Acoustic Gtr has unwanted Resonance

[Sonixx]

i've got a Takamine Acoustic Gtr and it's got a resonance problem with G# and A (~108Hz) and in the first 2 or so Harmonics.

when playing any G# or A on the Low strings the Notes die prematurely and basically sound terrible.

after some investigation, i've determined that i can apply pressure on the body just beyond the bridge and really decrease the resonance.

Question: anyone have any experience stiffening or adding mass this section of the guitar body. i don't want to change the tone very much with a mod so i'd like to do as little as possible. would glueing a small amount of wood to the inside just after the bridge be effective without altering to tone significantly?

any suggestions...thanks...

-kp-

 

[Treeline]

Nobody else has replied, so how about this:

Before you think about fancy fixes, remember Occam's razor: the statistically most likely solution to a given problem will be the simplest solution.

How do the strings sound open? If you have an imbalance that really shows up with a particular fretted note, try this:

Pull the saddle and place a little scrap of paper - say, standard copy paper weight, 1/4 " square (or smaller to fit the saddle opening) - under each of the offending string spaces, replace the saddle, string 'er up and then see if there are any changes. Once the paper compresses, it acts like a shim and puts a little more pressure on those strings than on the others.

 

[FZmontanaDF]

Bring it to a guitar tech.

 

[Sonixx]

thanks Treeline,

i tested the resonant frequency of the body and here's a graphic showing the problem. you can see the large resonance and some harmonics. i placed a mic inside the guitar body and swept a sine wave for external excitation. the guitar body really vibrates at certain frequencies as you can see.

i'm looking for a way to dampen this some. it may end up not being worth the hassle and i'll sell the guitar. if i apply pressure on the body, dampen it, the tones are much more even around the problem area.

thanks for the input...

-kp-

 

[Sonixx]

Bring it to a guitar tech.

thanks, but i generally do my own guitar work. if i can't fix it, i sell it.

 

[Fangar]

Sounds like you might need a Triphazer!

Fangar

 

[Major Tom]

I had a similiar problem with an old Ovation, I discovered that one of the small braces under the top had come unglued. I got out some glue and a C clamp and fixed it, problem solved. It is tough to work thru a small opening using a mirror, since all of your movements are reversed; I guess that's why Dentists make the big $$. Anyway, get out a small mirror and a flashlight, see if you can find anything loose. It might help to flex the top a little, otherwise if an unglued brace rests against the top you won't see it If you have room, reach in and pull on the braces. Thump on the top with the strings off and on; do you hear anything rattle? Also as mentioned in a post above, it is important that the bridge saddle is seated properly, sometimes the wood of the bridge can swell in such a way that the saddle is elevated in spots; it doesn't contact the entire bottom surface of the saddle. This will result in uneven sound, although in my experience the problem will be that some strings will have less volume and or sustain than others. If all else fails, take it to a Luthier for an "estimate", maybe he can tell you what is most likely wrong with a quick examination, and then you can take it back home and do it yourself.

 

[Treeline]

Is this a valuable guitar? Still under warranty? Is there anything special about it, or is it a workhorse instrument?

I'd try the simple thing first, and experiment in a way that is nondestructive. Then, if you locate the hot spot and can learn a little about how it responds, you will begin to see options. You've done a little of that already.

How does the guitar change if you roll up a t-shirt and stuff it into the soundhole? (I'm not kidding) Does the sound get dampened all across the board, or just a little in the bass end? Does it sound better?

Do you get the same unwanted resonance playing it tighter to your body than you are used to doing? In other words, does it change in any relevant way if you dampen the back only?

What happens if you tape a washcloth or small towel to the inside back of the guitar? (Use masking tape!!) Underneath portions of the top? Which frequencies change?

I'll echo Major Tom a little here. If you do your own work, you'll be familiar with the internal bracing. Look in there with a mirror and poke around to see if any braces are loose or broken. Something may not be doing its job.

The low resonant frequencies of the guitar will become enhanced with a thin or lightly braced top, and the trebles are enhanced (within reason) with a thicker or more stiffly (heavier) braced top. So if a brace is loose but isn't buzzing, it is not stabilizing the top as it should and you can get wolf tones popping up in odd places. Check to see the condition of the back plate (the extra reinforcement piece directly underneath the bridge, nestled in the lower central part of the X- brace).

You can experiment with adding a little mass to the top by using a small office supply clip on one of the top braces. Do this a little at a time; there will be a spot - somewhere - at which there may be a radical change in sound. Try to locate it with some precision and your options will open up. For instance, a cleat glued directly under a hot spot will enhance the mass of the top at that point and will change the sound.

Once you've gone through all the nondestructive ideas you have, you'll have developed a list of options. Then consider whether to take it to a tech; it is the easiest thing in the world to make a real mess with glue and a razor knife...

If you're really into this, you can experiment with clean glitter (or clean sand) and sound. Placing a speaker inside the guitar, you can send it a tone of a given frequency and measure the top's response graphically by sprinkling glitter on the top. As it resonates, the glitter will form a pattern that is a signature of that frequency and the particular build of the top (as affected by its attachment to the rest of the instrument). There are keys for different body types that contain signature information which may become useful to you.

Do a Google search for the Guild of American Luthiers (back issues of American Lutherie) and you will find technical discussions of this concept. Another source is the online Musical Instrument Makers Forum ( www.mimf.com ) which has highly moderated threaded discussions and a body of archives that will astound you. Another name to search for in this area is Rick Turner, a luthier who is active in the diagnostics and new ideas area of the craft. (The co-founder of Alembic and currently builder of the Renaissance guitar lines).

Finally, do a search of the term Helmholtz frequencies and it'll open up an acoustic can o'worms. Have fun!

 

[Sonixx]

I had a similiar problem with an old Ovation, I discovered that one of the small braces under the top had come unglued. I got out some glue and a C clamp and fixed it, problem solved. It is tough to work thru a small opening using a mirror, since all of your movements are reversed; I guess that's why Dentists make the big $$. Anyway, get out a small mirror and a flashlight, see if you can find anything loose. It might help to flex the top a little, otherwise if an unglued brace rests against the top you won't see it If you have room, reach in and pull on the braces. Thump on the top with the strings off and on; do you hear anything rattle? Also as mentioned in a post above, it is important that the bridge saddle is seated properly, sometimes the wood of the bridge can swell in such a way that the saddle is elevated in spots; it doesn't contact the entire bottom surface of the saddle. This will result in uneven sound, although in my experience the problem will be that some strings will have less volume and or sustain than others. If all else fails, take it to a Luthier for an "estimate", maybe he can tell you what is most likely wrong with a quick examination, and then you can take it back home and do it yourself.

thanks...i have already used the mirror and i did not see anything that looked suspect. although after reading yours and Treelines posts you've given me some new approaches.

i have already checked the saddle and made sure it is seated correctly and i've tugged on the inside braces for looseness. i think i'll check for looseness again. my hunch is this is somehow brace related. i can press on the body just after the saddle and the problem is almost gone.

thanks for the input...

 

[Sonixx]

Is this a valuable guitar? Still under warranty? Is there anything special about it, or is it a workhorse instrument?

It's not what one would consider valuable such as Vintage or Special in any way other than i like the way it plays records except for this problem.

How does the guitar change if you roll up a t-shirt and stuff it into the soundhole? (I'm not kidding) Does the sound get dampened all across the board, or just a little in the bass end? Does it sound better?

i haven't tried the t-shirt but i will.

Do you get the same unwanted resonance playing it tighter to your body than you are used to doing? In other words, does it change in any relevant way if you dampen the back only?

no i can stop the resonance problem by pressing on the top just behind the saddle

What happens if you tape a washcloth or small towel to the inside back of the guitar? (Use masking tape!!) Underneath portions of the top? Which frequencies change?

haven't tried it but i will

I'll echo Major Tom a little here. If you do your own work, you'll be familiar with the internal bracing. Look in there with a mirror and poke around to see if any braces are loose or broken. Something may not be doing its job.

i looked but think i'll look again and tug a bit harder on the braces

The low resonant frequencies of the guitar will become enhanced with a thin or lightly braced top, and the trebles are enhanced (within reason) with a thicker or more stiffly (heavier) braced top. So if a brace is loose but isn't buzzing, it is not stabilizing the top as it should and you can get wolf tones popping up in odd places. Check to see the condition of the back plate (the extra reinforcement piece directly underneath the bridge, nestled in the lower central part of the X- brace).

i'll check

You can experiment with adding a little mass to the top by using a small office supply clip on one of the top braces. Do this a little at a time; there will be a spot - somewhere - at which there may be a radical change in sound. Try to locate it with some precision and your options will open up. For instance, a cleat glued directly under a hot spot will enhance the mass of the top at that point and will change the sound.

i'll try it

If you're really into this, you can experiment with clean glitter (or clean sand) and sound. Placing a speaker inside the guitar, you can send it a tone of a given frequency and measure the top's response graphically by sprinkling glitter on the top. As it resonates, the glitter will form a pattern that is a signature of that frequency and the particular build of the top (as affected by its attachment to the rest of the instrument). There are keys for different body types that contain signature information which may become useful to you.

i'm going to try this. i've read about this before but now i'll try it.

thanks for you input...

-kp-

 

[TexRoadkill]

Just get a new guitar . I've had great results recording on my Larivee.


Does a different mic placement help? Are you close micing? Maybe back off the mic to get a bigger sound picture and it wont let those notes dominate so much.

 

[Sonixx]

hey TexRoadKill,

i may end up getting new guitar. i'm just not there yet, but it may not take a whole lot to get me there.

it's not in the mic or micing techniques. in my previous post i wrote about the guitar body and resonance and some testing i've done. the tonal differences are noticable when playing. it's worse when finger picking than with a pick. i'm a EE so i've got a pretty good take on the resonance thing, what i don't have a good take on is the remedy, if any.

thanks...