Planar Magnetic Speakers

What Are Planar Magnetic Speakers?

The planar magnetic speaker is a type of speaker closely related to the electrostatic speaker. It is nearly identical to an electrostatic loudspeaker in radiation concept, having a large, thin, vertically orientated plastic diaphragm as the vibrating element. Planar speakers use conventional magnetic principles, however, and are dynamic loudspeakers. Thin wires from a plastic diaphragm form an equivalent of a voice coil stretched out lengthwise. Magnetic strips are placed in the front and back of the diaphragm, where the stators would be in electrostatic loudspeakers. Magnetic interaction, instead of static interaction, causes the diaphragm to vibrate. Magnepan, a Minnesota company, is the best-known manufacturer of planar magnetic loudspeakers. The company founder, Jim Winey, designed the first model, the Magneplanar Tympani I, in 1971. It contained three tall panels that operated as dipoles.

The Construction

The term planar means that the speakers have roughly rectangular flat surfaces that radiate in a bipolar (i.e., front and back) manner. Planar magnetic loudspeakers with wide diaphragms, such as woofers and midranges, usually have vertical rows of magnets, with each row alternating between north and south poles. Narrow planar magnetic loudspeakers, or ribbons, have a diaphragm directly between the north and south poles of the magnet. Consequently, it has its place within the magnetic field. In this project, a loudspeaker with a 31/2‐in. The wide diaphragm has a ribbon configuration. The magnetic field is more linear with diaphragm excursion. It is distributed more evenly over the surface of the diaphragm. A smaller emphasis was placed on efficiency so that the speaker would have a diaphragm surface area and excursion comparable with cone‐type speakers.

Usage

The planar magnetic drivers found their use in some of the high-quality headphones. Moreover, they possess some advantages compared to other drivers producing the headphones. Planar magnetic drivers are like electrostatic speakers (ESLs) in that the moving mass of the diaphragm is extremely small, lighter even than the air it moves. Therefore, you can drive them delicately and accurately without the blurring effects of excessive inertia. Planar magnetics have a huge advantage over traditional dynamic drivers in that the "voice coil" spreads out over a large area rather than squeezed into a narrow gap within a massive chunk of metal. As a result, planar magnetic drivers can effectively dissipate heat that would otherwise build up in a traditional voice coil.

Power Handling

Planar magnetic speakers can handle much power without undue stress or audible strain. In fact, they can take many times the power of a traditional dynamic driver for a given surface area. Well-designed planar magnetic speakers present an almost perfectly resistive load to the power amplifier in the system since the voice coil is essentially a very long, thin wire. It acts like a simple resistor. This is the easiest possible load for the amplifier to drive and comparable to the one electronics manufacturers use during measurements designed to show how terrific their products perform. As a result, you can count on your amplifier sounding its best.