Moving Iron Speaker
Last Edited: Dec 26, 2023
What Is a Moving Iron Speaker
One of the earliest types of speakers is the moving iron speaker (also known as a moving-coil loudspeaker). These are still used in some small speakers, where developers prioritize small size and low cost over sound quality.
The Design
Their design mainly consists of a ferrous-metal diaphragm or reed, as well as a permanent magnet and a coil of insulated wire. The main characteristic is that the coil is wound around the permanent magnet in order to form a solenoid. A coil, called a voice coil, resides in a uniform magnetic field. When an audio current passes through the voice coil, there is an interaction between the magnetic field and the current, resulting in a force working on a movable coil. This force is proportional to the audio current. It causes vibratory motion (motion-like action) in the coil, making a conical paper diaphragm vibrate and producing pressure vibrations in the air, resulting in sound waves.
Iron Speaker Types
There are several types of moving iron speakers. Old, undamped, moving iron speakers have a characteristic sound and probably the worst sound quality of any known type of speaker usable for speech. Modern damped moving iron mechanisms can provide respectable sound quality, and various headphone types use them.
Moving Coil Loudspeaker
The moving coil loudspeaker consists of a voice coil (single-layer winding of fine enameled wire) wound on a cardboard or fiber cylinder. Audio current is fed to it through two terminals. The coil's location is within a magnetic field. The magnet is a p-type permanent magnet with a central pole (South pole) and a peripheral pole (North pole). Its structure contains high-grade magnetic material, like alnico (a composition of alnico is aluminum 10%, nickel 18%, cobalt 12%, copper 6%, and iron 64%), which retains the magnetism extremely well. The magnet is a shaper that gives a solid radial magnetic field in the annular space between the central and peripheral poles.
Permanent Magnet Type Speaker
The voice coil is free to move in the annular space, having a strong and uniform magnetic field. Because of the use of a permanent magnet, sound engineers and producers also call it a "permanent magnet type speaker." The coil is attached to the conical diaphragm, made of paper or parchment. It is called "Paper cone". The cone has circular corrugations. A flexible strip of rubber around its periphery supports it. The slider strings support the complete diaphragm and provide the required stiffness to restrain the motion. The spiders also keep the coil centered so that the cones move forward and backward only. Leads from the voice coil are cemented to the cone surface. From there, they are brought to the terminals and mounted to the metal frame or basket. When the motion of the coil is small, it will remain within the region of uniform flux density. Thus, the output is linear for the small motion of the coil. When the coil moves in the magnetic field, the electromagnetic action produces the back e.m.f. This makes the motion of the coil steady.
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