From: jeff@dirac.physics.sunsyb.edu (Jeff Althoff)
Newsgroups: alt.guitar
Subject: Re: Guitar Acoustics
Date: 10 Apr 92 15:59:20 GMT
Organization: Institute for Theoretical Physics, SUNY at Stony Brook

In article <29118.9204071755@thor.cf.ac.uk> scogpw@thor (Gordon Walker) writes:
>Well, after starting the Guitar Acoustics Dedate (anyone want to start
>alt.guitar.acoustics ? :-) - I had better respond to the responses back from my
>last posting. (So Jim Muller, Paul Ebersman and Jeff Althoff, watch this 
>space..............!)

   I watched, I read, I was not convinced :-) 

   Now that we're all in the glass house :-) .....
>
>
>[I wrote:]
>|>Regarding loudness vs sustain: you can't increase *both*. If you
>|>increase the overall response of the top plate in some way, the more
>|>efficient energy output, as you put it, means that it will die away
>|>quicker. You can't just get extra energy from somewhere for the sustain
>|>of the note. 
>
>[Althoff:]
> >Ever hear of damping, as in a "damped oscillator"? The braces certainly
> >damp the vibration of the top. Scalloping the braces reduces the damping.
> >Energy is not coming from nowhere, less energy is being dissapated by
> >the damping effect of the braces.
>
>Hmmmm - from someone on a ".physics" account too! Check out your text books on
>a "damped mass/spring oscillator". Changing the mass or stiffness does *not*
>change the internal damping of the system. You might indeed change the resonant
>frequency, which will of course alter the actual amount of time an oscillation
>takes to die away (just like playing a higher/lower note on a guitar). This is
>not the same thing!

  Hmmmm. I thought the amount of time a note took to die away was related
  to sustain!

  How do you define "internal damping"? 

>Scalloping a bracing would (locally) reduce more of its mass than its stiffness
>thereby (if it was as simple as this) increasing the resonant frequency of
>the system if anything. Thus, such a system would die away quicker.

  My statements are based on empirical knowlege of how the guitars sound. 
  It MAY be appropriate to think of
  the braces as a perturbation on the vibration spectrum of the unbraced
  top. (remember I've got a .physics account) If this is the case, then the 
  presence of the brace must to some extent damp the vibration of the top.
  Then it seems reasonable to think that the stiffer the brace, the more it 
  will damp the top. I think scalloping the brace makes it less stiff and 
  thereby reduces the damping effect on the top. Certainly there is a difference
  if I for example play an acoustic guitar with a guitar pick sitting on the
  soundboard versus playing one with a cinder block sitting on the
  soundboard. I think the presence of the cinder block will damp the
  vibration of the top more than the guitar pick. I think this is 
  an extreme example, but the physics is similar.
   Also I'm thinking about the thermodynamics: if energy is being dissipated
  thermally then this depends on the amount of material present. 

  Textbook examples are too simplified for this discussion, I'm afraid:
  these often assuming a simple (constant) damping force. 

  Remember that in a harmonic oscillator the frequency is proportional
  to the reciprocal of the square root of the mass. The potential energy
  and kinetic energy are both proportional to the mass times the square 
  of the frequency. The mass cancels out of the expression for the total 
  energy. It does however depend on the spring constant (stiffness).
>
>[Althoff wrote:]
> >I heartily suggest you go and play both a D-28 and an HD-28. This will
> >do a lot more toward understanding scalloped braces than any of this 
> >discussion.
>
>Have you got two to give away?! I fully admit to not having played both of
>them, but from the knowledge I have from my research a lot has been said that
>doesn't make sense to me. 

   You might try going to a music store. Or even the Martin Factory. 
   (well I guess since you have a .uk address this might be a little
   tough-- it would definitely help your research, though....)
   They have a room full of all kinds of Martin guitars with a sign that
   says "please play". It is dangerous to develop theories without any 
   knowlege of the experimental data. The difference between a guitar 
   with scalloped braces (e.g. HD-28) and a similar one with unscalloped
   braces (e.g. D-28) is *really* obvious. I'm not saying that the issue
   is uncomplicated or that every guitar of a given model is the same.
   However there are most definitely  general characteristics of the 
   sound that are always present. My explanations focus on what I've heard
   and an attempt to explain that. You've never even heard the difference.
   While it may be difficult to explain theoretically, you can hear the 
   difference immediately. This might be a valuable piece of data for
   your research.

>
>There are, no doubt, real differences from scalloping the braces, but I cannot 
>believe they are the ones talked about. The only way of proving anything would 
>be to scallop the braces of a D-28 (is that the unscalloped one?) without 
>changing anything else and compare (whether just subjectively or analytically).

  This is (almost) what one is doing by playing an HD-28 and a D-28. This
  is why I suggested this.

>Two hand-made guitars from a top luthier will *always* be different, no matter 
>how hard he tries (although some do get them bloody close sometimes). So 
>comparing two different guitars is not good enough in my book (although it 
>ought to give some indication if the guitars are really *that* close).

   How do you explain the general consensus among Martin owners (at least
   those I know) concerning the tonal qualities of scalloped vs. unscalloped
   braces?
>
>I hope all this proves more of a useful exercise than not. I don't wish to 
>enflame anyone with my arguments - I stated my defence and position to start 
>with I hope. Sometimes makers and scientists just have crossed wires on the 
>points they are trying to get across, in trying to talk in terms they don't 
>really understand (on both sides). I hope I've made sense on my side.

    A little personal experience with the instruments involved would 
    go a long way. I realize this is an extremely difficult topic to
    explain theoretically. All the more reason to get some experience
    with the physical realities and use that to guide the theory.

>
>Useful reading can be found in literature from The Catgut Acoustical Society,
>The American Guild of Luthiers, and the Acoustical Society of America.
>

   Useful guitars can be found at good music stores or at the Martin Factory.

  Thanks, Gordon, for a fun discussion...
  Best wishes,
  Jeff.
-- 

------------------------------
Jeff Althoff
jeff@max.physics.sunysb.edu