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...

Getting comfortable with charts – the difference between noise and music

I know there’ll be some of you who turn off when faced with graphs and charts. Because they’re so useful, though, I’d like you to persevere with them. You can get a lot from them once you can relate them to something real – that’s what I’d like to try and do here.

We can all tell the difference between noise and music (although where the boundaries lie can be a matter of opinion and circumstance). First an example of noise – the sound of surf on a beach:

And next the musical sound of a singing wineglass:

Now here are the sound spectra charts of each one for comparison, beach first:

Beach noise

Beach noise

Singing wineglass

Singing wineglass

The horizontal axis of the graph shows frequency (pitch, in everyday terms), and the vertical axis shows loudness at each frequency.

The difference between the two sounds is very clear to the ear, and you can see it is also very clear from their sound spectra. Noise blasts out fairly evenly across a wide part of the sound spectrum, where a musical note has a series of well-defined peaks, often evenly spaced as in the wineglass example.

The value of the charts is that you can actually read off the frequencies of the main frequency peaks, and so compare different sounds more analytically than listening allows.

Here is another example: the noise of an electric kitchen mixer compared to the sound of a tubular gong of the sort you’ll find in wind chimes.

Here’s how the two look when you analyse them, the kitchen mixer in blue and the chime in red (I’ll leave you to imagine the sounds):

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The chart shows the sound level put out by each. One is a maddening noise, the other is a pleasant ring (unless small children get hold of it), and the chart makes the difference as clear visually as it is to your ears.

The chime produces its loudest tones at well-defined frequencies, where the mixer blasts it out right across the spectrum, with one peak at around 400Hz which could well be related to the speed of the motor. We could all hum a note to match the gong, and maybe with the mixer too, probably somewhere between G and G#, which is at the 400Hz peak.

One interesting aspect of the gong signature is that each peak is double. That’s because the gong has a split along it at the bottom, giving it two fundamental frequencies instead of just one.

Charts like this can help a lot in understanding music. The singing wineglass is one of the purest sounds you’re likely to hear (this is a different wineglass than before with a different fundamental frequency, but the same type of spectrum):

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This shows that the singing glass puts out sound strongly at some very well-defined frequencies: 709Hz, 1664Hz,1816Hz, and 2496Hz. These correspond reasonably closely to the scale notes F5, G#6, A6, and D#7 (the number refers to which octave each note belongs to). Notice that even a “pure” sound contains more than one musical note.

Thoughtlessly, the makers of the glass didn’t take the time to tune the glass to the standard musical pitches.

By contrast, I tuned the 2nd string of my Jumbo 6 guitar to the note B3 at 246.9Hz:

Here’s the analysis of the note:

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The peak frequencies in order are:

  1. 246.1Hz    (B3 is 246.9Hz)
  2. 492.2Hz    (an octave above, B4 is 493.9Hz – double 246.6Hz)
  3. 738.3Hz    (F#5 is 739.9Hz)
  4. 990.2Hz    (B5 is 987.8Hz)
  5. 1236.3Hz  (D#6 is 1318.5Hz)
  6. 1482.4Hz  (F#6 is1480.0Hz)
  7. 1728.5Hz  (somewhere between G# and A6)
  8. 1980.4Hz  (B6 is 1975.5Hz)
  9. 2226.6Hz  (C#7 is2217.5Hz)
  10. 2478.5Hz  (D#7 is 2489.0Hz)
  11. 2724.6Hz  (somewhere between E and F7)

Those with a music theory background will recognise B, C#, D#, E, and F# as notes in the B Major scale. This is what’s known as a harmonic series – more about these in another blog entry. 

Matching frequencies to scale notes

When I analyse tap tones, I work in the unit for frequency – the Hertz (Hz). Frequency tells you how many times something vibrates in one second. With sound, that means the number of times in a second the particles of air near your ear push against your eardrum. The higher the frequency, the higher the pitch of the sound.

Musicians, naturally enough, don’t care nearly as much about this as guitar makers do. They know that if you go up an octave you’re hearing a frequency double that of where you started, and they know that the notes of the Western musical scale are at particular frequencies defined by a mathematical series known as 12 tone equal temperament.

Even if you aren’t a musician, you know that as well because you’ve listened to lots of music during your life. That’s what your ear has come to expect, especially if you’re from a Western culture.

Interestingly enough, guitars and other fretted stringed instruments aren’t very good at producing exactly the right frequencies to make a major scale sound perfect. If you have an electronic tuner, you can see this if you tune a string to its correct fundamental (eg the A to 440Hz) and then work your way up the fretboard fret by fret. Notice how cranky your tuner gets?

The frets aren’t right! That is one way to see it, but the explanation is actually to do with the physics of how real strings vibrate.

The reasons are something for another time, but musicians with really good ears – especially those who play fretless instruments like violins – are often driven crazy by what’s called the inharmonicity of a guitar. Fretless players can use subtle fingering adjustments to make their notes true.

The rest of us take it as it comes and put it down to being part of the sound of the guitar. Guitar makers pull their hats down over their eyes and quietly leave the room before they’re noticed.

So anyway, here’s a chart showing the notes in the diatonic major scale.

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The chart begins at 27.5Hz because the human ear doesn’t really hear anything much below that as a continuous sound (20Hz is considered the cutoff) . It ends rather arbitrarily at 7040Hz because most instruments don’t produce much sound at those high frequencies (but mainly because guitar makers don’t care about anything much over 5000Hz).

The standard tuning of guitar strings is shaded blue for your viewing convenience.

The octave number helps identify which note you mean: the B string on a guitar is tuned to B3 at 246.9Hz. Notice the doubling of frequency from one note to its octave above.

The little gradations on your guitar tuners that show how far above or below the string is are cents. The interval between any two notes is divided up into 100 parts, but as you can probably guess that isn’t simple because the frequency interval between two notes gets ever larger the higher you go.

Driving a guitar top

I have made the point that the reason behind my Yolande shape is that I want to drive the guitar top from the centre to achieve a particular sound. But where’s the evidence?

One way to show the difference between driving the top at the centre compared to the edge is to analyse the tap tones you get from doing just that.

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 In this chart the blue line shows the response of my Jumbo 6 being tapped just behind the bridge, near the centre of the lower bout. The red line shows the response when tapped right at the edge. There are two main differences:

  1. the peak at just below 100Hz is very much lower for the edge tap, as is the next peak at about 130Hz;
  2. the treble response from about 400 to 1000Hz is stronger for the edge tap.

The first peak is the low, boomy air body response. If you do this test on your own guitar – even if you don’t analyse it the way I have – you’ll be able to hear the difference. The edge will give you a slightly higher, thinner tone compared to the boomier centre tap.

So that sets the stage for an answer to why I design the way I do.

The next chart compares the response of one of my Parlour 6 guitars with a similarly-sized Martin 000-18 which has the bridge in the usual position for a dreadnought-shaped body, closer to the soundhole. The one on the right is the Martin, a lovely guitar.

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I tapped them each on the bridge where the strings cross the saddle. Keep in mind that this is not the best one-to-one comparison because of the other differences between the guitars (different top bracing, different-sized soundhole etc). I tried to make the taps as equal as possible.

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What’s remarkable is firstly the similarity of form between the two signatures – that’s because they’re both guitars.

It’s the differences that are interesting, though. The first peak – the airbody resonance – is slightly better for the Parlour 6, as well as occurring at a higher frequency because the airbody is a bit smaller than the Martin’s. From about 220Hz upwards, the Martin’s response is consistently stronger, and this corresponds to a very bright but slightly thinner sound. The Parlour’s tone is, for want of a better way to describe it, more like a smooth red wine compared to the Martin’s cheeky white. Both tasty, but definitely different.

You can see how the strength of the tap is important for this kind of comparison. Had I tapped the Martin less strongly, the form of the response would be the same but it might fall lower than the Parlour – or vice versa.

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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.

Some thoughts on guitar fetishism

No, it’s really not that kind of website. What I want to talk about is the relationship between guitars and guitar players, which in my view can sometimes be a little…strange. I know, because I’ve been there myself.

We all know that guitars can be beautiful things, and that our attention gets focused by names like Martin, Maton, Gibson, Taylor and all the rest. In our young (or old) and foolish phases of our lives, we can even convince ourselves that we’re better players, and perhaps even superior people, when we own one.

I don’t want to talk down beautiful guitars with gorgeous sound and silky playing actions. After all, that’s what I aim to make myself. Nor do I deny that a good guitar, compared to a horrible one, can help us play better. I find that a different guitar often helps me discover new things to play.

But let’s not get carried away with it. Some of the best guitar music I’ve ever heard was pumped out by a West African player on a guitar he’d made himself out of what was available to him. The strings seemed to be fencing wire, but maybe not. My point is that a guitar doesn’t make a player – it’s the other way around.

Sometimes cheap and nasty can be great. A guitar is just a wooden box – it’s the sound that it and its player make that’s important, not the beauty, the tonewoods, or the glossy finish.

Recently I saw a documentary in which three highly-accomplished concert violinists did a blind test to see if they could pick the Stradivarius from a couple of other high quality violins. None of  them could. It’s the sound that matters, not the name on the box.

Here’s one definition of “fetish” from the OED: something irrationally reverenced.

Hmm. Irrational reverence can be fun, though…

Does vibrating a guitar make any difference?

Guitarists have always believed that two identical guitars, each played exclusively by different players with different styles, will end up sounding quite different from each other. Others make sure to keep their guitars in front of their stereo speakers so that whenever they play their favourite music the guitar will vibrate in sympathy and take on the tone of the music it “hears”.

Is there anything in this?

Recently electromagnetic vibrators have been produced to artificially “play in” new instruments. Even I can tell that a brand-new, just strung up guitar sounds pretty raw compared to its sound a few weeks or months later, so I bought a Tonerite vibrator to experiment with.

The Tonerite fits between the strings down near the bridge and has a choice of settings:

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When the device is turned on, you can feel the whole instrument vibrate from top to bottom. Tonerite recommend you vibrate for about 3 days for a new instrument, and periodically repeat the treatment to “liven up” older instruments.

But does it do any good?

My trusty tap hammer says that it does, in fact, make a difference. Here is the tonal signature of one of my 12-string guitars before and after the vibration treatment at the very beginning of the instrument’s life:

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The blue signature is the instrument’s response before vibrating, and the red afterwards.

It’s pretty clear that the “after” picture is an improvement on the “before”. Entirely new formants (peaks in the response) have formed right across the spectrum from 200 to 1000Hz, and the sensitivity has improved pretty much across the spectrum. My ear agrees that the instrument sounded far richer and livelier after the treatment.

Some caution is needed in looking at the increased response overall, because the traces were produced by two separate and therefore not identical taps, so the “after” trace could have been a heavier tap despite my best intentions.

(This leads to the search for a standard tap device that delivers the same impulse to the guitar every time to make direct comparison more viable – but that’s the subject for another day.)

However, what makes me think that the two are similar enough is that the first peak at about 120Hz is what’s called the coupled Helmholtz response. I wouldn’t expect this to change much after vibration because it is produced by the overall “coupled” response of the top, the air in the soundbox, and the back. This peak is a little higher in the “after” response, which implies that my tap was a bit harder for this one.  But the two peaks are similar enough to show that the overall improvement in response after vibration is real, and not just the effect of bumptious tapping.

When you look at tonal signatures such as this, keep in mind that the response (the height of the trace) is measured in deciBels (dB). This can be tricky, because a difference of +10dB actually means a difference in sound power of 10 times. A difference of about 3dB means a doubling of sound power. This means that the red “after” line shows a hugely increased performance across the spectrum.

Why does it work? My belief is that when a guitar is built many localised stress points are set up in its structure. Both the resin glue and the wood will “creep” under the influence of mechanical vibration, smoothing out those localised stresses and improving its ability to respond evenly across the spectrum.

Because it’s not possible to have two identical guitars, it’s hard to say whether different players would have a different final effect – but it’s a reasonable thing to suggest.

In a separate blog I’ll explain what the peaks on a guitar’s tonal signature actually mean. On another I’ll show how my mighty hammer and I produce the traces, with the help of Audacity and Excel.

Jig and clamp world

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This is what the outside door to my workshop looks like. The plate was made for my birthday one year by my wife, Wendy – she always gets a laugh when I tell her I’ve just made a new jig to do something or other, and she knows that clamps are always welcome presents as well.

But the truth is that you just can’t make guitars without clamps and jigs. The problem is working out when the jig/clamp population is going to start crowding out everything else in the workshop.

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The scary thing is that jigs, which are devices that help produce an accurate shape in some way, always have some kind of clamp on them. For example, what would you call this:

It’s my side bender, so it’s a jig.  The white box is the temperature controller for the silicone rubber blanket that supplies the heat. But look at those sneaky clamps to hold the waist and ends down. Clamps are everywhere!

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And they come in  all different types. Like most luthiers I use go-bars to apply pressure to glue joints. They’re simple and elegant – all you need is an upper and lower rigid surface, then you jam bits of dowel end on between the two.

In this picture I’m joining two panels of Tasmanian Blackwood together for the back plate of a guitar.

I could give you a list of all the types of clamp a luthier needs, but I won’t. Wendy would laugh at me if I did. I plan to show how I build a guitar as these pages relentlessly procreate themselves, so you’ll see them all if you’re interested. But promise you won’t laugh.