From: scogpw@thor (Gordon Walker)
Newsgroups: alt.guitar
Subject: Re: Guitar Acoustics
Date: 7 Apr 92 17:55:12 GMT
Organization: University of Wales College of Cardiff, Cardiff, Wales, UK
X-Mailer: Cardiff Computing Maths PP Mail Open News Gateway

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..............!)

Sorry Tab fans if you find this boring, but you never know, you just might
find something of interest.......(although this might all drag on a bit - 
apologies in advance if it gets too long!)

Firstly, I think I ought to declare where my interest lies, as this seems
to be a "scientist vs makers" argument. Apart from being a keen amateur player
of both acoustic and electric guitars (which is why I started reading this
group), I come under the "scientist" label. I have done a lot of detailed
research on the classical guitar in one of the few dedicated labs in the
world set up to do this. (Note, classical, not steel string - this could prove
significant!) This has included liaising with other scientists in the field
*and* with some professional makers (those that will talk with us anyhow!). 

One of the biggest problems you get in this environment is the clash between the
makers and the scientist. The maker generally doesn't like having his toes trod
on by someone steaming into his speciality and saying he's talking rubbish.
Likewise the scientist isn't too impressed with some of the things a maker
claims he knows, "because it just ain't physics". At the end of the day, half
the problem is each trying to talk in terms they don't understand. A scientist
doesn't really know what a maker "does" to his plates when building a guitar,
and similarly a maker shouldn't make assumptions about the physics of a guitar
without really knowing, or thinking he knows, about the subject.

Therefore, whilst there are some things I *know* (from my research), there are
things which I will always learn from a maker. I can't deny things that really
do happen in a maker's experience, but I think I can argue about some of the
reasoning with more confidence.

And so (at last!) to some of the responses over the last few days:

Jim Muller muller@alliant.com wrote:

|[that I wrote: ]some
|good things about acoustic guitar design to which I'd like to add just a
|minor quibble or two.

Thanks for the words of support Jim!

I originally wrote:
|>Regarding loudness vs sustain: you can't increase *both*...  You can't
|>just get extra energy from somewhere for the sustain of the note. 

And Jim responded with generally good points about (1) you can do this by
changing various parts of the guitar with woods of different damping; and 
(2) that makers can intuitively increase the guitar's response in the 2-4kHz
range to create a "louder" guitar.

What I was originally replying to was the claim that loudness and sustain 
could be both increased by *just* scralloping the braces. Your point 1 
emphasises this by saying you need to change the material properties of the 
woods used to achieve this, you cannot do it just by changing the existing 
construction.
Your second point is certainly interesting - one I've not heard of before.
The 2-4kHz frequency range is a very "fuzzy" area in analysis of the guitar
response, and it would seem (to me) unlikely that a maker could affect this
range specifically without affecting things in other frequency ranges. But, if
there are results to show this, I would definitely like to see them. Any
references Jim?

Now regarding the reply given by Paul Ebersman (ebersman@uunet.uu.net) to my 
last reply to his first posting (if you follow me!)....

The point about "louder instruments are better": there may be a small amount of
mileage in this as a subjective assessment, but I would still argue against the
statement "acousticians generally agree" this to be so. (Backus may well have
suggested this - but that's only one acoustician.)

[Ebersman wrote:]
|If you have ever read anything on the Kasha (sp?) bracing system .....

I'm afraid the only thing ever published (that I know of) by Kasha is
universally recognised as complete rubbish. He was a chemist who made claims
that were just plainly wrong. If you are basing your understanding of guitar
acoustics on the theories of Kasha, or Kaman with Ovation for that matter, 
you are unfortunately being misled in many aspects.

[Ebersman wrote:]
|If you mike a guitar and put it on a scope, it has a waveform. You can
|compare two waveforms and the more closely they correspond, the more the
|sounds appear to be the same when put back through speakers. Therefore,
|you can compare waveforms for guitars and this would seem to be a nice,
|scientific way of comparing them.

I'm afraid it isn't. If you mean that you are playing a certain note to give a
waveform, that waveform contains a hell of an amount of information. This will
be mainly its frequency, with any information about the body modes being
excited and radiating the sound being quite subtle and vague. The same note on
a different string will give a different waveform. Moving the microphone will
give you a different waveform. Playing a different note on the same string will
obviously give you a different waveform. Playing identical notes in the same way
on two different guitars may give just as similar waveforms. Analysing guitars
involves an awful lot more to gain any useful information from them.

[I wrote:]
|>These paragraphs I'm afraid just show your ignorance of the subject - or
|>that what you've read has been misleading. For a start, "voicing" the
|>top (which I think I know what you mean) is just makers' jargon for
|>getting the right "feel" of a top plate, although they don't really know
|>what is happening acoustically (please put me right if you have evidence
|>to the contrary). It  would be certianly a quite complicated and subtle
|>effect to scallop the braces - and not necessarily making the instrument
|>just louder. The effect of braces to start with has been shown by
|>research to be fairly subtle and complex anyway, in the way they effect
|>the different "vibrational modes" of the top plate.

[Eberson wrote:]
|I guess building guitars doesn't count as knowledge of the subject...

Well, you know about building guitars, I know about their acoustics...

[Ebersman wrote:]
|No, it's not getting the "feel" right. You go by sound.....
 
You seem to do things differently from the makers I know - they certainly do go
just by "feel". Maybe it's a thing classical maker's do more, I don't know.
It's actually quite interesting that you say that you use a tap toning method
(more common in my experience with violin makers), and especially as you do it
with the top attached to the sides. I didn't know you could do that! (because I
would have thought that if you want to alter the top, you wouldn't have the 
back on; and if the back wasn't on it would fall apart, wouldn't it?).
It sounds by your description of your tapping technique like you're looking for
the lowest mode of the top plate. I slated you for referring to it as *the* top
frequency, because it sounded like you thought the top only had one resonant 
frequency. You can excite other modes of the top plate too if you know what 
you're looking for, by holding and tapping in different places.

The lowest mode of the top plate is certainly very important in determining the
tone quality of an instrument. Our research team is discovering, though, that
things are by no means as obvious as anybody has thought. How important its 
frequency is here we're not really sure yet. The frequency is important though
when it comes to considering "wolftones" which should certainly be avoided as
much as possible (although some effect will always exist somewhere). If you are
trying to adjust this frequency, the backplate definately needs to be in place,
as it's presence is vital to the final frequency of the lowest mode. (This is
what I originally meant by "finished" instrument - I know there's a long way 
to go but things aren't so important acoustically from then on).

[Ebersman wrote:]
|Strength vs mass *is* stiffness... 

Hmmm - I'm not exactly sure which physics books you read! "Strength"
isn't really a definable quantity as such, but I think I know what you
mean.

[Ebersman wrote:]
|The braces change the stiffness of
|the top and increase its strength without increasing it mass as much as
|just using a thicker top. Scalloping changes the mass and stiffness of
|the braces, so it changes how they stiffen the top. 

All sensible statements here - it's just that in your first posting you only 
talked about the mass of the braces in this respect. The next question is just 
how this extra stiffness (which is very localised, and probably quite subtle)
affects the different modes of the top plate. You can't just jump in with an
assumption about loudness and sustain at this point.

[Ebersman wrote:]
|Sanding the edge of
|the top also changes stiffness in different places on the top, changing
|the stiffness and the sound. Since different pieces of wood have
|different stiffnesses, this is yet another variable.

Again, reasonable things to say in the same context as above, but you can't
just say "and the sound". There is an awful lot in between.

[Ebersman wrote:]
|A factory has to think in worst case scenarios of the worst wood being
|used, so they overbuild. If you then hand voice the top, it allows you
|to shave more wood off of tops/braces with stiffer wood and still have a
|strong top. Unfortunately, as you noted many times, this is still a very
|touchy-feely thing and not terribly scientific, so robots just don't cut
|it.

I think we agree at last! I'm glad you mentioned needeing to have a "strong
top". With classical guitars, some of the braces which act principally as
structural elements also are important acoustically more than a maker might
expect. A lot goes into the acoustical makeup of the top plate - not least the
bridge which acts as a very important cross brace (having a much greater effect
than many of the braces makers normally concentrate on). That's something for
you to consider in future maybe.........


And finally.......(if anyone's got this far!)...

...Jeff Althoff (jeff@dirac.physics.sunsyb.edu)'s posting contained the
following:

[I wrote]
|>These paragraphs I'm afraid just show your ignorance of the subject ....

[Althoff]
|  Well, the phrase "glass houses" comes to mind.....

Well thanks very much! But I think the glass house is for you not me (read
on)......

[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!
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.

[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. 

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).
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).

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.

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

Sorry about the length of this - I hope you made it to the end!

Cheers ,

Gordon Walker (scogpw@thor.cardiff.ac.uk)
Newsgroups: alt.guitar