Electrodynamic loudspeaker
Encyclopedia
An electrodynamic loudspeaker or field coil loudspeaker is a dynamic loudspeaker in which the field is produced by an electromagnet
Electromagnet
An electromagnet is a type of magnet in which the magnetic field is produced by the flow of electric current. The magnetic field disappears when the current is turned off...

 rather than by a permanent magnet.

An electrodynamic loudspeaker therefore has two coils:
  • The voice coil
    Voice coil
    A voice coil is the coil of wire attached to the apex of a loudspeaker cone. It provides the motive force to the cone by the reaction of a magnetic field to the current passing through it...

     common to all dynamic loudspeakers, positioned in the air gap between the pole piece
    Pole piece
    A pole piece is a structure composed of material of high magnetic permeability that serves to direct the magnetic field produced by a magnet. A pole piece attaches to and in a sense extends a pole of the magnet, hence the name....

    s, the motion of which moves the loudspeaker cone.
  • A fixed field coil
    Field coil
    A field coil is a component of an electro-magnetic machine, typically a rotating electrical machine such as a motor or generator. A current-carrying coil is used to generate a magnetic field....

     which together with its magnetic core
    Magnetic core
    A magnetic core is a piece of magnetic material with a high permeability used to confine and guide magnetic fields in electrical, electromechanical and magnetic devices such as electromagnets, transformers, electric motors, inductors and magnetic assemblies. It is made of ferromagnetic metal such...

     replaces the permanent magnet of other dynamic loudspeakers.


The first electrodynamic loudspeakers were produced in the 1930s, to address the problem that strong permanent magnets of the time were extremely heavy. A compromise was therefore necessary between loudspeaker efficiency, which required the strongest possible magnet, and weight. The use of a strong but relatively light electromagnet solved this problem.

While now uncommon, electrodynamic loudspeakers were once common in top quality mantle radios and similar domestic audio
Audio
Audio is an electrical or other representation of sound.Audio may also refer to:*Audio, audible content in media production and publishing*AUDIO , an American R&B band of 5 brothers formerly known as TNT Boyz and as B5...

 applications, particularly in the post-war period of World War II
World War II
World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...

. In these appliances the field coil of the loudspeaker was also used as the main or only filter choke in the high tension
High tension
High tension is a synonym for high voltage.It can also mean:* The HT, high tension or B battery supply to a traditional vacuum tube circuit, see HT * Haute Tension, a French horror film released as High Tension in North America...

 power supply
Power supply
A power supply is a device that supplies electrical energy to one or more electric loads. The term is most commonly applied to devices that convert one form of electrical energy to another, though it may also refer to devices that convert another form of energy to electrical energy...

 to the valve
Valve
A valve is a device that regulates, directs or controls the flow of a fluid by opening, closing, or partially obstructing various passageways. Valves are technically pipe fittings, but are usually discussed as a separate category...

 anode
Anode
An anode is an electrode through which electric current flows into a polarized electrical device. Mnemonic: ACID ....

s.

Advances in permanent magnet technology rendered the traditional electrodynamic loudspeaker obsolete. The type survives in some high-end hi-fi applications.

External links

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