Understanding & Recording Guitar Speakers
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Technique : Recording / Mixing
Of all the factors that go into a great recorded guitar sound, the loudspeaker is the least well understood — but the right speaker can make all the difference.
When we talk about recording the electric guitar, we tend to focus first on things like mic selection, mic placement and how to position the guitar amp in the room, all crucially important parameters when it comes to capturing the sound we want. On the other side of the glass, meanwhile, guitarists have their own priorities. As the only group of people on the planet who can rival recording engineers for obsessing about their equipment, they will tweak and tinker with the settings on their amp, effects pedals and guitar in search of the perfect tone.
A loudspeaker is born in the Celestion factory. Here, the cone assembly is being constructed.
Yet in the centre of this little scene there is one crucial factor that is often completely overlooked by both parties: the loudspeaker that is actually making all the noise. In practical terms, the speaker is the voice of the instrument, the source of the sound we’re trying to capture. But how much do we really know about it? In this feature, we’ll be finding out how the speakers found in guitar cabs and combos actually work and discovering how much influence speaker choice has on the sound of the electric guitar. Hopefully, when we understand a little more about what makes guitar speakers tick, we can put this knowledge to good use in the studio.
Beyond simply looking at the theory, I want to find out what different speakers sound like in practice. To do this, I’ve had a rather unusual speaker cabinet built, equipped with four different drivers and a variety of input options, so I can directly compare their response when fed with the same signal. I used this cabinet to record a collection of audio examples, which can be found at www.soundonsound.com/sos/feb12/articles/speakersmedia.htm and are described in the ‘Hear For Yourself’ box.
Speaker Basics
A voice coil being wound.
On the face of it, the speaker in a guitar amp operates just like any other dynamic loudspeaker. The speaker cone is attached to a cylindrical coil of wire — the ‘voice coil’ — which sits in a narrow gap surrounded by a permanent magnet. When an electrical current passes through this coil, it becomes an electromagnet and, depending on the direction of the current, is attracted to or repelled by the permanent magnet. This moves the speaker cone back and forth, which in turn moves the air in front of it, creating sound waves. It’s this simple principle that lets us turn the alternating current originally generated in the guitar’s magnetic pickups back into something we can hear.
However, the speaker’s role in the resulting guitar tone is much more fundamental than merely being the last stage in the process of making that original signal much, much louder. Try comparing the signal from an electric guitar amp’s direct output with the sound coming from its speaker. It’s a bit like comparing the output of an acoustic guitar’s pickup with the sound of it miked up: the same basic information is there, but the depth, detail and character are missing from the direct signal. If this seems a rather obvious point, ask yourself why the electric guitar signal sounds better through the amp’s speaker than the speakers in your headphones or studio monitors, even though the latter are technically superior.
As we are about to discover, while the basic operating principles might be the same, there is one crucial difference that sets guitar speakers apart from almost every other variety. The speakers in your headphones, hi-fi system, studio monitors or PA rig are designed to be as free as possible from distortion and tonal coloration, but in the case of a guitar speaker, these things are not just tolerated but actively encouraged. Just as the amplifier shapes, drives, compresses and colours the signal from the guitar, the speaker stamps its own indispensable personality on the sound.
The completed cone assembly being inserted into the speaker frame.
I asked Ian White, Development Director for British speaker manufacturers Celestion (www.celestion.com), to explain more. “Designing guitar speakers is, in many ways, much more challenging than pro PA or hi-fi, because guitar speakers are so non-linear,” he says. “Hi-fi speakers are designed for linear operation mainly within what’s called their ‘pistonic band’, the region where the speaker is moving in and out in linear fashion. Above that band, the speaker goes into ‘break-up’ — instead of the whole thing acting coherently like a pump or piston, little bits of the cone are all doing their own thing — but then you’d typically move that part of the signal over to, say, a mid-range driver or tweeter. With guitar speakers, there’s almost no pistonic band. Within their usable frequency range, it’s almost all break-up.”
Given that, in a recording context, distortion is usually the sworn enemy, this concept might seem quite alien, but what we’re talking about here is not the nasty-sounding distortion you get from a fuzzbox or an overloaded channel. As Ian explains, it is the non-linear break-up of a guitar speaker and the complex modes and overtones this creates that give it its characteristic tone.
“Imagine I had a load of different panels made from wood, glass, polystyrene and so on,” he says. “If I got you to close your eyes and then hit each one in turn to make it resonate, you’d be able to hear immediately which was which. That’s because the different break-up pattern of each material gives a unique coloration to its sound. It’s the same with a guitar speaker: the break-up pattern gives the speaker its colour. Anywhere above, typically, 500Hz and certainly up in the higher registers, what you’re hearing as the character of the speaker is the tonal break-up. Your ear is latching on to that mess and interpreting colour, a flavour.”
Breaking Up Is Hard To Do
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Mon 21 May 2012
Sound On Sound : Est. 1985