Positive Quiddity: Planar Magnetic Speakers

Introduction

　

Ribbon and planar magnetic loudspeakers

A ribbon speaker consists of a thin metal-film ribbon suspended in a magnetic field. The electrical signal is applied to the ribbon, which moves with it to create the sound. The advantage of a ribbon driver is that the ribbon has very little mass; thus, it can accelerate very quickly, yielding very good high-frequency response.

Ribbon loudspeakers are often very fragile—some can be torn by a strong gust of air. Most ribbon tweeters emit sound in a dipole pattern. A few have backings that limit the dipole radiation pattern. Above and below the ends of the more or less rectangular ribbon, there is less audible output due to phase cancellation, but the precise amount of directivity depends on ribbon length. Ribbon designs generally require exceptionally powerful magnets, which makes them costly to manufacture. Ribbons have a very low resistance that most amplifiers cannot drive directly. As a result, a step down transformer is typically used to increase the current through the ribbon. The amplifier "sees" a load that is the ribbon's resistance times the transformer turns ratio squared. The transformer must be carefully designed so that its frequency response and parasitic losses do not degrade the sound, further increasing cost and complication relative to conventional designs.

Planar magnetic speakers (having printed or embedded conductors on a flat diaphragm) are sometimes described as ribbons, but are not truly ribbon speakers. The term planar is generally reserved for speakers with roughly rectangular flat surfaces that radiate in a bipolar (i.e., front and back) manner. Planar magnetic speakers consist of a flexible membrane with a voice coil printed or mounted on it. The current flowing through the coil interacts with the magnetic field of carefully placed magnets on either side of the diaphragm, causing the membrane to vibrate more or less uniformly and without much bending or wrinkling. The driving force covers a large percentage of the membrane surface and reduces resonance problems inherent in coil-driven flat diaphragms.

From: http://en.wikipedia.org/wiki/Loudspeaker

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Loudspeakers are available with planar magnetic technology and so are some very expensive headphones. Geoffrey Morrison of Forbes recently reviewed the Audeze LCD3 planar magnetic headphones, which cost $1,945 for a set.

Morrison tells us that these expensive headphones don’t look like other headphones and don’t sound like them either; "they sound much, much better," he writes.

His article in Forbes has a link to a video that shows how planar magentic drivers work to produce sound. He also notes how this sounds to the listener:
"Compared to most headphones, the LCD3s seem like they’ve taken a layer of film off the music. They’re not brighter then other headphones, specifically, or even more detailed (which would imply an overbalance of high frequencies). Full-size planar magnetic speakers have this same quality, and I figure it’s because of the speed at which the drivers react. Percussion hits, like the crack of a snare drum, are quick in real life and fade rapidly. Through the LCD3s these attacks just seem a little more lifelike in that speed and decay."Morrison also likes the way the bass sounds through these headphones; "There’s good bass and then there’s bad bass. These have good bass. It’s a very warm, full sound, that doesn’t overwhelm. There’s a tight accuracy to it: A bass drum sounds like a bass drum, a tuba sounds like a tuba. It’s not just a wall of low frequency energy like on many headphones."

Morrison also notes that although these headphones cost nearly $2,000, they are worth it. If the price is horrifying, though, there is the Audeze LCD-2 headphone available for half the price.

The Forbes article is online at: http://shopping.yahoo.com/news/audeze-lcd3-review---the-ultimate--1945-headphones--012301430.html