Speaker Geeks: Three-Part Harmony

When building a guitar speaker, one can make 101 micro-adjustments, whether it’s an extra turn of coil wire here or a thicker bead of glue there. However, these things are secondary to the three key elements that control most of a speaker’s tone, and it’s the speaker designer’s prime directive to get these three elements working together in harmony.

The membrane. Chief among these things is the sound-producing membrane—the cone. Guitar speaker cones are (still!) made from pulped and chemically treated paper pressed into the conical shape we’re all familiar with. After all these years, we still use paper because it provides the attributes we need: It’s lightweight, relatively (but not too) rigid, easy to shape, and cost-effective to produce.

Despite being made of humble materials, the cone is by no means a simple thing, and we can introduce variation into its production to control tonality. Thickness of the membrane is important. I’ve already touched on this in a previous column, but it bears highlighting again: A thinner cone deforms more easily under the pressure caused by coil movement. If you make the cone thicker, it becomes more resistant to those forces. However, these “deformations” are part of what contributes to the speaker’s tone. The skill of the speaker designer is to make a cone that’s thin enough to produce a musical tonality, while still being strong enough to withstand the application of a satisfyingly powerful kerrang.

The shape of the cone itself is also interesting. Way back in the days of radio, when a 1-watt amp was as powerful as you could get, straight-sided cones were used to tease as much output from the system as possible. The geometry of a straight-sided cone made it stiffer, so it could be constructed lighter, which enabled greater sensitivity and thus higher output.

It turns out that if you put enough signal into a speaker with a straight-sided cone, it eventually reaches a point where it suddenly and very quickly breaks up and resonates. Any audiophiles worth their salt would likely cover their ears in disgust, but that’s what we guitar players know as tone. And this explains why the modern guitar speaker evolved from the old-fashioned, straight-sided radiogram speaker.

Poles apart. When Celestion first started making moving-coil speakers for guitar amps in the late ’50s, we used an alnico magnet. This was the most commonly available technology at the time and what was used for all loudspeakers, regardless of application.

Forward to the 1960s and alnico became increasingly difficult to source, so guitar speakers were made instead with an iron-based ceramic magnet. While it was relatively simple to replicate the amount of magnetic force of an alnico magnet with the ceramic material, the two magnet types produced a noticeably different tone.

With its more aggressive, in-your-face sound, coupled with a high-end “graininess,” the ceramic magnet assembly was perfect for the music that rock ’n’ roll was evolving into during the ’60s. Compare this to alnico, which was relatively laid-back in its attack and delivered an overall smoother feel and a distinct chime at the top end.

After further experimentation, engineers and designers discovered just how much the size of the magnet affected tonality. By the end of the ’60s, Celestion offered their original ceramic guitar speaker in three different magnet sizes: the 50-ounce H (heavy), 35-ounce M (medium), and 20-ounce L (light). As magnet weight increases, so does the control of voice coil and cone movement. This has the effect of tightening the bass end and adding extra aggression in the vocal range.

In recent years, neodymium has become another magnet material used for guitar speakers. It’s very powerful for its size, and if properly controlled, neodymium can be used to provide a tonality that’s somewhere between that of ceramic and alnico, but with an additional level of note separation and fidelity that many consider desirable.

Wind me up. The voice coil is a length of copper wire wound around a “former.” It’s the moving part of the speaker’s motor whose configuration and performance is very much wrapped up in the amount of power a designer can make a speaker capable of withstanding. This, too, affects tone.

Consider the coil-former material: A paper former, as used in the 1950s, is less heat resistant than, say, fiberglass. The difference in material properties will impact the speaker’s tolerance of heat, which contributes to overall power rating (heat resistance directly correlates to power handling). And because various former materials sound different, this too contributes to the speaker’s tonal character.

Just as important is the diameter of the voice coil. A smaller coil flexes less than a larger one, but results in a larger cone length (the distance between cone neck to surround), meaning the cone is more likely to bend. A bigger coil will flex more, but the cone length is shorter, which means the cone is geometrically stiffer. A smaller coil will also have less mass and less inductance. All these physical attributes impact the sonic signature. Smaller coil speakers tend to feel brighter and break up more aggressively. Their fans consider them to be “sparkly,” whereas detractors would say they were “lightweight.”

More than anything, speaker tone is a direct result of a bunch of physical processes doing different things all at the same time. The better we can get these individual elements to work together in concert, the more desirable the resulting tone.