Category Archives: Tap testing

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:

VIB MODE MARIMBA

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:

VIB MODE ALONG

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:

VIB MODE ACROSS

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:

VIB MODE TWIST

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:

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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:

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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…

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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:

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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:

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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.

Link

Analysing the tap tones of an instrument is one of my most important tools as a guitar maker. A series of recorded taps will reveal a huge amount of information about an instrument, and as I develop as a builder I have a record of the characteristics of the ones I’ve already finished. This allows me to direct my efforts to make each one better than the last.

Tap-tone analysis is something anyone with access to a small hammer and a computer can do, and even if you aren’t a builder it can give a really interesting insight into guitars and what you want from them.

WHAT YOU NEED:

  • a small tap hammer (get one designed for tacks from a craft shop – mine is 17cm/6.5″ long and weighs 75g/2.5oz); stick a small self-adhesive felt pad on the head, OR  use your knuckle if you want to go all organic on me
  • Audacity recording software (available as a free download)
  • Excel spreadsheeting software if you want to make direct comparisons between instruments or taps on different parts of the same one.

HOW TO DO IT:

The first thing to do is to muffle the strings with a soft piece of cloth so they won’t ring.

Set Audacity’s sampling rate to 32000Hz to get good detail without overloading the buffer.

Sit with the guitar in a quiet-ish room in front of the computer so its microphone can pick up the taps. Open Audacity to a new project, press record, and tap the guitar top just behind the bridge about 20 times. Stop the recording, and you’ll see something like this:

2013-11-13 13:05:56 +11001

This is the record of the sound pulse produced by your tap series, and it contains a surprising amount of information once it can be unlocked. The key is a process called Fast Fourier Transform (FFT) that shows you all the frequencies present in the top response – in  other words, how the instrument responded in detail to you tapping it.

Select the pulse series, then go the Audacity’s Analyze menu, and select PLOT SPECTRUM. This is what you’ll see:

2013-11-13 13:06:50 +11001

For comparison with mine, select the Blackman-Harris window, size 16384, and Log frequency as I have. And that’s the spectral signature of your guitar. It’s like a sonic fingerprint – notice that there are a series of peak responses right across the spectrum, each of which is produced by a different mode of vibration. Complex? Oh, yes!

Note that there’s a peak at 50Hz. This has nothing to do with the guitar – it’s the 50 cycle hum that pervades a house where electrical appliances are at work.

Given the nature of the dB scale, though, anything reading less than say -60dB is kind of suspect. Remember that the difference between -40dB and -60dB is a difference of 100 times the sound power.

All guitars have a similar spectral response because that’s what makes them guitars (a guitar is a sound, not a wooden box). But as with fingerprints, each guitar is unique in detail. And unlike a person’s fingerprint, a spectral signature can tell you a great deal about the guitar that makes it.

The format of the Audacity file makes it hard to compare taps directly. For that you need to export the data to Excel and plot a graph of it, but I’ll explain how to do that on another occasion. You can see the idea if you go to my blog entry on vibrating guitars.

And the next question is: what does the signature mean? Ah, another time, another blog.

One last comment, though. There’s no substitute for your own ear when you choose a guitar – what’s right is what you like.