mass compared to copper. This raises the resonant frequency of the speaker and increases its efficiency. A disadvantage of aluminum is that it is not easily soldered, and so connections must be robustly crimped together and sealed. 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 ring of the gap is one pole, and the center post (called the pole piece) is the other. The pole piece and backplate are often made as a single piece, called the poleplate or yoke. Magnet 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 complication is 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 larger magnet for equivalent performance. Electromagnets were often used in musical instrument amplifiers cabinets well into the 1950s; there were economic savings in those using tube amplifiers as the field coil could, and usually did, do double duty as a power supply choke. Very few manufacturers still produce electrodynamic loudspeakers with electrically powered field coils, as was common in the earliest designs. Alnico, an alloy of aluminum, nickel, and cobalt became popular after WWII, since it dispensed with the problems of field-coil drivers. Alnico was used almost exclusively until about 1980, despite the problem of alnico magnets being partially demagnetized by accidental pops or clicks caused by loose connections, especially if used with a high-power amplifier. After 1980, most driver manufacturers switched from alnico to ferrite magnets, which are made from a