Loudspeaker drivers, switch mode power supplies, chain motors, dynamic microphones, throne shakers, disk drives and step-down transformers. What ties these pro audio staples together? The answer is magnetism. Magnetism, magnetic behavior, and magnets are everywhere in professional audio. If one needs to control electricity or drive physical motion using electricity, magnetism is almost always involved.
As magnetism is not a singular property, but rather a number of related physical effects, we will overview the most common magnetic properties. Then we’ll dive deeper into neodymium-based permanent magnets, the kind that are commonly used in loudspeakers, high-output dynamic microphones and power supply fans. Let’s get started.
The Many Faces of Magnetism
Describing the physical effects behind magnetism quickly brings one into the very heart of “modern” physics. It does not take much reading on the topic to run into Einstein’s theory of relativity, quantum physics and, specifically, a branch of 20th century physics called quantum electrodynamics. Modern physics is very good at precisely describing what behaviors are observed, but it is not spectacular at providing intuition about the “why” of physical phenomena. And so it will be with magnetism, which arises from quantum considerations.
Electrons dominate the observed magnetic effects in materials. Electrons exhibit an inherent response to a magnetic field, and this phenomena is called the “spin” of an electron. Spin is classified as “up” and “down.” Now the electron isn’t actually spinning, any more than it “orbits” around the nucleus of an atom. The terms are merely for convenience. It is the combination of the electron’s orbit and spin that gives the overall magnetic behavior of a given electron. The spin “magnetic moment” of a given electron is approximately equal to a quantity called the Bohr magneton (named for Danish physicist Niels Bohr). The Bohr magneton is essentially the smallest unit of magnetism in materials.
In most materials the magnetic behavior of electrons cancels out, as electrons tend to pair together in each orbit as a “spin up” and “spin down.” Spin pairing cancels out any macroscopic magnetic effects. In certain materials, though, there are a number of unpaired electrons, and those electrons give rise to the bulk magnetic properties we observe. The magnetic behavior of electrons is always there, but it takes certain configurations for all the electron spins to not cancel each other out and give rise to a net magnetic effect. Magnetic behaviors include ferromagnetism, ferrimagnetism, antiferromagnetism, paramagnetism and diamagnetism.
The Anisotropic Effect
Ferromagnetism is generally what people refer to when they discuss magnetism. Ferromagnetic materials show a spontaneous net magnetism in the absence of an external magnetic field. In other words, ferromagnetic materials are “permanent” magnets. One thing that helps give rise to permanent magnets is so called magnetic anisotropy. Anisotropy is a fancy way of saying that properties are not the same in all directions. For instance, a soda can is very strong when you press on it from above and below, but not very strong if you press on the side of the can. A soda can exhibits anisotropic strength, with a strong preference along the vertical axis of the can.
Materials that have anisotropic behavior make it easier for there to be a permanent misalignment of electrons spin, and therefore a material that has net magnetic behavior. Materials that exhibit this anisotropic effect generally have crystal structures that lack uniform properties, like atomic spacing, in all directions (i.e., x, y and z). The arrangement of the atoms in the crystal structure gives rise to the ability to have asymmetry of the electrons, which then results in a permanent magnet.
The quest for materials that exhibited “magnetocrystalline anisotropy” had its last big breakthrough in the early 1980s. Today that technology is commonly called neodymium or “neo” magnets. Neodymium magnets are not pure neodymium (Nd) but rather a chemical compound, Nd2Fe14B, neodymium-iron-boron, or NiB. The discovery of NiB’s magnetic performance and methods for making Nd2Fe14B were developed by General Motors and the Japanese firm Sumitomo in 1982.
The development of neo was driven partly by market volatility in sourcing cobalt. Cobalt is mined in the occasionally unstable African country of Zaire, and is used to make samarium cobalt (SmCo) permanent magnets, which dominated before neodymium was discovered. Neo magnets are stronger than SmCo magnets, and Nd-based magnetic materials grew to replace SmCo in many applications where strong, light magnets are needed, including professional loudspeakers.
Neodymium and Permanent Magnets
Neodymium is only one of the chemical elements in high strength permanent magnets, but it is the one that requires the most effort to obtain. Neodymium is a “rare earth” element. Rare earths consist of elements 57 to 71 on the periodic table (neodymium is element 60). Despite being labeled a rare earth, neodymium, and most of the other rare earths, are not particularly scarce, but instead spread out in low concentration from specific ores.
The extraction of metals from ore is messy business. The production of the rare earth base alloy, called mischmetal, is no exception. The rare earths are extracted from two different mineral ores called monazite and bastnaesite. The process involves acids, large amounts of water, and extensive leftover mineral tailings. Mischmetal production from monazite commonly begins with dissolving the raw ore in hot sulfuric acid.
The earth’s crust contains similar amounts of neodymium as copper, but one will not find a vein of neodymium. It instead exists distributed throughout the monazite and bastnaesite, which must be patiently concentrated from large amounts of starting ore into mischmetal alloy. The various rare earths are then separated from the alloy via a complicated chemical technique known as solvent extraction. After solvent extraction of Nd, one is left with a chemical compound known as a neodymium “salt.” That salt goes through one further chemical reaction, called reduction, to produce pure Nd metal.
Neodymium metal is far removed from the ore and much processing is required to reach the finished product. The large amount of processing involved in extracting Nd is one of the central reasons production switched to China, a country with comparatively lax environmental standards. From 1965 to 1995 most of the world’s rare earth metals came from a single mine in California, the Mountain Pass rare earth mine. If you have driven from Las Vegas to Los Angeles on Interstate 15, you have driven by this mine. Today, the majority of the world’s rare earth metal supply is both mined and processed in China. Environmental restrictions were a key component in the growth of the Chinese rare earth dominance. Molycorp, the owner of the Mountain Pass mine, filed for bankruptcy protection in 2015.
Producing neodymium is only part of the equation for the professional audio industry. The metal must then be alloyed and formed into magnets that are used in loudspeaker products. There are two major classes of neo magnets: “bonded” and “sintered.” Sintered magnets are the ones relevant to the world of professional loudspeakers. To manufacture sintered magnets, the starting materials (iron, boron, and Nd) are melted in the absence of oxygen. The resulting liquid is cast into alloy ingots. The ingots are then ground into powder with small particle sizes. This loose powder is then placed in an aligning magnetic field and pressure is applied to form a “powder compact” of the desired shape and size. Because of the aligning magnetic field, the magnetic domains of the powder particles are oriented in a specific direction that facilitates the later creation of the permanent magnet.
After alignment and pressing, the powder compacts are heated to chemically bond the individual particles together. The process of bonding small particles together via heating, but not melting, is known as “sintering.” During sintering, solid particles fuse together as atoms from adjacent particles diffuse into other neighboring particles as they vibrate from the applied heat. Sintering is a common process in the formation of ceramic materials like porcelain or clay tiles.
Neodymium magnets undergo a special type of sintering, called “liquid phase” sintering. In liquid phase sintering, a thin layer of liquid forms on the surface between adjacent particles, bonding them together. After the small neo particles coalesce by sintering, the magnet is carefully cooled, and the surface is coated with a metal (such as nickel) to prevent corrosion. The finished, unmagnetized neo magnet is then sent to a loudspeaker manufacturer, who places it in the loudspeaker driver’s motor structure. The motor structure is then placed in a strong electromagnet to cause permanent magnetization.
Magnet Parameters
Just as there are key concepts in magnetism, there are other important parameters to describe the physical performance of real magnets used in loudspeaker drivers. The first is the “remnant induction.” The remnant induction is a measure of how strong the permanent magnet remains after an external field magnetizes it. Loudspeaker designers generally desire the strongest magnet possible, and therefore the largest remnant induction. A second parameter is the “coercive force.” The coercive force is a measure of the magnet’s resistance to being demagnetized. Clearly, the demagnetization of a loudspeaker’s magnet would be undesirable, so a high coercive force is preferred.
A third parameter used to describe the magnet’s behavior is the “Curie temperature.” Above the Curie temperature, the magnet permanently loses its intrinsic magnetic field. Even below the Curie temperature, heat reduces the magnetic performance. As heat is a fact of life for loudspeaker drivers, improving the temperature-dependent performance of neodymium is of interest to loudspeaker driver manufacturers. Temperature stability is improved by adding small quantities of other rare earth elements, in a process called “doping.” Magnets doped with terbium or dysprosium are often utilized for pro audio driver applications.
Performance Dilemma
Manufacturers must go through the calculus of where to utilize neodymium-based drivers. On the demand side of the fence, manufacturers must determine where the price/performance curve stops supporting the use of neo drivers. Only a few years ago, when rare earths were very scarce and prices were high, neo was reserved for higher-end drivers. Today, with very attractive pricing, the ability to use smaller magnet structures and facilitating the development of more compact magnetics, neodymium drivers can be found across a broad range of price points and are widely used in both cone and compression drivers.
Phil Graham is FRONT of HOUSE’s regular technical contributor and resident scientist. His formal education includes graduate research in high temperature materials.
