04 March 2011

Urs Langenbacher: Tradition and Applied Physics for the Guitar II



This is the second part of the interview between me and Urs Langenbacher. The first part is here.


Hajo:
Time to put it positively: How does the sound of your ideal guitar look like?
© Urs Langenbacher


Urs:
This is not an easy question. I imagine an instrument with a fundamental sound characteristic. This means that the Helmholtz resonance frequency is very low - f sharp or below. The fundamental sound is a very important criteria in order to classify an instrument. At the same time it should have a good discant excitation, but I'm not a fan of very weak soundboards. These lack often the core or center of the sound.



Hajo:

I think we need to dive deeper into some technical parameters. How is the Helmholtz resonance determined at a guitar?


Urs:
The Helmholtz resonance is determined by four factors:


  1. The enclosed static air volume and pressure in the cavity.
  2. The length of the neck of the outlet. (This factor plays a minor role with the guitar since the strength of the cover at the soundhole hardly varies. An exception are of course guitars with a Tornavoz.)
  3. The outlet area (sound hole diameter) (With these three factors, I can not so easily change influence on the resonance as in the formula that describes them, these three factors appear under a root. One must change them exponentially, in order to achieve a linear change.
  4. The Spring constant, which behaves in a linear manner.

So, to achieve a specific resonant frequency, we can only act within these parameters. For me it makes little sense, to change the height of the ribs, in order to lower the Helmholtz resonance. This results very fast in playability problems. The most effective way to influence the resonance is to change the spring constant,
which is derived from the corpus construction. (This factor in the Helmholtz formula is before the root and has a linear behaviour.)  So I can influence much easier the position of the cavity resonance by structural changes in the stiffness of the plates. The Helmholtz resonance is not only the lowest resonance frequency of the instrument, but it also affects the harmonics lying over it. Just as Matthias Dammann has expressed it in his interview: It establishes a sound characteristic over the entire spectrum. The full-bodied or warm tone affects everyting up to the high registers.


Hajo:
Something practical: How can I determine the cavity resonance in my instrument?

Urs:
You have to sing into the sound hole starting from deep E continuously rise the tone. If you hit the air resonance the entire corpus starts to swing. It is usually between F (6th string) and A.

Hajo:
How do you achieve a deep Helmholtz resonance?

Urs:
The vibration system consists of top, bottom and sides and neck. The back side, ribs and neck are for me almost constants at the moment . I have a stiff, pre-stressed back whose resonant frequency is clearly higher than with most traditional spanish instruments. Also, my ribs are pretty solid. Basically, I have the conception that the soundboard plays the leading role.
© Urs Langenbacher

Hajo:
Theoretically it is me clear, but let me ask again: The essential design element for the tonic tuning is the soundboard whose size is determined by the size of the Plantilla. The back and the sides are relativly stable.
How do you shape your soundboards?

Urs:

I am looking on two fundamentally different approaches or types of instruments. At present I use a Manuel Ramirez Plantilla, inspired by one of Segovia's Instrument from 1912. It is compared to most modern instruments a small model. With this small model I only come on my deep adjustment if I elaborate the soundboard soft. The soundboard functions like a very softly suspended small-scale loudspeaker. The cover has a moderate curvature not least for safety reasons (climatic problems). The bracing is very traditional after Santos Hernandez, who has built the "master" instrument of the Ramirez workshop.

With the other type of instrument the soundboard surface is relatively large. One can elaborate this large soundboard comparable rigid and achieve a deep Helmholtz resonance. These instruments however have a cooler character.

Hajo:
What is a soft suspension?

Urs:
A soft suspension is a weak material strength at the edge and therefore greater flexibility in the boundary region. This is the most important question: Is the soundboard, working as the membrane, capable to produce  a large amplitude or is it rigidly coupled?

Let me say it differently. There are two different approaches:

  • A more rigid suspension of a larger soundboard with a small amplitude
  • A softer suspension of a small area soundboard with larger amplitude

I opted for the latter. The main reason is the sound design but also playability. Let me explain it with a comparison.
I anchor rigidly a rope (in our case the string) rigidly to a concrete wall. I stretch this rope now with a certain strength and pluck it. The tension is directly noticeable.
Now I fasten the rope to a down-hanging branch of a tree. It now has a soft suspension and I stretch it with the same force as before: The overall tension over the construction will not be the same compared to the rope fixed on the wall. If we transfer this example to the guitar it means the following:

When we regard a guitar whose soundboard has a "soft" suspension the strings do not seem to have a too strong tension. I am designing the string action rather height going from 4,5 mm to 3,8 mm. The guitar can be played very comfortable and allows a wide range of amplitudes and a great dynamic. At the same time this suspension is offering less intonation problems. For a softly hung up soundboard I need less compensation at the bridge., because the string tension-changes ,when pressed down on the fingerboard, does not occur as high as usual.
Bridge © Urs Langenbacher


Hajo:
Where are the limits of this tuning?


Urs:
The handling of the instrument is giving a limit.I already had reactions  by players that the strings, which feel very soft are harder to control. The Helmholtz resonance is an aspect with which I play quite consciously. I am guided by the reaction of the guitarist. If a player wants to have the deep fundamental sound, and is prefering a more resistant string reaction at the same time, I would propose a larger plantilla.  The Helmholtz resonance  should not precisely lie on a half-tone, because it would produce a great response and projection, but no sustain.

Hajo:
Do you use some technical devices in order to tune the soundboard?

Urs:
Yes and No. During the building process I try to maintain a completely controlled path. I make only small changes, in order to be able to interpret the reactions more safely. I take this control also by using measurement procedures, so that I know where I am in the production process. For me Chladni figures are a tool  to work more controlled.  They are not a key to a good instrument. But if I try to reproduce one of my instruments, I have reference data, which serve me as guideline for necessary changes. It is very helpfull that I am mainly using my self cut Spruce for the tops, so that the quality is comparable.



Hajo:

Where do you see opportunities for further development in the building of  guitars?

Urs:
There are many possibilities to change the concept, but I have no ambitions to follow a revolutionary way. I'm sure that improvements that satisfy me and the musicians can be made using traditional paths. The skill lies in the coordinate fine tuning of the components. These 7-8 hours, in which one deals with these details are the best invested in the entire production process.


Hajo:

Guitarists who play your instruments claim that you do not build your instrument, but their personal instrument. How do you manage the implementation of the requirements of the player?

Urs:
I observe the player extensively, how he deals with the instrument. Subsequently, I try to accordingly set the variables of my building method accordingly. Usually guitarists cannot at all imagine at all that they exert a large influence on me by their way of playing.

Last year I have built an instrument for a guitarist with an extremely light attack. On the basis of a model,  the  player prefered, I selected the timber of the top as light as possible.  The wood was cut by myself  and split without sap wood. With this topI ended up with 20 grams less in weight at the same rigidity of the  soundboard compared to another instrument.

Hajo:

How does the construction affect the playing characteristics of the instrument?

Urs:

The differences in construction of a soundboard have a very large influence on the playing characteristics of the instrument. An example: Some customers did bring me instruments with fretboard difficulties. Deep E-string was buzzing on the first frets.) Even an increase of the string height did not change the problem. There was no obvious mistake to be found on the fingerboard and set up. The  instrument was not able to realize lower frequencies than its Helmholtz resonance, which was quite high. The ability of the soundboard to consume and to project the energy of a string decreases significantly below the Helmholtz resonance. The amplitude, which the string shows after the attack, remains wide for a long time. The energy transition to the Soundboard did not function properly. It was a conceptual problem of the instrument.  The problem mainly occurs on instruments wearing stiff tops. An instrument with a lower Helmholtz resonance does not show this particular problem,  because it enhances the radiation in the critical frequency range.

Hajo:

Depending on the construction the strings have a higher amplitude. Do you have to increase the string height at your instruments?

Urs:

When playing my instruments for the first time, many guitarists say, Many guitarists say, when they see the instrument for the first time: "Oh, this is a very high string action. Can you play this at all?"  Players who just take the instrument and start to play do not notice the "problem". On a "softly" tuned instrument you can play without problems with a string height of 4,5 mm in the bass and 3,8 mm in the treble without problems. This does not manifest itself in a arduous tension for the left hand, what a surprise!.

As I have just explained, the string/bridge coupling on a softly hung up soundboard does not produce an extreme feeling of tension. This provides better playability. The softer the coupling of the string to the soundboard, and the softer the soundboard is suspended, the less extreme tension occurs on the left hand of the player. The sense of playing for the guitarist is not determined only by the absolute string action. Besides the sound quality remains for me a good playability of my instruments remains a priority for me.