How can I lower the body resonance of an acoustic guitar?

[Frank]

How can I lower the body resonance of a solid-spruce-topped acoustic
guitar? I have already shaved some of the back braces which didn't
make any differnce. So now, I guess my only course of action is to do
something to the top. What I can try? Thin the top near the edges?
Shave braces? I have never before tried shaving top braces, but I'm
willing to try it if I stand a chance of success. It's not a vry
expensive guitar, but it's one that I love the look of. Trouble is,
there are 'dead' notes around middle A, due to the body's resonant
frequency. Those notes are converting into physical vibrations that I
can feel being absorbed by my body - rather than going outwards, as
sound waves.

Thank you

Frank

[Nick Odell]

Post by Frank
How can I lower the body resonance of a solid-spruce-topped acoustic
guitar? I have already shaved some of the back braces which didn't
make any differnce. So now, I guess my only course of action is to do
something to the top. What I can try? Thin the top near the edges?
Shave braces? I have never before tried shaving top braces, but I'm
willing to try it if I stand a chance of success. It's not a vry
expensive guitar, but it's one that I love the look of. Trouble is,
there are 'dead' notes around middle A, due to the body's resonant
frequency. Those notes are converting into physical vibrations that I
can feel being absorbed by my body - rather than going outwards, as
sound waves.

By "body resonance, do you mean the air resonance inside the body?
Reduce the size of the soundhole. Stick a temporary baffle over the
soundhole with double-sided tape and experiment with different sized
cutouts in it. If that solves the problem you will then know the
optimum size you require.

HTH

Nick

[Frank]

Post by Nick Odell

Post by Frank
How can I lower the body resonance of a solid-spruce-topped acoustic
guitar? I have already shaved some of the back braces which didn't
make any differnce. So now, I guess my only course of action is to do
something to the top. What I can try? Thin the top near the edges?
Shave braces? I have never before tried shaving top braces, but I'm
willing to try it if I stand a chance of success. It's not a vry
expensive guitar, but it's one that I love the look of. Trouble is,
there are 'dead' notes around middle A, due to the body's resonant
frequency. Those notes are converting into physical vibrations that I
can feel being absorbed by my body - rather than going outwards, as
sound waves.

By "body resonance, do you mean the air resonance inside the body?
Reduce the size of the soundhole. Stick a temporary baffle over the
soundhole with double-sided tape and experiment with different sized
cutouts in it. If that solves the problem you will then know the
optimum size you require.
HTH
Nick
--
real e-mail is themusic dot workshop at ntlworld dot com

Hi Nick,
I did try this but it didn't improve it. It actually seemed to make it
worse if anything. Does that mean that enlarging the soundhole might
be the answer? I'm hoping there might be another method.

Frank

[Nick Odell]

Post by Frank

Post by Nick Odell

Post by Frank
How can I lower the body resonance of a solid-spruce-topped acoustic
guitar? I have already shaved some of the back braces which didn't
make any differnce. So now, I guess my only course of action is to do
something to the top. What I can try? Thin the top near the edges?
Shave braces? I have never before tried shaving top braces, but I'm
willing to try it if I stand a chance of success. It's not a vry
expensive guitar, but it's one that I love the look of. Trouble is,
there are 'dead' notes around middle A, due to the body's resonant
frequency. Those notes are converting into physical vibrations that I
can feel being absorbed by my body - rather than going outwards, as
sound waves.

By "body resonance, do you mean the air resonance inside the body?
Reduce the size of the soundhole. Stick a temporary baffle over the
soundhole with double-sided tape and experiment with different sized
cutouts in it. If that solves the problem you will then know the
optimum size you require.

Hi Nick,
I did try this but it didn't improve it. It actually seemed to make it
worse if anything. Does that mean that enlarging the soundhole might
be the answer? I'm hoping there might be another method.

No! No! No! No! Enlarging the soundhole a)will shift the air resonance
upwards and b)is invasive and pretty well irreversable. It sounds as
if the problem is not the air resonance but elsewhere and I'd strongly
suggest you try Al's experiments to determine if it might be the top.
But please don't chop any more bits off the instrument until you are
absolutely sure that it's going to work!

Nick

[Frank]

Post by Nick Odell
But please don't chop any more bits off the instrument until you are
absolutely sure that it's going to work!

Have no fear... this guitar is worth too much to me. I'm gonna be
careful. ;-)

Frank

[Al Carruth]

Iirc this is a small guitar and probably has a short scale (I really should
review the first posts!), in which case it will have less string tension than a
full-sized one. If that's true you could get away with the thinner top. Most
steel strings these days have tops around 3mm or maybe a bit less, and
ful-sized classicals can go thinner than 2.0, although it starts to get a
little dicey below about 1.8. Remember that the stiffness of the plate goes as
the cube of the thicness, so a 10% reduction in thickness will reduce the
static stiffness by about 30%.

Even though you got the best results with the weight just behind the bridge I'd
be careful about removing too much wood there. All in all I'd start to thin the
top along the lower edge, and work up toward the bridge. If ou can get a mirror
inside and shine a bright light on the top you can gauge the thickness to some
extent by how much light shines through. To really do it right you'ld need a
piece of the same spruce of known thickness, but even without the calibration
it's a useful technique. I'd thin the top in preference to shaving the braces:
they contribute more of the stiffness that resists the bridge torque. Sadly,
this technique won't work with cedar.

It's good that you kept track of the frequency as you tried the weight: now you
will know when to stop! I'd actually go about halfway with the thinning and
try the guitar out for a while. You may find that it will work better thinned
out that it did with added weight. You migtht also find that some combination
of thinning and added weight will get you a good enough sound without weakening
the top too much.

Good luck!

Alan Carruth / Luthier

[Frank]

Post by Al Carruth
Iirc this is a small guitar and probably has a short scale (I really should
review the first posts!), in which case it will have less string tension than a
full-sized one. If that's true you could get away with the thinner top. Most
steel strings these days have tops around 3mm or maybe a bit less, and
ful-sized classicals can go thinner than 2.0, although it starts to get a
little dicey below about 1.8. Remember that the stiffness of the plate goes as
the cube of the thicness, so a 10% reduction in thickness will reduce the
static stiffness by about 30%.
Even though you got the best results with the weight just behind the bridge I'd
be careful about removing too much wood there. All in all I'd start to thin the
top along the lower edge, and work up toward the bridge. If ou can get a mirror
inside and shine a bright light on the top you can gauge the thickness to some
extent by how much light shines through. To really do it right you'ld need a
piece of the same spruce of known thickness, but even without the calibration
they contribute more of the stiffness that resists the bridge torque. Sadly,
this technique won't work with cedar.
It's good that you kept track of the frequency as you tried the weight: now you
will know when to stop! I'd actually go about halfway with the thinning and
try the guitar out for a while. You may find that it will work better thinned
out that it did with added weight. You migtht also find that some combination
of thinning and added weight will get you a good enough sound without weakening
the top too much.
Good luck!
Alan Carruth / Luthier

Al,
It's a full sized jumbo with steel strings. Thank you for the advice.
Can you just answe two moe questions:
What are suitable tools for thinning? Is an orbital sander OK? Or do
you guys use scrapers or something? Also, from your experience, would
you hazard a guess as to how much of the 2.5mm of the solid spruce top
I'll need to remove at the thinnest place (i.e. the back end)?

Thanks again for your help.

Frank

[Al Carruth]

Frank wrote:
<<Trouble is, there are 'dead' notes around middle A, due to the body's
resonant
frequency.>>

Do you mean the A on the G string, second fret? That's usually the 'main top'
part of the bass reflex couple between the top and the 'Helmholtz' air
resonance about an octave lower. Generally speaking the only way to move that
one is to shave braces or thin the top.

One way to see what might happen is to try adding some weight to the top.
Poster adhesive, such as 'BluTac' or 'FunTac' is good for this: I'd steer clear
of modeling clay since that can leach mineral oil into the wood. Knead a gob of
the stuff up to make it nice and sticky and slap it on somewhere to see how it
changes the tone. Adding weight will usually make the guitar a little quieter,
but you want to ignore that and concentrate on the tone change. The middle of
the bridge tie block is a good place to try first.

Adding weight in a certain spot has the same effect in dropping the resonance
frequencies as thinning the wood there. You can try different amounts of weight
in different places to find the most effective spot, and then concentrate your
thinning there.

Thinning the top or the braces is not something to be approached lightly,
though. I'd be particularly careful in removing material right around the
bridge, as that tends to weaken the top the most. Unfortunately, that's also
the spot that usually has the most effect on the frequency, so you're in sort
of a quandry. The 'safest' place to remove material from the top is back by the
tail block, but you obviously want to taper the thing all the way back up
toward the bridge. Try to avoid removing anything from the area between the
bridge and the soundhole: that's the most critical area structurally.

Finally, there may be more going on here than just the 'main top' resonance.
There are a couple of other things that can get into the act, and they may be
contributing to the problem as well.

Alan Carruth / Luthier
http://www.alcarruthluthier.com

[Frank]

Post by Al Carruth
<<Trouble is, there are 'dead' notes around middle A, due to the body's
resonant
frequency.>>
Do you mean the A on the G string, second fret?

Yes - or the 5th string, 12th fret.

Post by Al Carruth
That's usually the 'main top'
part of the bass reflex couple between the top and the 'Helmholtz' air
resonance about an octave lower. Generally speaking the only way to move that
one is to shave braces or thin the top.
One way to see what might happen is to try adding some weight to the top.
Poster adhesive, such as 'BluTac' or 'FunTac' is good for this: I'd steer clear
of modeling clay since that can leach mineral oil into the wood. Knead a gob of
the stuff up to make it nice and sticky and slap it on somewhere to see how it
changes the tone. Adding weight will usually make the guitar a little quieter,
but you want to ignore that and concentrate on the tone change. The middle of
the bridge tie block is a good place to try first.
Adding weight in a certain spot has the same effect in dropping the resonance
frequencies as thinning the wood there. You can try different amounts of weight
in different places to find the most effective spot, and then concentrate your
thinning there.
Thinning the top or the braces is not something to be approached lightly,
though. I'd be particularly careful in removing material right around the
bridge, as that tends to weaken the top the most. Unfortunately, that's also
the spot that usually has the most effect on the frequency, so you're in sort
of a quandry. The 'safest' place to remove material from the top is back by the
tail block, but you obviously want to taper the thing all the way back up
toward the bridge. Try to avoid removing anything from the area between the
bridge and the soundhole: that's the most critical area structurally.
Finally, there may be more going on here than just the 'main top' resonance.
There are a couple of other things that can get into the act, and they may be
contributing to the problem as well.
Alan Carruth / Luthier
http://www.alcarruthluthier.com

Hi Al,
Best results were obtained by sticking a 2.25 oz weight at the back
edge of the bridge. I added weight bit by bit. When I reached 2.25 oz,
the problem became barely noticeable...

The places where I feel the most vibration were at the bridge itself,
and to some extent about an inch behind the bridge. Adding weight at
the back of the bridge seemed to be the most effective.

So if I start thinning the top behind the bridge, where should the
most thinning be done: immediately behind the bridge, or right at the
back edge of the top? Actually, the top is already pretty thin to
start with (2.5mm).

Have you had any experience with those ghizmos that bolt to the
underside of the bridge plate and transfer some of the tension to the
back side of the guitar? I believe they were designed to counteract
top-distortion from string tension, but the designer also claims it
improves volume, IIRC... (was it called the Baggs system, or Boggs
system, or something??) I was wondering if one of those might be worth
trying - it might just dampen that A vibration sufficiently at the
bridge. What do you think?

Frank

[Frank]

PS

Just as a matter of interest, When I was adding weight just behind the
bridge, when I was up to about 2 oz, I noticed the problem frequency
had moved down to the G note. Adding another 0.5 oz cured this and the
guitar sounded very nice, except for being too quiet due to the added
weight.

So any additional advice on brace-shaving or top-thinning would be
much appreciated. (Bearing in mind that the top is already only 2.5mm
thick (under 0.1").

Can a belt-sander be used to thin the top? Any tips on telliing how
much you've taken off? If the anser is to shave top-braces, any tips
on how to get to them? Craftsmen with thin flexible arms are in short
supply around here. ;-)

Regards

Frank

[Rodney Myrvaagnes]

Post by Frank
Can a belt-sander be used to thin the top?

AARRGH!! Sputter sputter . . . . . Harumph

Rodney Myrvaagnes NYC J36 Gjo/a

"WooWooism lives" Anon grafitto on the base of the Cuttyhunk breakwater light

[Al Carruth]

I must have been thinking of anoter thread, probably on some other list. Sorry.


2.5mm is a bit thin for a Jumbo, so I'm not sure you would want to take off
much thickness. I use a scraper, and only sand lightly at the end to smooth
things out. If you had a lot to remove you might use a very sharp block plane
to begin with, but that's a gamble. They can tear out on spruce with very
little provocation, and leave you with a mess.

Alan Carruth / Luthier

[Frank]

Post by Al Carruth
I must have been thinking of anoter thread, probably on some other list. Sorry.
2.5mm is a bit thin for a Jumbo, so I'm not sure you would want to take off
much thickness. I use a scraper, and only sand lightly at the end to smooth
things out. If you had a lot to remove you might use a very sharp block plane
to begin with, but that's a gamble. They can tear out on spruce with very
little provocation, and leave you with a mess.
Alan Carruth / Luthier

As a luthier you are probably familiar with the JLD Bridge System
(http://www.jldguitar.com/) I was wondering of one of those might
help. I guess it would change the dynamics of the guitar in some way -
and possibly cure the problem. I guess the only thing os to try one
and see, yes?

Frank

[Al Carruth]

I've only seen one or two of the JLD setups in guitars. It should cut down on
the torque load on the top, which is what kills guitars over the long term, and
will certainly have _some_ effect on the tone, if only because of the added
mass. I'm always skeptical of sweeping claims ("Makes _every_ guitar better!"),
but it might help in your case by allowing you to thin the top more without
exceeding structural limits. And then again, it might not.

Alan Carruth / Luthier

[Sbtypesetter]

I've used it. Think their claims just
might be true. Nothing but good to
say about the system. Every flattop
I build will have the JLD system
installed.
-Rick

[l***@rubbishearthlink.net]

Post by Sbtypesetter
I've used it. Think their claims just
might be true. Nothing but good to
say about the system. Every flattop
I build will have the JLD system
installed.

May I assume you will make some modifications to guitars having the
system?

Jim L

[Sbtypesetter]

Just to lighten the bracing a little.

-Rick

[Frank]

Post by Al Carruth
I've only seen one or two of the JLD setups in guitars. It should cut down on
the torque load on the top, which is what kills guitars over the long term, and
will certainly have _some_ effect on the tone, if only because of the added
mass. I'm always skeptical of sweeping claims ("Makes _every_ guitar better!"),
but it might help in your case by allowing you to thin the top more without
exceeding structural limits. And then again, it might not.
Alan Carruth / Luthier

Just wanted to add a comment about my latest experiments. I spent some
time today experimenting further with the use of weights stuck to the
body. After quite a bit of trial and eroor, using different weights in
different positions on the top and back, I found I could really
fine-tune the sound of this guitar in various ways. It's quite a
fascinating thing. By sticking an object of the right weight in the
right position on the top, I can cut out the unwanted harmonics that
were sullying the guitar's real potential - and without much loss of
volume. I have truly cured the rogue A-vibe on this guitar now, using
a stuck-on weight. An even more interesting thing is that by moving
the weight to different positions, I can alter the tone of the guitar
quite considerably - it's like having a tone control on the thing -
but better, because the weight thing changes the characteristics of
the sound in ways that en electonic tone control wouldn't. I can now
make this guitar sound like on of about three different guitars,
depending on where I stick the weight. Simple but effective.. Cool!

Frank

[Ralph & Diane Barone]

Post by Frank
Just wanted to add a comment about my latest experiments. I spent some
time today experimenting further with the use of weights stuck to the
body. After quite a bit of trial and eroor, using different weights in
different positions on the top and back, I found I could really
fine-tune the sound of this guitar in various ways. It's quite a
fascinating thing. By sticking an object of the right weight in the
right position on the top, I can cut out the unwanted harmonics that
were sullying the guitar's real potential - and without much loss of
volume. I have truly cured the rogue A-vibe on this guitar now, using
a stuck-on weight. An even more interesting thing is that by moving
the weight to different positions, I can alter the tone of the guitar
quite considerably - it's like having a tone control on the thing -
but better, because the weight thing changes the characteristics of
the sound in ways that en electonic tone control wouldn't. I can now
make this guitar sound like on of about three different guitars,
depending on where I stick the weight. Simple but effective.. Cool!
Frank

Perhaps you should buy a couple of small rare earth magnets. They are
strong enough to stick to each other through the thickess of the top and
they should stay there with no adhesives.