Acoustics

by Wikibooks, open books for an open world

Available in 123 free installments

Owner:

The Magnet Motor Drive System

The main purpose of the Magnet Motor Drive System is to establish a symmetrical magnetic field in which the voice coil will operate. The Magnet Motor Drive System is comprised of a front focusing plate, permanent magnet, back plate, and a pole piece. In figure 2, the assembled drive system is illustrated. In most cases, the back plate and the pole piece are built into one piece called the yoke. The yoke and the front focusing plate are normally made of a very soft cast iron. Iron is a material that is used in conjunction with magnetic structures because the iron is easily saturated when exposed to a magnetic field. Notice in figure 2, that an air gap was intentionally left between the front focusing plate and the yoke. The magnetic field is coupled through the air gap. The magnetic field strength (B) of the air gap is typically optimized for uniformity across the gap. [1]

Figure 2 Permanent Magnet Structure

When a coil of wire with a current flowing is placed inside the permanent magnetic field, a force is produced. B is the magnetic field strength, $l$ is the length of the coil, and $I$ is the current flowing through the coil. The electro-magnetic force is given by the expression of Laplace�:

$d\underline F = I\underline {dl} \times \underline B$

$\underline B$ and $\underline {dl}$ are orthogonal, so the force is obtained by integration on the length of the wire (Re is the radius of a spire, n is the number of spires and $\underline e_x$ is on the axis of the coil):

$\underline F = 2\pi R_eBnI\underline e_x$

This force is directly proportional to the current flowing through the coil.

Figure 3 Voice Coil Mounted in Permanent Magnetic Structure

The coil is excited with the AC signal that is intended for sound reproduction, when the changing magnetic field of the coil interacts with the permanent magnetic field then the coil moves back and forth in order to reproduce the input signal. The coil of a loudspeaker is known as the voice coil.