The most common type of driver uses a lightweight diaphragm, or cone, connected to a rigid basket, or frame, via a flexible suspension that constrains a coil of fine wire to move axially through a cylindrical magnetic gap. When an electrical signal is applied to the voice coil, a magnetic field is created by the electric current in the voice coil, making it an electromagnet. The coil and the driver's magnetic system interact, generating a mechanical force that causes the coil (and thus, the attached cone) to move back and forth, thereby reproducing sound under the control of the applied electrical signal coming from the amplifier. The following is a description of the individual components of this type of loudspeaker.[citation needed]
The diaphragm is usually manufactured with a cone- or dome-shaped profile. A variety of different materials may be used, but the most common are paper, plastic, and metal. The ideal material would be stiff, to prevent uncontrolled cone motions; light, to minimize starting force requirements; and well-damped, to reduce vibrations from continuing after the signal has stopped. In practice, all three of these criteria cannot be met simultaneously using existing materials; thus, driver design involves trade-offs. For example, paper is light and typically well-damped, but not stiff; metal can be made stiff and light, but it is not usually well-damped; plastic can be light, but typically, the stiffer it is made, the less well-damped it is. As a result, many cones are made of some sort of composite material. This can be a matrix of fibers, including Kevlar or fiberglass; a layered or bonded sandwich construction; or simply a coating applied to stiffen or damp a cone.[citation needed]
The basket, or frame, must be designed for rigidity to avoid deformation, which would change the magnetic conditions in the magnet gap and could even cause the voice coil to rub against the walls of the gap. Baskets are typically cast or stamped metal, although molded plastic baskets are becoming common, especially for inexpensive drivers. The frame also plays a considerable role in conducting heat away from the coil.[citation needed]
The suspension system keeps the coil centered in the gap and provides a restoring force that causes the speaker cone to return to a neutral position after moving. A typical suspension system consists of two parts: the "spider", which connects the diaphragm or voice coil to the frame and provides the majority of the restoring force, and the "surround", which helps center the coil/cone assembly and allows free piston-like motion aligned with the magnetic gap. The spider is usually made of a corrugated fabric disk, generally with a coating of a material intended to improve mechanical properties. The name comes from the shape of early suspensions, which were two concentric rings of Bakelite material, joined by six or eight curved "legs". Variations of this topology included adding a felt disc to provide a barrier to particles that might otherwise cause the voice coil to rub. A German company, Rulik, still offers a spider made of wood. The surround can be a roll of rubber or foam, or a ring of corrugated fabric (often coated), attached to the outer circumference of the cone and to the frame. The choice of suspension materials affects driver life, especially in the case of foam surrounds, which are susceptible to aging and environmental damage.[citation needed]
The wire in a voice coil is usually made of copper, though aluminium—and, rarely, silver—may be used. Voice-coil wire cross sections can be circular, rectangular, or hexagonal, giving varying amounts of wire volume coverage in the magnetic gap space. The coil is oriented co-axially inside the gap; it moves back and forth within a small circular volume (a hole, slot, or groove) in the magnetic structure. The gap establishes a concentrated magnetic field between the two poles of a permanent magnet; the outside of the gap being one pole, and the center post (called the pole piece) being the other. The pole piece and backplate are often a single piece, called the poleplate or yoke.[citation needed]
Modern driver magnets are almost always permanent and made of ceramic, ferrite, Alnico, or, more recently, neodymium magnet. A trend in design—due to increases in transportation costs and a desire for smaller, lighter devices (as in many home theater multi-speaker installations)—is the use of neodymium magnets instead of ferrite types. Very few manufacturers use electrically powered field coils, as was common in the earliest designs. The size and type of magnet and details of the magnetic circuit differ, depending on design goals. For instance, the shape of the pole piece affects the magnetic interaction between the voice coil and the magnetic field, and is sometimes used to modify a driver's behavior. A "shorting ring", or Faraday loop, may be included as a thin copper cap fitted over the pole tip or as a heavy ring situated within the magnet-pole cavity. The benefits of this are reduced impedance at high frequencies, providing extended treble output, reduced harmonic distortion, and a reduction in the inductance modulation that typically accompanies large voice coil excursions. On the other hand, the copper cap requires a wider voice-coil gap, with increased magnetic reluctance; this reduces available flux, requiring a slightly larger magnet for equivalent performance.[citation needed]
Driver design—including the particular way two or more drivers are combined in an enclosure to make a speaker system—is both an art and science. Adjusting a design to improve performance is done using magnetic, acoustic, mechanical, electrical, and material science theory; high precision measurements; and the observations of experienced listeners. Designers can use an anechoic chamber to ensure the speaker can be measured independently of room effects, or any of several electronic techniques which can, to some extent, replace such chambers. Some developers eschew anechoic chambers in favor of specific standardized room setups intended to simulate real-life listening conditions. A few of the issues speaker and driver designers must confront are distortion, lobing, phase effects, off-axis response, and crossover complications.[citation needed]
The fabrication of finished loudspeaker systems has become segmented, depending largely on price, shipping costs, and weight limitations. High-end speaker systems, which are heavier (and often larger) than economic shipping allows outside local regions, are usually made in their target market area and can cost $140,000 or more per pair.[11] The lowest-priced speaker systems and most drivers are manufactured in China or other low-cost manufacturing locations.[citation needed]
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