Author Archives: Michael Williams

About Michael Williams

I was born several centuries ago in 1950. Music has always been important in my life, so I'm spending my retirement making guitars and trying to write songs. I spent my working life as a teacher, so it's important to me to pass on knowledge to other people who might benefit from my experience. That's what this site is for. My idea of fun is asking basic questions about things that most people don't worry about. This means I'm only impressed by traditions that are actually helpful - which quite a lot are, of course. As a guitar maker it means that I look for new things to do rather than following along with what tradition suggests is right. So I hope you'll find something useful here, and you'll find it in your heart to forgive my bad jokes...


As I do with every component of a guitar, I want to understand the purpose of the back plate so I can build it in the best way I possibly can. Gore and Gilet’s book Contemporary Acoustic Guitar has been very useful to me (as always) by raising the possibility of building what they call a “live back” – in other words, a back plate that contributes to the overall tone of the instrument by vibrating along with the top when the strings are sounded. This adds a complexity to the tone that wouldn’t be there with a non-live back, so should be desirable.

I’m not convinced I have succeeded in this yet. In this page I’m going to show how I make and brace the back plate, and evaluate my current success in the quest for the live back. I have adopted the back brace plan used in Trevor Gore’s instruments as a starting point, as you’ll see. At the end of this entry I’ve put some evidence that indicates I haven’t breathed life into my backs just yet. Perhaps I need a thunderstorm and lots of electric arcs and an assistant called Igor.

But now I want to show you some test results that have got me thinking more about this concept.

What is the purpose of the back plate?

“Traditional” steel string guitar backs use a ladder bracing system that I also used in my early attempts. It looks like this:

Traditional ladder bracing

Traditional ladder bracing

The purpose of the back braces has always been seen as mechanical rather than tonal. Thin panels tend not just to flex under load but to distort as they absorb and lose water. Wood always expands much more across the grain than along it, so this can lead to weird shape chances unless stiffly braced. The back centreline join can also be pulled apart by dimensional changes in the wood if the guitar is moved into a dry climate after being built in a humid one.

Many people argue that when you play the guitar your body is in contact with the back and damps out all the vibration anyway, so attempting a live back is pointless. This isn’t at all true, as I think I can show you.

The T(1,1)1 “breathing mode

A guitar’s simplest mode of vibration is called the “breathing” or T(1,1)1 mode. You can see it in this animation as Mode #2:

This simple mode would not exist if the back were not there. It’s as simple as that. The “Breathing” mode is low frequency (usually around 100Hz for a guitar) and contributes bass boom and depth to the sound.

But can the back contribute higher modes to the overall guitar sound as well? This is what we mean by “live back”.

Can I convincingly show you a live back in action? Sadly, no. But I live in hope…maybe this guitar will achieve it. Stay tuned!

What does the back actually do when a guitar is played?

This isn’t as simple a question as it sounds. Gore and Gilet’s analysis shows that a soundbox is made up of three separate resonators: the top, the back/sides, and the air contained in the box.

Each of these has its own fundamental frequency and an associated string of harmonics or partials, as does every resonator. Here, for example, is the frequency spectrum of a singing wineglass – the highest peak is the fundamental, and the other peaks represent the inevitable series of higher frequency partials produced by different modes of vibration.


Here is the string of a guitar string ringing. It behaves in a very similar way, although in this case we can call the partials “harmonics” because they have a simple mathematical relationship with the fundamental, which partials do not:


Where it all becomes complicated is when you start to join simple resonators together, as you do when assembling a guitar. Because they’re connected, each one influences the others, and not in a simple way.

Okay, enough physics for now. Let me show you what happens when you start to mess with one out of the three joined soundbox resonators. I’m going to use some data from tapping one of my Jumbo guitars to illustrate.

Firstly, here’s the spectrum produced by tapping the top with the guitar held up by the neck and no contact with the soundbox:

Jumbo top response held free from body

Jumbo top response held free from body

Notice that it isn’t a simple spectrum like that for the wineglass, although there are a number of peaks that stand out. Notice particularly the first one at around 100Hz, which is the total resonance of the top, back/sides and airbody (the T(1,1)1 or “breathing” mode we’ve already seen). Now look at what happens when I hold  the guitar in my normal playing position resting lightly against me:

Jumbo top response in playing position

Jumbo top response in playing position

I have kept the original signature (the red line) so you can see the effect of body contact (the black line). Very little at all, wouldn’t you agree?

Okay – one last graph before we get on with construction details. This one shows what happens when I aggressively damp the back by pressing my hand firmly against it while tapping the top:

Jumbo top, back heavily damped

Jumbo top, back heavily damped

See the difference? Again, I have left the first freely-held response (red line) for comparison. The most striking effect of heavy back damping is the complete disappearance of the first peak in the response, the “breathing” mode. Immobilising the back has broken the link between the three resonators in a dramatic way. The tap sound was an anaemic thunk.

What does this tell us? Well, it makes clear a number of vital things about how a guitar works:

1. Messing with one of the three coupled resonators affects the other two, sometimes dramatically. This understanding opens up possibilities for tuning the soundbox if you know how.

2. The response with the guitar held in the playing position shows very little difference to the guitar held freely – the T(1,1)1 airbody peak is still strongly there. This suggests that the back continues to have a strong influence on sound production when playing, and that the common wisdom (that the player’s body contact completely damps the back and makes it useless to strive for a live back) is emphatically wrong.

3. The “breathing” mode is important to the production of sound by a guitar, and it isn’t there unless there is a back to contain it and importantly that it is free to respond to the strings. When tapping  with the back aggressively damped, the tap sound lost its depth and became a rather pathetic thud. This is because with the back immobilised, the box can’t “breathe”.

To sum up:

a) the guitar back is a very important factor in determining the overall sound of the guitar, and deserves close design attention;

b) all guitar backs are “live” in the sense that they are essential for the guitar to produce a sound with any depth (damp it aggressively and the “breathing” mode dies);

c) I haven’t yet achieved the extra Gore/Gilet step of building a back that further enhances the sound by adding complexity through its own resonant response on top of the “breathing” effect.

Building the back

This is the bamboo guitar back nearly finished, showing the Gore-pattern ladder/radial bracing. Here I have been cutting the “gable” with a brace chisel (just visible bottom left). The back is resting is the 20ft radius dish to preserve the curvature as the braces are fitted.

The Gore back bracing design

The Gore back bracing design

These are the spruce brace blanks machined to size:

Brace blanks (back braces on the right)

Brace blanks (back braces on the right)

The are the steps in putting a back together are:

1. thinning the panels to the right thickness;

2. joining the two panels to get enough width for a back plate;

3. bracing the back plate;

4. attaching the back plate to the sides.

Thinning the panels

I thin the back panels using a drum sander before joining them:

Drum sander

Drum sander

I check the thickness often with vernier calipers. A trick I have learned in using a drum sander is that you don’t need to adjust the drum height for each pass, especially when you’re nearing the thickness you want.

Vernier calipers

Vernier calipers

This way I can control the thickness to an accuracy of 0.1mm. In another page (The bamboo guitar – Part 3) I have explained how I chose what thickness I would work to for the bamboo. My aim was to produce a back with properties as close to my usual blackwood backs as I could.

Once the panels are the right thickness it’s time to join them together. Though initially it daunted me, it’s surprisingly easy to get a good invisible joint. Here’s a top I’ve joined using my method (the chalk marks help keep the two panels in the right relationship while I’m working on them):

A top joint

A top joint

These are actually spruce top panels, but the principle is the same and I forgot to take photos for the bamboo back.

The first step is to plane the joining edges straight on a shooting board, then fitting them together on a flat surface to look for gaps. Using an old plane with sandpaper stuck to the sole,  with care you can remove the high spots and get an invisible joint. The trick is to get one edge as straight as possible, then adjust the other to fit it. The shooting board allows you to keep the edges square as you work on them.


Joining the panels

Once you’ve got the edges fitting perfectly, it’s time to join them together. I use a simple but effective setup that I got from somewhere I can’t remember. I have a flat table with a batten fixed along each side, one of them adjustable so I can clamp different size pieces.

The jointing table

The jointing table

I slip a small batten under the the panels where they join so the edges are slightly lifted, and adjust the edge battens to just hold them in place:


To make sure the panels stay in vertical alignment, I hold the joint down with cork-backed blocks and go-bars.

When the batten is slipped out the edges are gently forced down to make a beautifully tight joint. Oh, and don’t forget to put glue on first, and put wax paper down so you don’t glue the panels to the table…

Once the glue is set, you can take the joined back plate out of the clamps, clean up any dried glue, and cut out the correct shape.

Adding the back braces

Time to get out the old 20ft sanding dish again, last seem while profiling the side assembly. This time it will be used for two things:

a) sanding the braces to the right curve;

b) acting as a mould to make sure the back conforms to the right curvature.

The first bracing piece is the marriage strip, a piece of 20 x 4mm spruce kept from past top panel production. It is glued along the centre joint to reinforce it, so it’s important to cut it so that its grain crosses the joint at right angles – you can see the strip across the bottom of this photo:

Marriage strip

Marriage strip

The marriage strip is glued on and the whole back pressed into the dish using go-bars. Glue is slippery, so keep a sharp eye on the strip to make sure it doesn’t absent- mindedly wander off before the glue grabs. I also make sure that I know exactly where the marriage strip must end to meet the end blocks neatly once the back is glued to the sides.

The back brace blanks have already been machined to 20 x 10mm, keeping the grain vertical. I cut each one to length, mark out the scallops, then sand the bottom surfaces to the 20ft curve by rubbing them in the sanding dish, I the cut out the scalloping on my bandsaw and finish them with a small sanding drum in my drill press.

I use a sharp X-acto knife to cut through the marriage strip and a sharp chisel to remove wood so the braces can be glued in.

Here’s how it looks with the go-bars in place:

Braces held in place with go-bars

Braces held in place with go-bars

Next to go in are the radial tone bars:


The scalloping of the braces reduces the mass of the back, but importantly also allows some later tuning of the back resonances once the guitar is finished. (Remember we’ve found that messing with one of the three resonators will affect both the others.) The general principle is that the thicker the braces the higher the fundamental resonance of the back plate will be. If I want to lower the back resonances later, I can carefully thin down the central scallops working through the soundhole.

By the way, the idea that you can tune each brace separately doesn’t work in practice (or even in theory, for that matter – sorry Roger Siminoff) because the back responds as a whole, not a simple sum of its individual parts. Maybe there’s a saying in that somewhere.

Finally, I use a very sharp chisel to carve the top edge of each brace at an angle so it comes to a point like a gabled roof. This reduces their mass but pretty much preserves their elasticity.

Joining the back to the sides

The sides of  the soundbox have already been shaped to match the curvature of the back plate (see The bamboo guitar – Part 6) so it’s now a matter of joining the two assemblies. Because I don’t use an edge binding to hide the joint, I need it to be a perfect fit and firmly clamped:

Joining the back to the sides

Joining the back to the sides

I use cylindrical screw clamps from Stewart-MacDonald (  Once the glue has set, I can carefully trim the overlap down and sand the edge so it is smooth and slightly rounded off.

I now put on a coat or two of epoxy to seal and harden up the grain, leaving the guitar looking like this:

Epoxy sealing coats on

Epoxy sealing coats on

Not bad for bamboo, wouldn’t you say?

And now we return to the live back issue…

So how will I know if I succeed in producing a live back? The evidence would be there if the spectral signature of the back, or at least parts of it, showed up clearly in the overall signature of  the guitar. This sounds simple, but actually isn’t.

We’ve seen that the three resonators, the airbody, the top, and the sides/back that make up the soundbox of a guitar, link together as a whole but not in a simple way – each one changes the others in subtle ways. So if they’re linked so intimately, how can we sort out if any part of the overall sound (other than the breathing mode) comes from the back specifically? Can we isolate what the back is up to and compare it to the overall tonal signature?

There is a way to decouple the top signature from the back signature: block up the soundhole. This takes the airbody out of the mix because it can’t breath, leaving the top and back much less intimately connected.

Here is a comparison of the overall tonal signature of the guitar (the top tapped with the soundhole open) – the red line –  in the normal playing position. The blue line is as close as we’ll ever get to knowing how the back would respond on its own. What I’m looking for is a convincing overlap between the two, showing that the back is contributing to the overall sound.

No evidence of a live back here

No evidence of a live back here

The outstanding feature of the back response is the peak at 210Hz, and maybe the overall response is higher there as a result. Maybe. The airbody response isn’t present because the soundhole is blocked.

Except for a teensy little peak at 175Hz and a fat-looking feature between 210 and 230Hz, I can’t see any convincing evidence that the back is having any influence on the overall sound. Oh, well.

And for my next trick…

Next I’ll describe making and fitting the top panel, the most crucial element in any guitar.

The bamboo guitar Part 6 – the wedge, blocks and splints

A strangely painful-sounding title, wouldn’t you say? I’m a bit worried how this will turn out, but I promise you won’t end up with a wedgie or a splint as a result of reading it.

Top and tail blocks

The end blocks hold the two halves of the soundbox together. They are important for the structural integrity of the guitar, particularly the top block that has to absorb the stress of the neck joint under string tension of around 70kg weight. It has to be a substantial chunk of wood accurately fitted to the soundboard, sides and back. That 70kg weight is another reason I like laminated top linings – imagine an adult man standing on the end of the guitar.


I make mine from two blocks of Niugini Rosewood joined at right angles and pinned vertically with two 9mm dowels, all held with Titebond glue. The reason for the shape is that the guitar will have a bolt-on-bolt-off neck, so there needs to be space for horizontal and vertical bolts to land. (In this picture the guitar is resting in sanding dish because I am profiling the bottom edge to the right curvature before fitting the side splints and the linings.)

The front face of the block is shaped to fit by temporarily taking the sides out of the construction mould and sticking some adhesive backed sandpaper where the block will land. The block then gets sanded to an exact fit – and I do the same with the tail block before putting the two half sides back into the mould.

Both blocks then need to have a channel cut into them to fit the 18x6mm top linings. Some careful shaping is needed to get a good fit, because of course the linings have a subtle curve to them. When there’s a good fit I glue them both in place  with firm clamping.

The tail block will have to support the strap button and the jack for the pickup, so again I laminate it up out of four 4mm thick pieces with grain crossed to stop it splitting if the guitar is ever dropped on its end.


In this picture you can clearly see the notches in the top lining waiting for the side splints.

The tail wedge 

It’s customary to cut away the sides where they join at the tail and replace them with a nicely-contrasting wedge of wood – here I’ve used blackwood. To get a good fit. I make the wedge first, clamp it to the guitar and carefully cut into the sides, and then remove the waste with a chisel. The subtle shape means the when you tap the wedge into place it fits very tightly for a hardly-visible joint.


Profiling the back edge

Once the tail wedge is in and trimmed off, I profile the bottom edge of the soundbox before I glue in the side splints (in all the pictures so far, the guitar is resting in the 20′ radius sanding dish). No matter how hard I try, somehow the sides never match the designed profile and I have to adjust them.


Side splints

The side splints in a guitar are small strips of wood glued vertically to the sides between the soundboard and the back. They reinforce the side panels by crossing the grain to stop splitting, and they add extra rigidity. They are quite important, as Gore and Gilet point out, because when a guitar is played, at the simplest level of analysis the soundboard and the soundbox necessarily move out of phase with each other. The more rigid the soundbox back and sides are – that is, the more the behave like a single unit – the more efficiently the soundboard can vibrate.

So I take my splints seriously now, where before I used to slap in a few little matchsticks made from scrap spruce and move on. I now laminate them from hardwood and notch them carefully into the top and bottom linings. I make them from three 2mm laminations of leftover Blackwood, so they’ll be the same 6mm thickness as the top linings. They’re 9mm wide, and carefully shaped to fit the  curvature of the sides.


It’s much easier to fit the splints a bit over-long, then trim them with a saw to match the depth of the side panels – otherwise you’re left sanding end grain when doing the final profiling of the back.



The last task before fitting the back panel is to install the kerfed lings. I fit them by bending a length of the lining around the curve between each side splint, mark the length, and cut the length off with a bandsaw. If they’re cut slightly overlength you can fit them exactly by sanding the ends.


Each piece is glued in, and they need to be clamped. I use spring clothes pegs, with the odd more powerful small spring clamp when needed.

The thing to look for here is that the lining fits well against the sides. Because you’ll be working with the soundbox face down, you can sometimes get an unpleasant shock when it’s all finished – you proudly turn it the right way up and find the top edges of the linings are standing slightly away from the sides.

With a bit more work in the sanding dish the linings and the ends of the side splints will be profiled exactly to mate up with the back plate when it’s ready to fit.

The bamboo guitar Part 5 – laminating the top linings

OLYMPUS DIGITAL CAMERAThe top linings form the joint between the soundboard and the sides of the guitar. Generally guitar makers use kerfed linings like the one above. They’re easy to bend around the curves, and they act like a series of small reinforcing blocks to tie the top and sides together. They work well, and I use them for the back-to-side joint.

But I have a theory that solid top linings are better at transferring vibrational energy from the soundboard to the soundbox body. They also add greatly to the overall strength and structural rigidity of the guitar. So I laminate them out of thin strips of Australian oak, a wood that takes to heatbending well. In future I think I’ll use bamboo instead, because the trees the “oak” comes from, Eucalyptus regnans or Mountain Ash, are precious. They’re one of the tallest tree species in the world:


I cut the strips on my table saw from a blank that I have thicknessed to 18mm in my drum sander. I then thickness the strips to 2mm in the other dimension (actually, I stay in this dimension to do it) by feeding the cut faces through the drum sander. Using good old Titebond  glue, I laminate three 2mm thicknesses in a mould to end up with a pre-shaped lining of 18x6mm.


To make it easier I prebend the strips using the side bender, otherwise they put up a hell of a fight.

Here’s what it looks like out of the mould:


What I’m doing here is cutting notches to house the top of the side-splints that will reinforce the sides. The splints also add rigidity to the sides, and a bit of useful mass as well.

I then glue the linings onto the sides, after which I can saw the ends flush with the construction mould. After that I bolt the two sides of the mould together ready to take the top and tail blocks that unite the sides. Notice the very clever placement of baking paper to avoid gluing the guitar to the mould. Yes, we guitar makers are a cunning lot.


The bamboo guitar Part 4 – bending the sides

I use a mould and heat blanket to bend my sides rather than try to do it freehand with a heated pipe. Lately I’ve been thinking about trying a temperature-controlled electric bending pipe instead, in the hope (or is it delusion?) that it might give me more control over the final curvature.

When people ask how I bend the sides, I tell them I use a bender. Here’s what I think flashes through their minds before they hit me for being a smart-arse:


No, but seriously folks, people are often overly impressed by bent sides on a guitar. It was the thing that intimidated me the most when I started building, I know, but I’ve found that while there’s a very bountiful source of possible stupid mistakes available to choose from, it’s actually not that hard.

My side bender is a hollow mould on a base, with a waist clamp to draw the wood down into the concave bend and two clamps each at the head and tail to draw the side around the convex curves:


The sides of the mould define the shape, and it’s useful to slightly exaggerate the curves because the bent wood always springs back a bit when it’s removed. Around the edge of the mould you can see the ends of the rods that go from from one side to the other to form the actual bending surface.

The clamps are made from threaded rod and wingnuts from the hardware shop. You can’t see the end clamps that pull the front and back downwards to put the whole piece under tension, but they’re just eyebolts with a wingnut on each that act on the undersurface of the mould base.

There is a thin metal sheet above and below the wood that make up a metal/wood/metal sandwich to stop the grain breaking around the top surface, which is under tension. I use aluminium, though springy stainless steel is better in some ways but very hard to work.

Here are the first two metal/bamboo layers of the sandwich, ready for the blanket and then the second metal sheet on top. I put baking paper in between and give it a squirt of water to help the bend:


The heat is provided by a silicone rubber thermal blanket connected to a temperature controller. In the picture below you can see the thermocouple temperature sensor and the controller that allows heater to work safely and accurately.


Here is what the bent side looked like after the first attempt at 180 degrees C, which is what the manufacturers recommend. You can see that the waist doesn’t conform to the mould shape at all well, so I had to re-bend it several times at higher temperatures before it came right, ending up at 230 degrees:


Here is one bent side clamped into the building half-mould while still warm to allow it to keep the bend:


The bamboo behaved really well during the bending, and showed no sign of delaminating. So far, so good.

In my scheme of things, the next job is to make up the laminated top linings and fit them.

A bamboo guitar Part 3 – bamboo compared to blackwood

In my last entry, I had just finished measuring the fundamental frequencies of my bamboo panels in three different vibrational modes:

  1. along-grain free beam (marimba) mode: 109.0 Hz
  2. across-grain marimba mode: 226.1 Hz
  3. twist (torsional) mode: 80.1 Hz

What I am aiming at is to compare the stiffness of the bamboo material with my usual blackwood, so I can feel smug that what I get from the bamboo guitar will be worthwhile acoustically as well as using a readily renewable resource. With a little help from Gore and Gilet, I can convert these frequencies into measurements of:

  1. stiffness (Young’s modulus) along the grain
  2. stiffness across the grain
  3. shear modulus, or stiffness in the twisting mode

And if I’ve done the same tests on a blackwood panel (and you can bet I have), I can begin to compare the properties of the two materials. So here are the figures for blackwood:

  1. along-grain marimba: 130.1Hz
  2. across-grain marimba: 182.4Hz
  3. twist mode: 80.1Hz

Before I can work out the stiffness moduli for the materials, I need to allow for the fact that the bamboo and blackwood panels were slightly different sizes as well as different in density.

The next step was to calculate the density of each panel (density is mass divided by volume, measured in kg/cubic metres). I weighed the panels and carefully measured their length, breadth and thickness, and came up with these results:

  1. density of bamboo: 490.3 kg/cubic m
  2. density of blackwood: 697.3 kg/cubic m

So blackwood is about one and a half times denser than bamboo.

Using three more equations from Gore and Gilet (Equations 4.5-2, 4.5-3, and 4.5-4, page 4-59) I get:

  1. stiffness along grain: bamboo 10.6 GPa; blackwood 19.8 GPa
  2. stiffness across grain: bamboo 1.4 GPa; blackwood 1.6 GPa
  3. shear (twist) modulus: bamboo 1.3 GPa; blackwood 2.5 GPa

To sum up (pay attention up the back!):

  1. blackwood is 1.9 times stiffer along the grain than blackwood
  2. blackwood is 1.1 times stiffer than bamboo across the grain
  3. blackwood is 1.9 times stiffer in twist than bamboo

I’m now very close to answering my question about what thickness to make my bamboo back panels to match the stiffness of my blackwood ones. Back to another Trevor Gore equation (4.5-7 on page 4-61), which will tell me how many times thicker the bamboo back must be to have the same stiffness as blackwood. After some nail-biting, hair-pulling writing of the expression into a spreadsheet (Jeez, Trevor!), the answer is:


Oh, alright. Settle down. The excitement is too much, I know. Talk about suffering for the sake of art.

A bamboo guitar Part 2 – properties of bamboo

Before I start building my experimental bamboo guitar, I want to know a bit more about the properties of the bamboo sheets I’ll be using. In particular, I need to know how it compares with the Tasmanian Blackwood I normally use. I want to build a guitar that’s as similar as possible to my successful blackwood-bodied Parlour 6 model, so the first thing I need to know is how thick to make the two panels I’ll be joining to make the back of the soundbox.

Even within one species of timber, each piece is different from every other piece, so I’m expecting huge differences between bamboo and blackwood. Gore and Gilet (Contemporary Acoustic Guitar Design and Build, Trevor Gore, Australia 2011)  offer a very useful way of predicting the thicknesses of panels of different density and stiffness so they will all perform in a similar way when built into a guitar (page 4-60). I can’t recommend this book too highly if you have an interest in how guitars work.

Because I aim for a “live back” instrument (one in which the back vibrates and contributes extra complexity to the tone of the guitar), I need to create a bamboo back as close as I can the the blackwood ones I know give me the effect I want.

What I want to measure is the stiffness of the bamboo so I can compare it with blackwood. The way to do this is the tap the panels before they’re joined, and measure the frequencies at which they ring when tapped in three different modes – one indicates the stiffness along the grain, the second across the grain, and the third in its twist or torsional mode.

Gore and Gilet give an equation that combines these three factors with the material’s density and panel size to give an estimate of how thick to machine a panel (Equation 4.5-7, page 4-61), but more of that later.

So back to the tapping. Which mode you sound when you tap is determined by:

  1. where you tap (that is, excite) the panel; and
  2. where you hold (that is, damp) the panel.

The first two I want are called “marimba” or “free bar” modes, one of them along the grain and the other across. The marimba mode is the way a bar will vibrate when it’s not fixed at either end, so the ends and middle are free to vibrate strongly. It looks like this:


You can see that if you support the bar at the nodes, you won’t damp this vibration as long as the rest of it is free – in a marimba, each bar is supported at 22% of the length from either end, and struck at the antinode in the middle.

So for my first along-grain marimba mode I can hold it by pinching it between my thumb and finger 22% of the length along the panel near its edge, and I’ll strike it dead centre:


It rings at quite a low frequency, and has good sustain because there’s lots of mass in motion.

For the second marimba mode I want to do the same thing, but this time using the width of the panel instead of the length:


If I strike the panel at 22% of its length from the end, and in the middle, I won’t be exciting the first marimba mode too much because I’ll hit it on a node. So, I pinch the panel at 22% of its width from the edge and about halfway down the length of the panel. This tap won’t produce much of a ring, and it will have a higher frequency than the first mode it’s effectively a short bar and the amplitude of the vibration isn’t great. This one is the hardest to get a clear result for.

Now for the third mode. This one is a twist or torsional mode:


This one has two nodal lines, so I support it halfway along the length and strike one corner. This one gives me a lower tone with good sustain.

For each panel, I use Audacity with a sampling rate of 11025Hz to record 12 or more quick but careful consecutive taps for each mode of vibration:


I select all the taps, and use Audacity’s analyze/plot spectrum function to arrive at a spectral signature for each mode, something like this:


This plot shows the average of my twelve taps – each one is slightly different, so it’s better to amalgamate them all rather than trust to one on its own.

I then use the Export button in this Frequency Analysis window to create a .txt file, which I can then import into Excel. Why bother? I can plot my own graph in Excel which lets me zero in on the frequency bands I want – in this case from 0 to 300Hz.

This all sounds complicated, but it’s become quite a quick process now I’ve practised a bit.

Here’s how my bamboo 0-300Hz data turned out:

Bamboo panel tap response

This shows all three modes plotted against each other (another reason for using Excel). You can see the main resonances as peaks on the line as usual. Looking at the blue line (showing the vibration along the grain of the panel), you can clearly see the fundamental frequency is at a little over 100 Hz (109.0 Hz to be precise).

The lowest frequency peaks for the red (across-grain) and green (twist mode) aren’t as easy to sort out. Because it’s very hard when you’re testing one mode to damp out the other two completely, there is usually some doubling up that I need to sort out. The strongest response for the across-grain mode is at 226.1Hz, but you can also see I haven’t successfully damped out the along-grain or the twist modes. However 226.1 Hz is clearly the strongest response for the across-grain mode.

The green (twist mode) has its lowest peak at 80.1 Hz, but I failed to completely damp the across-grain response. The along-grain response was completely damped out because I was holding the panel right where the antinode falls – no vibration possible.

So here’s what I now know about my bamboo panels:

Along-grain mode fundamental frequency: 109.0 Hz

Across-grain made: 226.1 Hz

Twist mode: 80.1Hz

By themselves, these figures don’t mean much, but when compared with figures for blackwood they give me the start I need. More in my next exciting instalment…

A Bamboo Guitar Part 1 – why bamboo?

I’m certainly not the first person to think about using bamboo to make a guitar – Yamaha produced an acoustic 6 string called an FGB1 a while back, and people have used thick bamboo laminates for electric guitar solid bodies.

The first reason I have for wanting to try bamboo is to do with its merits as an easily renewable material, unlike the hardwoods usually used for acoustic guitars. Many of these are very hard to get hold of as humanity tramples across the planet: Brazilian Rosewod and Koa are just two examples.

Most people know how good a flooring material bamboo makes because of its hardness and pleasant appearance. It’s now available in thick laminated panels for benchtops and furniture making as well.

One thing I don’t yet know about bamboo is how stable it will be with changes in humidity and when it comes under the prolonged stress of string tension. Users of the Yamaha FGB1 talk about having to adjust the neck every week to keep it playable.

Another thing I need to investigate is its acoustic properties – particularly the along grain, cross grain, and torsional moduli of elasticity for the panels I’m using.

Other question from a builder’s point of view are:

  • how well I can make the bamboo bend in the side mould?
  • will the formaldehyde laminating glue hold up while bending?
  • how well will bamboo take machining to fit edge bindings?
  • will the formaldehyde glue outgas under heating?
  • what thicknesses should I use for the soundbox?

As a starting point, I’m going to make the sides and back of the soundbox out of bamboo while keeping the rest of the structure and materials the same as I usually use – spruce for the top, and Niugini Rosewood for the neck.

I have now got hold of some 4mm thick by 200mm wide vertically laminated panels:


I got it from Bamboo Australia Pty Ltd in South East Queensland. You can order from them on line. Durnford is very interested to see how the bamboo guitar turns out.

So using a renewable resource is my first reason to try bamboo. But I have another reason as well.

What I want to find out is how much effect the timber species the soundbox is made from has on the overall tone of the guitar. The reason I want to try and test this is because I have a suspicion that, when stacked up against all the other factors that determine the tone of a guitar (top bracing, soundboard timber quality, soundhole size, strings, neck bracing system, etc) the effect of the wood making up the sides and back of the soundbox is minimally important.

This is not what most people think, so perhaps I’m quite wrong. Some will spend a fortune on a Brazilian Rosewood guitar, for example, because they believe the tone is unique. But then, concert violinists apparently can’t reliably pick out the sound of a Strad in a blind test. Everything we sense is filtered through our expectations – if you don’t believe me, search out some Derren Brown videos on YouTube (don’t be put off by the Svengali stuff – he’s brilliant at illustrating how much at the mercy of our own expectations and senses we are).

I have read of people who claim they can tell an instrument made with hide glue from one made with plastic resin glue by listening to it. Maybe I’ve just got a dud ear.

Making one bamboo guitar won’t give me a definitive answer, of course, but it’s a step on the way.

I intend to document the building of my bamboo guitar on this blog, and I’ll include my thinking on how to evaluate the results – not a trivial question!

How to tap test a guitar – part 2

In a previous page – How to tap test a guitar Part 1 – I talked about the first steps  in producing a tonal signature by tap testing, and how to use Audacity software to record and analyse the tap.

In How to tap test a guitar Part 2 I want to complete the description of how I analyse the spectral signatures I end up with, using Excel to produce charts that allow direct comparison between taps. Just looking at Audacity Fast Fourier Transform charts like the one below is all very well, but they’re so detailed and spread across the whole audible sound spectrum that it’s hard to make sense of them, even though it’s obvious there’s pattern and structure there as you’d expect from something musical.

Guitar makers are mainly focused on the 80 to 1,000Hz range because that carries information we can actually use to take control of the sound. Players wanting to compare guitars, though, may be interested in the whole range from 80 to around 3,000Hz because that’s the whole response of the instrument we can hear.

Just a reminder: remember we’re measuring response in deciBels (dB) where a difference of around 3dB is a DOUBLING of sound level. 10dB is a difference of 10 TIMES, and 20dB is a difference of 100 TIMES.

Just to quickly run over what I’ve covered already:

  • use a small padded hammer, or if you want to go all organic, your knuckle to tap the guitar top, with the instrument held in the usual position, pointing at the computer microphone (the built-in microphone on my MacBook seems okay, and I’ve compared it with a reasonable quality Apogee USB microphone)
  • tap as consistently as you can, and in the same spot on or close to the bridge (unless you’re interested in how the response varies across the top)
  • record the tap series – around 20 of them – using Audacity (try to keep the recorded pulses from “clipping” – that is, hitting or overlapping the upper and lower track borders)
  • select the tap series, go to the ANALYSE menu and select PLOT SPECTRUM
  • marvel at the wonders of modern technology (not so long ago that analysis would have taken hours) and scratch your head while thinking “What does that mean??!”

You will have ended up with something like this from one of my guitars:


One nice thing is that this window allows you to scroll the cursor along the tonal signature and it will tell you not only what frequency you’re on, but also the exact frequency of the nearest peak. Each of these peaks relates to one aspect of the guitar’s structure, which is why we guitar-makers like it. For example, the first large peak at just under 100Hz is the Coupled Helmholtz response that comes from a combination of the top, back, and air body “breathing”. The frequency of it is determined by the size of the soundbox, the vibration of the top and back, and the size of the soundhole. So you can see what I mean, here’s a tap from a soprano ukulele:


Because the ukulele has a much smaller body, the Coupled Helmholtz peak is higher at 129Hz. (Does the smaller soundhole of the uke also help shift the peak upwards? Weirdly, no – it’s pulling the other way. Ah, physics…don’t you love it?)

It would be perfectly reasonable to stop at this point and admire all the different tap signatures you collect one by one just using the Audacity window, but I think you’ll quickly see the limitations of that. You can save the tap tracks on Audacity if you want, and come back to analyse the recording as often as you like. Or, you can take screen shots and keep the signatures as JPEG files. What you can’t easily do is overlap different taps so you can compare directly, as you might for example if you want to win an argument that says Matons are better than Martins, for example (a silly example, because you can’t prove any such thing – but you get my drift I hope).

So the next step is to export all this information in a form that will allow you to plot it as a graph using Excel. At this point I’m going to assume you know how to use Excel charts reasonably well, but I will perhaps give more detailed instructions later on if anybody wants – you can contact me through the Contact me page on this site.

The format I use for exporting is .txt because it’s easy to import into Excel, which will be the step after this.

So, as you already guessed while I’ve been droning on, now hit the Export… button. Save the data file somewhere handy, making sure it’s in .txt form.

Create a new Excel workbook. I usually start out with two tabs, one labelled DATA and the other GRAPH. In the DATA worksheet, select Data menu from the toolbar, then click on Get External Data/Import Text File… . Follow the string of Excel-ish dialogue boxes through to its inevitable conclusion, and you should find a very large set of data magically appearing in the worksheet.

Here’s where I assume you know how to create a new chart and plot the data. I normally plot the horizontal Frequency axis from 0 to 500 or 1,000Hz to suit my purposes, and use a linear rather than logarithmic scale. If you want to see all the data range (up to around 3,000Hz usually) you might want to use a logarithmic scale instead.

And on that probably very annoying note this page reaches its end, gets into its jimjams, and stumbles off to bed with a hot water bottle, because here in Canberra tonight it’s FREEZING!

As part of my series on building a bamboo guitar, I’ll go into the tap tone analysis of the instrument in detail.

Why the Balrog in Moria got really cranky

The Balrog who lived in the basement of the Mines of Moria wasn’t usually a cranky type. Indeed, he was having a quiet few centuries off after ridding Moria of its dwarf plague.

Always a believer in self-improvement, he’d decided to use the time constructively and build himself a guitar. His brother had taught him some way cool riffs you could play on the E string, and so he was humming “DANT DANT DAN,  DANT DANT DA-DAN, Smoooooke on the waaaater...” quietly to himself as he worked on the new Super Oliphaunt model.

Of course, his brother’s guitar had elf-gut strings, which everybody knew were the best if you could get them. Troll-gut wasn’t bad, but it was best for basses. Man-gut never lived up to its promise, he deemed. He’d have to settle for orc-gut – you couldn’t even get dwarf-gut these days. Oh well, he thought, they’d had to be got rid of. The greedy bad-tempered little sods with their lousy folk songs and their axe competitions and their horrible hairy little faces (to say nothing of the males).

It hadn’t been easy working out how to make the Super Oliphaunt fire- and slime-proof, but perseverance always pays off, and now he was just about to…

“…Pippin felt curiously attracted by the well…Moved by a sudden impulse he groped for a loose stone and let it drop…then far below, as if the stone had fallen into deep water in some cavernous place, there came a plunk, very distant, but magnified and repeated in the hollow shaft. “What’s that?” cried Gandalf… “Fool of a Took!…Throw yourself in next time…!” *

…and now, as the Balrog was just about to…PLUNK!!!!!! 

” What’s that?” cried the Balrog, nearly dropping his whip. The little team of orcs in front of him quailed, and nervously shifted their grip on the sweaty leather ropes that attached them to Grond, the Hammer of the Underworld. The Balrog had borrowed it from the Chief Nazgul, who had sniffed and hissed at him from under its black hood and told him to have it back inside a week. Or else – there were big plans for Grond. The Chief Nazgul really should get something done about its breath. It was, like, whoah!

So the Balrog was under pressure, no doubt about it. He took a deep breath.

Forget the plunk, he told himself. There’s no time to worry about plunks.

“Heave!” he called to the orcs. “Not too far…now, let go!” Grond swung toward the Super Oliphaunt’s bridge section and connected with a satisfying tap, followed by an echoing -tom.  Moria really did have nice acoustics. “Again! Let’s keep it uniform!” he roared. Tap-tom, tap-tap, tom went the hammer.

This guitar was going to be great! So resonant! The coupled Helmholtz resonance was awesome, and the T(2,1) cross dipole was singing. Singing! If only he could get hold of an elf, or even a few dwarves for a set of strings. (Dwarfs! Dwarfs! Not “dwarves”! he chided himself. He always forgot what the correct plural form was. Or was it dwarrows?)

And many levels above…

Gimli’s like: “That was the sound of a hammer, or I have never heard one.” 

“Master!” squealed a breathless orc as it skidded to a halt at the bottom of the Endless Staircase.”The upper levels…there’s…there’s…a wizard, an elf, a dwarf, two humans, four little guys with hairy feet, and another creepy little guy with bug eyes following them!”

“Be cool,” said the Balrog. “An elf, you say…and where did you say I can find our visitors?” A wizard, eh? Perhaps he’d make a wizard set of strings! “Keep tapping, lads, while I go and see to our visitors.”

“Yes, sir, whatever you say, sir!” cried the orcs. They drew back on Grond and let fly with a couple more big taps. DOOM! DOOM! the Super Oliphaunt roared.

“Not so hard, you idiots!” shouted the Balrog, and burst into flame.

Now I’m not saying that needing a set of strings is a good reason for attempted murder of a wizard. Maybe it is, maybe it isn’t. Getting between me and a new set of strings is like getting between a cruise-ship passenger and the smorgasbord, so I do sympathise. But Gandalf could have let him have the elf. Just saying.

And I know when I get interrupted while tapping I get pretty cranky too. The Balrog never got to play Smoke on the Water for Sauron on his awesome Super Oliphaunt, once Gandalf had finished with him. So was the Balrog evil? Perhaps, but I blame society.

* The Fellowship of the Ring, JRR Tolkien, Houghton Mifflin NY 2002