Wednesday, February 29, 2012

Speaker Driver Evolution?

I am a computer geek, among other things. I jumped on the PC band wagon way back in its infancy and watched as Windows grew out of MS-DOS, straying for about a decade into UNIX and then back again to Windows. Today, my taste in operating systems has changed as has the playing field. My smartphone is Android and my notebook Windows 7. My gigabit network is powered by Cisco and I have 15 devices connected to it, 4 hardwired and 11 wireless. What started out back in the 1970s as a fad is today the backbone of the Internet. It has evolved, as it should.

But such an evolution has not occurred in speaker technology. While there are new piston materials, new magnet materials, new approaches to damping resonances and increase power handling capacity, new surrounds, and new modeling parameters that simplify cabinet design, the speaker driver of choice by the overwhelming majority of manufacturers is still the old dynamic driver.

Components of a Dynamic Speaker Driver

Dynamic drivers have been around since 1924 (invented by General Electric engineers Chester Rice and Edward Kellogg) and have what is called a "linear motor" at its core.

The linear motor, the "engine" behind this type of speaker driver, has been kicked around since 1840 (Charles Wheatstone at King's College, London) with refinements to its design ongoing. The operation of a linear motor used in a dynamic driver is simple. Basically, a coil of wire is suspended inside a magnetic field to which a diaphragm (aka piston) is glued. Applying an electrical signal to the coil moves the piston in and out as the coil tracks the signal back and forth inside the magnetic field.

There are numerous places for error to occur with such a design (flexibility of the glue, non-linear distortion introduced by the surround and spider restricting movement to name just two), and yet we persist in trying to force more from a 90 year old technology.

The main issue is that air is passively pushed by the piston. This is sort of like passing gossip along to friends where eventually the original story is very different from what was first told. Any time you try to push something in a passive manner, there is a non-linear reaction to the push. That is, the air does not instantaneously respond to the movement of the piston because of its inherent flexibility. So even if the characteristic distortions of dynamic driver design could be completely eliminated, the elasticity of the air still produces additional distortions beyond the control of the driver.

The best one can hope to do is to more efficiently couple the air to the piston (use an acoustic matching transformer called a "horn") and keep the piston excursions small so these inherent distortions of dynamic drivers are minimized.

OR, and this is where science is presently stumped, OR find yet another way to move the air that does not require these limitations (somehow eliminate the piston). Diaphragm-less drivers have been bantered about using plasma schemes (aka "flame" or "RF" loudspeakers), that produce amazing clarity while introducing ozone into the air that can harm you after prolonged exposure.

A Plasma Tweeter by Corona Acoustics, GmbH

So we have a way to go in perfecting this technology if what we want to do is to move the air in such a way. Unless we find a brand new way to move air without such inherent limitations, we will be pretty much stuck with the nearly century old dynamic driver.

If there is something to invent, this is it. Hopefully, just as computer operating systems have evolved, so will speaker driver technology.

Yours for higher fidelity,
Philip Rastocny

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