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Copyright © 2018 by Wayne Stegall
Created July 24, 2018.  See Document History at end for details.

Guitar Shape

Mathematical modeling of guitar shape leads to modeling program


Some time ago I wondered if the shape of a classical guitar was mathematical and further if a mathematically modeled shape would be better in some way.  I imagined this to be a polar plot.

Manual Attempts

When I began to explore these ideas, I first tried for a peanut (or hourglass) shape with the following equation and succeeded.  This resulted in upper and lower bouts of same width with the expected waist in between.

 r = a0 + a2cos2θ

I already had in mind that if I added a1cosθ to the equation the lower bout could be larger than the upper.

 r = a0 + a1cosθ + a2cos2θ

Now the width of the outline was unavoidably narrow relative to the length without some further change.  The required change was to stretch the y axis of the polar plot.

 x = r·cosθ
 y = ymult·r·sinθ

Now that could get the right bout widths, the outline seemed too roundish.  Adding a sixth harmonic then allowed the familiar shape with considerable tinkering with the coefficients.

 r = a0 + a1cosθ + a2cos2θ + a6cos6θ

Computer solution

When I came to want a program to calculate the coefficients and plot the curve, I was stuck on the idea of and empirical method of refining the coefficients but failed to imagine a final algorithm.  Then I thought of the min/max algorithm used to calculate digital filters and thought it right.

Min/Max algorithm process

  1. User specifies length, lower bout, waist, and upper bout dimensions and a4 and a6 coefficients
  2. Choose reasonable θ values for minimum and maximum points
  3. Calculate coefficients a0, a1, and a2 from current min/max θ values by solving simultaneous linear equations.
  4. Find actual min/max points for next calculation.
  5. Repeat 3 and 4 until convergence.

Equations on which simultaneous linear equations are based

r = a0 + a1cosθ + a2cos2θ + a4cos4θ + a6cos6θ
x = r·cosθ
y = r·sinθ/ay
r = ay·y/sinθ
a0 + a1cosθ + a2cos2θ + a4cos4θ + a6cos6θ - ay·y/sinθ = 0
Length = 2(a0 + a2 + a4 + a6)

Matrix input to Gaussian elimination






Length – 2(a4 + a6)
–(a4cos4θ0 + a6cos6θ0)
–(a4cos4θ1 + a6cos6θ1)
–(a4cos4θ2 + a6cos6θ2)

Program and its operation

Program name is guio.exe

Download program version 1.0.0.

Run program and a default program shape appears.

Menu->Edit->Input Shape brings up dialog to define your own shape loaded with default parameters.

Figure 1:  Input Shape dialog.

Enter all required values experimenting with small values of a4 and a6.
in->cm and cm->in buttons convert units.
Press Calc to calculate and draw shape.
Press OK to do the same and exit dialog.

Figure 2:  Guitar outline in horizontal orientation

Menu->View->Vertical toggles between vertical and horizontal modes with vertical mode indicated by check mark.

Figure 3:  Guitar outline in vertical orientation

Help in adjusting coefficients a4 and a6.

A4 and a6 should only have small experimental values.  Figure 4 below shows positive and negative values of a4 and a6 added to circles to help you to understand their effects.  Starting from the inside working out they are:

Figure 4:  Graph of the effects of coefficients a4 and a6.

Lutherie use

For those who want life sized drawings of guitar shapes the number displayed in the upper left hand corner of the plots indicates the number of pixels per unit measure.  Then follow this procedure.

In GIMP this is done as follows:
Menu->Image->Scale Image
Change X resolution and Y resolution to <your value> pixels/in (or pixels/cm)

Dreadnoughts too

Although I originally sought to model the Spanish guitar the following shows an attempt at a dreadnought.

Figure 5:  Input and results of attempt to model a dreadnought.

Document History
July 24, 2018  Created.