Murphy Custom Guitars
Handcrafted Acoustic Guitars
Drogheda, Co Louth, Ireland.
Modal Tuning
The difference in sound between guitars is mainly due to difference in either size, shape, amplitude and the frequency of its modes.
As I wrote about on the TONE section of this site, the sound of a guitar can be altered by changing either the size, shape, amplitude or the frequency at which it vibrates.
When a guitar vibrates, it vibrates in a specific way. The different way in which a guitar vibrates for different frequencies/notes can be separated into categories called modes.
Torrified Spruce Fan braced guitar top
The Picture & table below you can see the 6 main modes of vibration that you will see on all guitars. But the frequency of the modes change from guitar to guitar. This is
why guitars of the exact same size, make & model can sound different.
If we can change the frequency at which these modes appear.
We can alter the sound/tone of the guitar.
Changing the frequency of these modes is no easy task and starts from the very moment that you pick up that top or back plate. So many things alter the modes frequency, from selecting the correct plate thickness, monitoring the carving for the braces, having the correct stiffness and weight of your bridge, stiffness and mass of your sides, the volume of your soundbox, the diameter of your soundhole, using the correct finish.
Controlling all these things during the building of a guitar brings you to this point.
Image from designofaclassicalguitar.com
Monopole | Cross Dipole | Long Dipole | Long Tripole |
T(1,1)1 T(1.1)2 T(1.1)3 | T(2,1) | T(1,2) | T(3,1) |
100 180 224 | 347 | 384 | 457 |
A B C | D | E | F |
The Circles on the guitar top represent the nodal line. On either side of this nodal line, It will be in opposite phase to each other. So when one is moving up the other is moving down and vice-versa.
You can also see the frequency for each mode for one of my meduim body guitars.
The table on the left shows the 6 main modes that can be seen on all guitars.
These first 3 monopole modes contribute to the majority of the sound of the guitar.
T(1,1)1 - Main air frequency.
T(1,1)2 - Top Monopole Mode.
T(1,1)3 - Back Monopole Mode.
The voicing methods that i use can alter the 3 monopole modes independently.
T(1,1)1 - Main air frequency:
This frequency can be altered by adjusting the soundhole diameter.
T(1,1)2 - Top Monopole Mode:
This is controlled by:
1)The top plate density & stiffness.
2)Carving of the braces.
3)density/weight of the sides.
T(1,1)3 - Back Monopole Mode.
This is controlled by:
1) Back plate density & stiffness.
2) carving of the braces.
Above is a Spectrum Analysis of one of my Medium body guitars. This is achieved by analysing the tap tones of each guitar. With this information i can alter the frequency that the modes resonate at. This gives far greater consistancy from guitar to guitar & ensures a good sounding guitar everytime.
It is my preference to use a live/active back bracing design. This brings T(1,1)3 back frequency into the equation. and adds a bit of extra colour to the sound as it works with the top of the guitar. To the left you can see the radial live/active back. The centre portion of the brace in the ladder brace in the lower bout is carve out. this can be carve more when the soundbox is assembled to fine tune its frequency a few semitones from the top, so the guitar doesn't produce wolfnotes.
A non-live back can still be done on request