05 April 2011

Review: David C. Hurd - Left-Brain Lutherie

This book has a long subtitle and I will present it to you:

Using Physics and Engineering Concepts for Building Guitar Family Instruments: 
An Introductory Guide to Their Practical Application.

When I built my first guitar I found in the workshop a book from Jürgen Meyer in the bookshelf. The owner told me that it is "hard stuff", but I am a radio-amateur and used to deal with waves, resonances, filters, damping, ... so it was nice reading, nothing really new, but lots of data which I could not remember. But I think I got the main points and THAT was absolutely interesting. Later on I made a lot of measurements of different guitars to find the footprint of these instruments.

I searched the literature for other books but only found the usual suspects. Good books, but not really dedicated to stringed instruments.I had to wait until 2004: Left-Brain Lutherie was published.

And the first look - Oh dear, I thought I spent the money for the wrong instrument: the ukulele. But I only had to read on and understood: It covers Guitar Family Instruments.

But another warning: It is not a basic book about how to build a guitar or ukulele. It describes in mathematical formulas and diagrams the physics and the engineering techniques you should know to understand better what you are actually constructing. So this is a book for a luthier who has learned his craft, knows how to work with a plane and chisel, but digs into modifying his concept to get a reliable basis for further improvements.

But beware: It is not a book to be read during a longer session in the bathtub. It is a working book with tons of data and formulas. But the author did better than that. He organized this wealth of material in spreadsheets. This sort of presentation does not disburden you from understanding but it makes things a lot easier to grab the sense.

And you are invited to make measurements on your own instruments. So you can put them into these diagrams and see where your instrument is situated in comparison to the research Hurd has done. (For the faint at heart - some mathematics is involved and it takes some time to understand: But believe me it is possible - no PhD needed.) What else do you need: A computer and some instruments which you can buy or build by yourself. The spreadsheets and formulas are on a CD, so you do not have to retype them. And you find a lot of pictures on this CD. This alone is worth the price.

There is another advantages of the book. If you want to make measurements on your own instruments and compare them to the results in this book the parameter you use must be identical. So Hurd puts a lot of effort to describe the setup for the measurements, so that they can be reproduced. Well done and it does not cost you a fortune nor a laboratory from space industry.

So what is covered in the book:

Before I dig further into the content: This is not a novel how to build, but a scientific almanac of problem definition and their solutions. As in every good text it is cross-referenced with

  • Table of Contents
  • List of  29 Tables with exact descriptions
  • List of 107 figures 
  • List of all used symbols with definitions
  • References
  • Additional Sources of Information
  • Software used
  • Appendices: Static and Dynamic MOE Determination, ...
  • Index

I must admit, that I never read the book from the first Page to the last. I only used the cross-reference to find that piece of information which I needed. And I always found it.

I can not list all themes covered. So I will give you my personal list of things which interested me and where I found solutions in the book
  • Body Size and Air Resonance
  • Mechanical Top stiffness
  • Creating a Top-Deflection Model 
  • Calculating String tension
  • Saddle Height Torque Test
  • Description of Methods for Wood-Properties Measurements
  • Measuring Density
  • Deflection Models
  • Different Bracings and the torque curves of a plane (disk) 

In Chapter 5  he applies his findings to several ukuleles and two classical guitars (Kenny Hill Palo Escrito #3027 ,Ramirez R31993) . Each instrument is described overall measurements, bracing system , compliance map of the soundboard with maximum deflection, plots of thickness, string tension. The measurements are set in relation to his deflection model. After these calculations instruments were comparable and conclusions could be drawn. BUT: Every builder should understand the underlying model and its properties but he can not take this data and apply them to his guitar. He has to measure his own e.g. E(ave), string tension, bridge torque and top thickness and feed it into the spreadsheet. Then he can create a compliance map for the individual instrument and can go on constructing.

(Have a look at these pages. I could not copy the videos. And there is more.)

An own chapter is devoted to resonance coupling an the work of Jürgen Meyer. Here you find hints how to measure the Helmholtz resonance and top and back resonances during construction. A special notice is given to the fact that it is necessary to normalize your data, so that they can be compared to e.g. Meyers frequency ratios. The pool of information which Meyer provided is very complex. So Hurd put them for your convenience in a special spreadsheet ready to use.  Of course you first have to determine the Helmholtz frequency of your instrument, determine what mechanical compliance range your top should be, ... You have something to do. And you can do it with another spreadsheet the other way round: Copy an instrument with a particular set of final resonance ratios.
But let us stop. It is too much condensed information to be covered in this review: neck, bridge, gluing, humidity, wood, bracing, string diameter....

In the beginning I said, that this book is for the experienced luthier, for he will find for problems which can be described in mathematical or physical terms a method to describe his instrument in a more precise way. For the amateur luthier this book presents an insight of information which will not help to initially build the instrument, but give a better understanding of all mechanical and acoustical processes involved.

Conclusio: This book is the bridge between the empirical approach of guitar building and the highly scientific mathematical and physical descriptions of instrument modelling given at universities. It is not down to earth concerning the mathematical requirements, but it is well worth to revive one's own knowledge and to start building instruments in a more acoustically controlled manner.
More information you can find on the website of David C. Hurd.

And don't forget to have a look into the Left-Brain-Lutherie mailing list, which is quite active and sticks most of the time strictly to the subject.