerned with, and means that the material has a direction of magnetic orientation.  When the magnet is made, it is subjected to an intense magnetic field that causes the magnet to have a uniform magnetic property.
    How the material's magnetic properties are oriented is our main concern.  You can have all the strength in the world, and it won't do you any good unless the magnet's line of flux are in the right direction.  I found this out the hard way.  I would have had a working ribbon mic days before i finally got mine to work if I had known the orientation properties of my magnets.
    In Anisotropic magnets, the magnetic orientation can be though of as magnetism in a 3 dimensional space.  Even though there are as many orientations as you can think of, for this project there are 3 basic directions, and these are:
    Through the length:  Also known as Parallel to length.  The magnet's lines of flux run through the material from one end to the other, with one end being North and one end being South. 
    Through the width: also known as Parallel to width.  The flux lines run from one side of the width to the other side of the width, with one side being North and the other side being South.
    Through the thickness:  A.K.A. Parallel to thickness.  The flux runs through the thickness of a magnet with the North on one side and the South on the other. 
    I found that most manufacturers knew what you were talking about if you used these orientation phrases.  They may help in a mis-communication situation. 
    I used some that were magnetized through the width.  If I'm not mistaken, the added flux width also extends further out the sides of the magnet, where the magnetization through the thickness would not extend as far.  The Through the length orientation really does us little good, because we need at least two inches for the length of the ribbon, and if we turned the magnet on it's side to use this orientation, we'd have to cut off the magnets or let them stick out the sides.  So it's easier to get magnets that are magnetized through the width and start from there.
    It would also be fair to mention that many old ribbon mic designs used horse shoe magnets or circular magnets.  The horse shoe magnets had the ribbon suspended in the gap, and the circular magnets had a notch cut out where iron bars would fit in to make the ribbon gap.  Keep these in mind just in case you have some lying around. 

    More about magnets...
    *****If you use magnets that you find, they may not have the appropriate pole characteristics.  Magnet poles can be in any pattern at all.  You can have multiple poles on the same surface.  So unless you are positive that the poles are consistent, it might be better to order some magnets that are uniform in the pole structure.  
         But if you decide to try to use the ones you have anyway, It may be difficult to find out where the lines of flux are.  It's something I haven't figured out an easy way to determine EXACTLY.  I did find a way around it (sort of).  One way is to take the magnets you have and use them in a ribbon structure.  At this point it might be a good idea to use a regular tin foil diaphram because they are easy to make and accept more abuse.  I would hate for you to go through all the trouble of making a silverleaf ribbon and destroy it on accident trying to figure out some magnet;s pole pattern...
        So try this.  Take the magnets and face them so that they are attracted to one another, and check the element through a record player preamp.  The mic will work fairly well if the magnets are in the right position and have the right orientation.  It may take flipping them over a couple of times to find the highest output, though. 
         Also, I found that magnets can be somewhat identified by holding the magnet a FAIR DISTANCE from a TV set, and rotating it horizontally.  The colors of the TV will shift with the magnet's field.  I found that if the North side is pointing at the TV, then the color distortions are concentrated at the center of the TV screen.  If the South pole is pointed at it, then the color distortions are more on the outside of the screen.  The areas in-between the poles are not as effective as the poles on the screen, so they are easily ruled out.  I have found that unusally strong isotropic magnets can fool me into thinking that they are Anisotropic, but I think I'm getting better at identifying them.
    With this test and the "rough ribbon" test combined, I can usually figure out the oreintation fairly easily.
    *****PLEASE DO NOT DO THIS TO A TV YOU LIKE.  I'm not responsible for what you do to your TV, so do it at your own risk. 
    I've also read the you can take a magnet and suspend it with a string or suspend it in water and it will natrually orient itself with the North pole facing the Eath's geographic South pole.  In fact, there is no reason why it won't work, but I haven't tried it.  Basically it is the making of a big-ass compass.