TONE SCIENCE
An in-depth look at how an acoustic guitar works from the scientific perspective of aerospace engineer and luthier Eddie Green BSc(Eng)
Eddie Green has worked in the aerospace industry for many years, contributing to the construction of jet fighters such as the Jaguar, Tornado and Lightning, as well as specialising in stress analysis at English Electric and BAE Systems. But he’s also a keen luthier, at one time working at Fylde Guitars with Roger Bucknall. It’s a rare combination you have to admit. Furthermore, Eddie is also the author of The Mechanics And Construction Of The Acoustic Guitar, so who better to ask for a wholly individual view on the inner workings of the humble acoustic guitar?
Whereas acoustic guitarists might spend hours in deep conversation about the effects of the different timbers and methodologies of the world’s guitar builders, as well as the usual talk of strings, nut materials, scale length and so on, Eddie’s attention is focused more on what he calls the ‘acoustic path’ – that is how a guitar produces tone from the initial vibration of a string and how those vibrations travel through the instrument’s body to produce the tones we know and love. We began by asking Eddie what he believes lies at the heart of an acoustic guitar’s tone…
“The strings play a vital role in the guitar’s tone – they are in effect the ‘heart’ that pumps the soundboard to produce the sound, and it is important to choose the right strings to give you the sound you want. Play them for a few months and the sound becomes dull because those important overtones have been lost.
“Stringed instruments are also unique in that the sound usually contains strong, even harmonics. This is because as the strings vibrate there is a slight increase in tension each time they reach their maximum amplitude, which is twice each cycle. The saddle/bridge assembly then transfers these tension fluctuations into face movements via the main X-braces, which in turn energise the secondary
and finger braces in order to spread the vibrations into as much of the soundboard as possible. The fundamental frequencies are generated simply from the stringtension increments making a slight angle to the saddle, up and down, while at the same time there being a forward ‘tug’ on the bridge that generates a rocking motion twice each cycle.
“The attached soundboard should have no dominant resonant frequencies itself since these would produce annoying ‘wolf’ notes, the ideal situation being that the forced fundamentals and higher harmonics generated by the strings are evenly transferred to the soundboard to produce the sound we hear. The final quality or ‘tone’ comes from a mix of the fundamentals and higher harmonics coming from the strings being subtly blended by local face and body responses, which varies with the materials and the skill and craftmanship of the maker.”
So the relationship between the string and the bridge is far more complex than most players will realise?
“The rocking bit is common to instruments that have steel strings because the steel is stiff and you do get this little added increment of tension twice a cycle, which rocks the bridge at twice the frequency of the fundamental. That was one of the things I discovered and I actually proved it with a little experiment, which is written up in the book. That’s the basics of what the strings do but then understanding how the forces from the bridge get into the bracing of the instrument and then pass around the soundhole into the neck to balance out, that’s a separate stress analysis. Nevertheless, if you understand that, you understand how the soundboard vibrates to produce the sound we hear.”
Talking about transferring the string vibration into the guitar itself, how does the material used at each end of the string length – the nut and saddle – make a difference to the sound?
“It’s best to think about this in the extremes because in between there are lots of materials for saddles and nuts that have a fairly even strength, density or stiffness. If you go for cheap plastic, in particular for the saddle, as the strings go over the plastic will bend forward and backwards as the strings vibrate. This puts a little ‘spring’ into the acoustic path, which will dampen the sound. Bone will be perhaps 10 times stiffer than plastic and that will allow the high frequencies to get through better. If you go to a solid brass saddle or nut then you’re right at the other end as nothing will move and you’ll certainly notice a difference in the sharpness and clarity. But you probably wouldn’t like the sound as the sound we enjoy from a guitar is a mixture of vibrations and resonances of the instrument here and there – the bending of the woods – and if you introduce something that is really hard and harsh it breaks that character.”
Moving onto the elements of the guitar body itself, two contrasting choices for top woods are cedar and spruce – why is there such a difference between the two?
“If you tap a cedar half blank you will get a really rich, warm resonance with a few highs. But if you tap a quality, fine-grained Sitka spruce then the sound is totally different. There will be a higher pitch resonance, more overtones and a lot more attack, and that’s really the difference between spruce and cedar.
“If you’re playing fingerstyle and you want a lot of warm response and you’re not bothered about heavy strumming, a good cedar soundboard is the ideal thing. But if you want to cut through a band or if you want to really attack the strings, you’ll get more what you might call ‘harshness’ from a hard spruce. You can tell right away when you pick up the wood. I like European pine, which is softer than spruce and almost sits in the middle between cedar and Sitka and so you get a bit of both. Douglas fir is actually stiffer than Sitka so you need to make the top a little bit thinner but you get an awful lot of attack. It’s up to everyone’s preferences as to what they want but the two ends for comparisons are cedar and spruce.”
“If you tap a cedar half blank you get a rich resonance. But if you tap a quality Sitka spruce the sound is totally different”
What is the primary job of the back and sides of an acoustic?
“The main job is to support the soundboard. When you’re making a guitar and adding the braces to the top, if you tap it you can hear the different sounds at various points – a bit like a xylophone. Attach it to the body and all that resonance disappears because you’re supporting the edge of the soundboard by the structure and mass of the ribs. If the edge is not held solidly, you end up with problems. The
binding that you put around the edge of the top, the purfling and so on does free the soundboard up, so that it can locally bend more freely, but it doesn’t stop the fact that it’s being held vertically by the ribs. Some manufacturers cut a little shallow groove around the lower bout of the top, close to the edge, and that frees the edge up and you get a little more response.
“The higher frequencies radiate mainly from the area behind the bridge, whereas the lower and mid frequencies bounce around inside the body and come out through the soundhole. The back vibrates to a certain extent and gives you more warmth at the lower frequencies. You can tell that this happens because if you strum the guitar resting on a pillow there is a distinct loss of low-frequency warmth because you have stopped the back from resonating on its own and modulating the sound that comes from the top. So the back and sides’ main job is to support the soundboard and their other job is to modulate the sound that comes out of the soundhole and give that extra warmth.”
Does the shape of an acoustic guitar’s soundhole have an effect?
“The soundhole itself is really a port for the low-to-mid frequencies and the smaller it gets, the higher the pitch of the mid-frequency hump becomes. When you get a very small hole, like on the Maccaferri instruments, it almost sounds like an archtop. With a much larger hole you get the sound that you’d recognise as being from a dreadnought or 000size guitar. I don’t think the fact that the soundhole is, say, oval would make much difference, because if you look at the basic Helmholtz formula, it’s the area of the hole that counts.”
The neck joint for an acoustic guitar tends to be either via a dovetail joint or a bolt-on system. Is there a difference in the way that this would affect the tone? “Technically, there’s not a lot of difference because with a good dovetail, where it fits well up to the last millimetre of fit, you clamp it down and it all squeezes together and effectively you’ve got a continuous path down the neck and through to the body. With the bolt-on it’s the same thing and with the usual two bolts you’ve got an equally continuous path. This is very important since the string-tension fluctuations generated from the vibrating strings are felt at the headstock and these in turn have to be passed back down the neck, across the joint and through the face as compressive fluctuations, eventually being balanced by the forces coming from the bridge. If the joint is anything but solid this will hinder the vibration transfer and affect the clarity of the sound. So the dovetail and bolted [neck joint], if they’re well constructed, will give you a really solid path for the vibrations to pass through into the body.”
Is it true that the shape of the headstock can have an influence on a guitar’s tone?
“The shape is not critical, what is important is the bending stiffness. Because the strings are attached to the tuners on the headstock, the string-tension fluctuations effectively bend the headstock twice every cycle – it bends it up and down – so you will get more sustain from a stiffer headstock because there is less bending. If it’s larger in area it doesn’t make as much difference as increasing the thickness does, but you’re limited to about 15mm in thickness because of the machineheads. With slotted headstocks where the thickness is greater because of the side-fitting machineheads, even though there are slots, it’s much stiffer because the strings are attached to rollers that are within the thickness of the headstock, so there’s a much more direct line of force from the rollers to the neck. A few people say they prefer the slotted headstock because it seems to give them a lot more clarity or sustain and I suspect that it’s the way the strings are attached.”
“The soundhole is a port for the low-to-mid frequencies and the smaller it gets, the higher the pitch of the mid-frequency hump”
So if all these elements that we’ve discussed are present – nut and saddle, top wood, a solid back and sides, good neck joint and so on – the guitar will be performing as well as it can?
“Understanding how the vibrations start and how you end up with a sound is fundamentally linked to understanding how the string tensions are carried within the guitar’s structure. This is all part of what I call the ‘acoustic path’. Understand this in detail and eliminate any weaknesses and you have a good guitar.”
Eddie Green’s book, The Mechanics And Construction Of The Acoustic Guitar, is available now via Purple Parrot Publishing www.purpleparrotpublishing.co.uk