01 November 2011

Review: Richard M. French Engineering the Guitar: Theory and Practice

This book is for the luthier in between. It is not a book for the amateur who wants to build a guitar the first time. There are better books out there ( e.g. Roy Courtnall - Making Master Guitars ). It is not for the one whose brain is purely structured from too much mathematics (e.g. David C. Hurd - Left-Brain Lutherie ).

It is for the luthier who wants to improve his guitar building with engineering methods and is not afraid of using mathematical knowledge which is taught at the university. As a basis he can use the book from Fletcher: The Physics of Musical Instruments (page 239ff).

After a short history of the guitar ,acoustic and musical theory is introduced. You will find the "normal" introduction to scales, temperament, frequency ratios and fret spacing. Here no math is required. It is all layed down in tables for a quick look up. Sound radiation and plate modes are also covered. Next follows a the human perception of sound. He looks at different possibilities of filtering concepts and the representation of sound (time, frequency) and demonstrates and explains e.g. the envelop of a plugged note. This is more of a warm up of the knowledge which you should have before you continue reading the book.

French describes now the guitar. Remember I used the word engineering. The author tries to describe the guitar in formulas which will be so accurate, that you not only get a model but are able to reproduce what you have described. You could drive with mathematical descriptions a CNC-machine and you will get the machined parts identical down to 0,0001 mm.

So he introduces the compressive forces which work on the guitar neck and models a neck which is capable to withstand the forces. He describes the profile, deflection and deformation of a tension rod. All this with mathematical formulas. But you will get the results as graphics and tables.

After that you will find an overview of construction methods for classical, steel string acoustic and electric guitars.

But back to some theory about dynamic behavior of the guitar or better the strings and the body. After introducing some methods to measure frequency responses of a guitar you will find theoretical and practical dimensions of the dynamic behavior of strings. This is not always easy to follow but you will get in the end some down to earth measurements with which you can work. Of course the sound radiation of a guitar body and the correlation between the resonant frequencies and the corresponding mode shapes of the guitar are explained.

Now comes the part which was most interesting to me: The Analytical Models.

He describes discrete models were three elements in the structure are coupled together and create the modes:

  • top of the body
  • back of the body
  • enclosed volume of air

He develops a two and a three DOF ( Degree of Freedom) model and also gives a set of parameters with real values of a guitar. So it is possible to calculate the effects of design changes. For me the geometric models were rather convincing. He describes methods to develop a classical guitar curve fit so that it can be reproduced without losses. Well done. I have never seen this before.

As already told the book aims at the luthier who wants to speed up the manufacturing process. So French describes the pros and cons of machines and computers in the production process and the possibilities of testing the instruments for acoustic responses.

Sound quality is the main important thing for a guitar. But what is sound quality? This is not an easy question and good and bad sounds are sometimes in the hands of the guitarist. But there are some characteristics which can be described and measured. Based on the time domain it is the attack and decay, which describe a sound. In the frequency domain it is the loudness of frequencies, harmonics and resonances which can be made visible in spectrograms and waterfalls.

So the energy flow in the guitar (after Rossing) is triggered by the string and splits into High frequencies (Bridge, Top-Plate) and Low Frequencies(Top Plate - (Air Cavity, Soundhole), (Sides, Back). With this knowledge sound quality can be influenced by varying the active parts.

In the chapter guitar electronics my mind was kind of out of tune ;-)

So putting it together: I am a little bit biased. The book covers a lot of ground and you get an idea where engineering methodology can and will be used. to produce guitars. To use the mathematical and physical background of this book I think you will need more that that is covered in this book. The way from formulas to functional computer programs is a long way. And you have to go this way to develop a model for your guitar. So it is a pity, that some of the equations are not accompanied by working algorithms, so that you can   experiment with them and see what happens if you change a parameter. But at least French has given some hard facts and you can start going that way on your own. But it will be a way full of stones.

And still I think that a guitar modeled by an experienced luthier who talked about your dream guitar, who knows his wood will build an instrument that is superior to a machine factored guitar. But that is an endless debate.