What is a Moving Iron Speaker


Moving iron speaker (also known as a moving-coil loudspeaker) could be considered as one of the earliest types of speakers. Their use is still present in some small size speakers where developers are not turning to sound quality but rather small size and low-cost production of the speakers.


The Design


Their design mainly consists of 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 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 and hence causes vibratory motion (motion like action) in the coil, which make a conical paper diaphragm to vibrate and produce pressure vibrations in the air, resulting in sound waves.


Iron Speaker Types


There are several types of moving iron speaker. Old undamped moving iron speakers have a characteristic sound, with 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 it.


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 pot-type permanent magnet which has 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 so shaper as to give a strong 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 spider strings support the complete diaphragm and also provide a 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 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 for the small motion of the coil, the output is linear. When the coil moves in the magnetic field, the electromagnetic induction produces the back e.m.f. This makes the motion of the coil steady.


Source Texts