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Use The Force… Inside Moving Coil Loudspeaker Motors
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What makes a moving-coil loudspeaker tick? What are some of the major variations in loudspeaker motors?

As I discussed here, a wire carrying an electric current in a magnetic field will experience a force.

The maximum force for a given current and magnetic field will occur when the current is flowing perpendicular to the magnetic field.

In a moving coil loudspeaker, a coil of wire is immersed in an air gap in a magnetic circuit. The magnetic field is “radially” oriented, meaning that its direction is across the gap in the radial direction.

As you can see in Figure 1 (below), this means the current is perpendicular to the magnetic field (blue arrows) in the gap.

The force is also proportional to the magnetic field strength, the current, and the length of wire in the gap. For a given magnet, the wider the air gap the lower the magnetic field strength in the gap.

Narrow gaps thus yield the strongest field, but the coil has to fit in the gap with some reasonable clearance to prevent it from rubbing against the side of the gap when moving.

Figure 1—Magnetic flux passing through a voice coil in a loudspeaker motor. (click to enlarge)

For a given wire size, the longer the length of wire in the gap, the thicker the coil and the greater its resistance, thus reducing the current. Using thicker wire will reduce resistance, but increases coil thickness and mass, requiring a wider gap, and lowering the magnetic field strength.

The majority of woofers used in professional sound applications use coils with winding lengths longer than the thickness of the air gap. This keeps the gap full of coil turns as it moves in and out, provided that the coil never moves so far that the ends of the coil enter the gap.

If that happens, the number of wire turns in the gap begins to decrease, also decreasing the length of wire in the gap, and reducing the driving force.

Roughly speaking, the distance the coil travels before the end of the coil enters the gap is called the Xmax (Figure 2).

Figure 2: Xmax as determined by geometry in an overhung coil design. (click to enlarge)

What about the magnet circuit? How is it made and what are the tradeoffs? Some of the earliest moving coil loudspeakers used a field coil to create the magnetic field. This was basically an electromagnet made from a large coil of wire wrapped around the pole piece and carrying a DC current.

Sometimes the field coil was used as a large choke to smooth the DC voltage of the amplifier. In the 1930s a new magnetic material called Alnico (made from Aluminum, Nickel, and Cobalt) was invented and soon became the popular choice for providing the static magnetic field.

Alnico provided a good energy/weight ratio, but a civil war in Zaire in 1978 closed down the only commercial source of cobalt in the world. As a result, ferrite magnets quickly replaced Alnico in most applications.

Ferrite magnets are a ceramic material made of iron and either barium or strontium oxides. They are relatively inexpensive, but have a fairly low energy/weight ratio meaning a speaker with a ferrite magnet will typically be heavier than one with an alnico magnet of the same strength.

Very strong, lightweight magnets made of neodymium, iron, boron, and small amounts of several other elements were first patented by Sumitomo, General Motors, and the Chinese Academy of Sciences in 1983.

Neo magnets are “scary strong” for their weight, but the earliest forms of neo magnets lost their magnetism at temperatures low enough to be a problem for high-powered loudspeakers.

More patents followed in later years covering improvements to neo magnets to increase their strength and resistance to high temperatures.

Due to these improvements and their high energy/weight ratio, neo became a more popular magnet choice for speakers around 2000.

Neo speakers are still quite popular, of course, but rising prices for neodymium and other components of neo magnets have recently caused considerable concern and will continue to result in increased prices for neo speakers until other rare earth mines resume or commence operations in the next few years.

Samarium cobalt magnets are also very strong and expensive, although not as strong as neodymium, and can operate without damage at very high temperatures.

What about coils? How are they made, what are they made of and why? Naturally since the coil is meant to conduct electricity it should be made of a material with high conductivity.

By far the most popular materials for conductors are copper and aluminum. Silver is actually the metal with the highest electrical conductivity, but is too expensive to be of practical use in a loudspeaker.

Aluminum has a lower conductivity than copper by volume, but higher by weight, so it is often used when it is important to keep the coil weight to a minimum.

Some manufacturers use straight aluminum, but aluminum will develop an insulating oxide coating almost immediately upon exposure to air. This oxide coating can actually be used as the insulation instead of coating the wire with enamel, but also makes it difficult to make an electrical connection by soldering.

For this reason, many manufacturers use copper-clad aluminum wire, which is only slightly heavier than bare aluminum and much easier to work with.

The most common coil construction is known as a two-layer coil. In this construction wire (usually round in cross-section) is wound on a cylindrical paper or plastic bobbin starting at the top and spiraling down to the bottom of the coil. There, it is turned around and a second layer is wound on top of the first layer back to the top of the coil.

One of the big advantages of this kind of construction is that both the lead-in wires are left at the top of the winding. They are dressed up the bobbin and connected to flexible tinsel lead material that carries the current to a terminal mounted on the loudspeaker frame.

A four-layer coil is the same as a two-layer coil except that two more layers are wound, down and back up a second time.

Naturally, it is possible to wind any even number of layers in this fashion, but it is rare to have more than four.

A four-layer coil would normally be used in a woofer or subwoofer where the higher coil mass is not detrimental.

Copper is the most common conductor used in two- and four-layer coils and copper-clad aluminum or aluminum are used if necessary to reduce coil mass.

Any coil wound with round wire will have air spaces between the turns of wire. For a given wire cross-sectional area and number of layers, this increases the thickness of the coil.

Square or rectangular cross-sectional shaped wire can eliminate these air spaces, thereby decreasing the thickness of the coil, allowing for a narrower air gap and higher magnetic field strength. These advantages however, are at the expense of more difficulty and expense in winding the coil.

Most coils of this type use one layer of rectangular wire wound with the wire standing on its thin edge, like a Slinky, and are often called “edge-wound” coils. With a one-layer coil, one end of the wire is left at the bottom of the winding and is usually returned up the inside of the coil in a split in the bobbin.

Figure 3: At left, a flat wire coil, and at right, a round wire coil. (click to enlarge)

An edge-wound coil of the same thickness as a multi-layer coil will usually be stiffer due to the increased glue bond area between turns. Because of the expense, edge-wound coils are usually used only when high magnetic field strength and low-mass are paramount, such as in compression drivers, and thus are usually aluminum or copper-clad aluminum.

While not involved in the generation of motor force, the bobbin must transfer this force to the diaphragm or cone, except in the rare case of a bobbin-less transducer where the coil is attached directly to the diaphragm.

Bobbin materials are chosen for a combination of characteristics including mass, stiffness, and thermal characteristics.

Paper has a good stiffness/mass ratio and is very inexpensive, but can’t be used in high power applications because it cannot handle the heat. It is commonly used in low to medium power electric guitar speakers because of its desirable acoustical characteristics.

Nomex is a synthetic paper-like material which can handle more heat and is somewhat more expensive than paper. A very high-temperature material called polyimide is a very popular choice of bobbin material due to its combination of stiffness/mass ratio and its ability to withstand temperatures of more than 7000 degrees F.

Aluminum is another bobbin material that can withstand very high temperatures, but in some cases its high thermal conductivity can transfer these high ternperatures to the cone, spider, and adhesives used in the cone/spider/coil joint that cannot handle them. Fiberglass is yet another material sometimes used as a bobbin material.

There are many trade-offs to get the proper balance of motor strength, coil mass, adequate clearance for reliable operation at high power, Xmax, etc.

It is the loudspeaker designers’ art to blend all the different motor construction options with the choice of cone and other components to give the desired result.

Rob Gault is with Eminence Loudspeakers, based in Kentucky.

 


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