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Fixing resonance dead spots on an ABG.

Discussion in 'Luthier's Corner' started by Rockin John, Jul 27, 2017.


  1. Hello!

    My avatar shows me with my Tanglewood TW55 FR. It's a bass that has pronounced resonance dead spots on all E, F, F# positions, except on the low E string, low positions.

    It's quite a long story how I came to buy it (new) and keep the bass in this condition ... but I did! As a result it's lived in its case virtually unplayed until recently because I just couldn't get on with it for that reason. So it's now time to sort the issue or move it on: it'd be nice to sort it because I do like the bass, otherwise.

    I'm long enough in the tooth to have made, tinkered with, modified, etc, a variety of stringed basses so the practical side of any work required should be possible.

    I just need guidance as to what I can actually try to do. So far I've experimented with adding mass to the headstock, playing with mutes, adding a bass bar (temp fixture). A G clamp on the headstock eased the E problem a bit but the F and F# remain.

    Thoughts / wisdom gratefully accepted.

    Tks.

    J
     
  2. The remedy to this issue may be a fret leveling in the spots that are dead. Then some fret clean up/polishing of course.

    Is it isolated to the upper register or a few of the strings?
     
  3. Thank you. As you will note, this bass in an unlined fretless.

    The spots are A - 7,8,9. D - 2,3,4. G - 9,10,11

    Thanks.

    J
     
  4. Ok then, sorry about not noticing that.

    Perhaps someone with more expertise on fretless will chime in....
     
  5. Bruce Johnson

    Bruce Johnson Supporting Member Commercial User

    Feb 4, 2011
    Fillmore, CA
    Professional Luthier
    This is a tough one, with no real easy answer. You've isolated it as a real resonance deadspot problem. That is, the structure is going into resonance at those particular frequencies, deadening them regardless of where they are on the fretboard.

    The solution is going to be to slightly change the stiffness of the structure. But that's not going to be easy. On a full acoustic instrument, it's all much more complicated. It's going to be hard to identify just where it needs to be stiffened, and how much. And, stiffening it may just shift the problem around a few notes, without reducing it.

    On that type of instrument, the softest part that moves the most is the top with its bracing. So you may want to start there. Maybe try experimenting with a soundpost, as used in violin family instruments? Cut and fit a small stick that lightly wedges between some point on the bracing on the back and the bracing on the top. Try it in different locations and see what it does. As long as you aren't forcing it, you won't be harming anything, and can take it back out. It may reduce the overall volume, but it may even it all out.
     
    reverendrally and Jazz Ad like this.
  6. Hello Bruce, and thanks for your comments. I did try a soundpost (I stupidly called it a "bass bar" in the opening post, which of course is a completely different thing!). Tho no real difference, I will give that another shot.

    I've some understanding of the root problem of resonance, and the mechanics that's behind such things as stiffness. I also appreciate that I'm more likely to move the resonant point than fix it. Evening out the volume is something I didn't think of, tho, and that's worth consideration if it can be achieved: of course, it'll either happen or it won't!

    Yet to try is stuffing soft material into the body - Eg. Foam rubber. May help / work, may not!

    Thanks again.

    J
     
  7. andruca

    andruca

    Mar 31, 2004
    Madrid (Spain)
    Haven't tried it on any ABG but on electric basses a Fat Finger does the trick (basically a metallic mass that attaches to the headstock). It shifts neck resonance so that those dead spots shift to somewhere else where they don't bother. I think it's a good idea to try cheap non-invasive stuff such as this first. You could even try attaching a small metal C clamp you might have around (put some cloth underneath to avoid scratching the wood) and see if it does something, then eventually shell out the 10/12$ that a Fat Finger costs (much nicer looking and less bulky than a C clamp).

    fender_07.
     
  8. rwkeating

    rwkeating

    Oct 1, 2014
    Chicago
    none
    Have you tried different gauge strings? Strings with different tensions may have a different affect on how the bass responds ... I think?????
     
  9. Bruce Johnson

    Bruce Johnson Supporting Member Commercial User

    Feb 4, 2011
    Fillmore, CA
    Professional Luthier
    Try the soundpost idea, but not in the location that a soundpost usually goes. That particular resonance could be caused by one area of the top being too soft, and it goes into resonance at that frequency. The solution may be to slightly stiffen up one quadrant of the top. The problem is figuring out where.

    If you are handy with tools, try making up a little jacking soundpost; a wooden post with a screw in the middle to adjust the length. Then you can experiment with putting it in different places inside, pressing lightly against various spots in the top. That could help you narrow down where the problem is. Then you can work out a more permanent solution, like an additional brace or post. You are trying to find the point where the largest vibration motion is happening, at that frequency, and then brace it so it can't move so much. Test and adjust; trial and error. But a fun challenge.
     
  10. Hello Andruca. No, I've not tried that proprietry gadget but I have tried a 6" G clamp / C clamp on the headstock. It helped the E a bit but not really the F or F#. This surprised me a bit because I was expecting more if a change with such a large lump of cast iron!

    Bruce, thanks once again. An adjustable sound post us a great idea. Naturally it's very difficult working inside the body via the sound hole. I could remove the Fishman, I guess, to give me more access.

    I have a decent workshop for wood (and for electronics which was my job before retirement). I suppose it would not be out of the way to capo the note to be plucked then try to resolve the resonant points with a suitable transducer: I can do all that stuff. Another way might be not to capo the note, mute the strings totally and feed electronic signals into the body. Then look for the resonant points again with the transducer. Hmm. Food for thought.

    RWKeating, I've only tried different brands of strings. They'll all have varying tensions but it made no difference!

    Best.

    J
     
  11. Bruce Johnson

    Bruce Johnson Supporting Member Commercial User

    Feb 4, 2011
    Fillmore, CA
    Professional Luthier
    You may learn something by just planting your thumb at spots on the top while plucking the strings, capo'd at the notes. You may find a spot where pressing lightly stops the deadening.
     
    T_Bone_TL likes this.
  12. Yes, sir, nothing ventured, nothing gained. I'll give it a go! Tks
     
  13. Jazz Ad

    Jazz Ad Mi la ré sol Supporting Member

    What I was going to suggest is basically what Bruce explained better.
    Don't bother with the headstock, your issue is likely the body.
    First super simple hack though, try to plug the whole hole with a sheet of cardboard and see what it does.
     
  14. Ok, Jazz Ad, and thanks. You are correct in that the headstock experiment appears to achieve nothing! I will fill the sound hole and see where that gets me.

    In the meantime I've discovered that part of the reason for the dead spots appears to be stimulating the other strings into vibration. Eg. playing the dead spots on the G string stimulates the E, A, D strings; one or more of them, anyway! This is a natural feature of stringed instruments, of course, but it was interesting that the dead spots improve slightly when E, A ,D are fully muted. I'm not sure what this implies ... yet!
     
  15. Bruce Johnson

    Bruce Johnson Supporting Member Commercial User

    Feb 4, 2011
    Fillmore, CA
    Professional Luthier
    Seeing the other strings go into resonance confirms that the resonance is happening, and that the problem is in the top. The top starts shaking and shaking the bridge with it, which makes the other strings vibrate. They aren't the cause, they are a side effect.

    You can use this in your diagnostics. While plucking the offending note and watching the other strings resonate, press on spots on the top to see what spots reduce the resonation of the open strings.
     
    chapito likes this.
  16. Jazz Ad

    Jazz Ad Mi la ré sol Supporting Member

    Linking the top and back of the instrument is a good way to get rid of this kind of resonance. If makes both sides stiffer.
    You can test try it by GENTLY using a spreader, AKA reversed clamp through the soundhole.
     
  17. Thank you. Yes, I understand that parasitic resonances are an effect, not the cause. I must find nd that capo.....
     
  18. Ok, and thanks. The sound hole experiment was interesting. Using the cover appears to make other dead spots without doing much to fix the existing ones!! I didn't have much time to try this properly, but there's definitely a new one appeared at B, 4th 'fret' G string. Will have another go later today and see what I can find out.
     
  19. Another general observation from closer investigations is that my original observations weren't quite correct.

    The dead spots aren't truly dead. They're only partially dead in comparison with notes that ring out correctly.

    Also different notes are affected on different strings. On the D string, 'frets' 2-E, 3 - F, 4 - F# and 5 -G are affected. On G string the string open and the octave ring as clear as a bell.
     
  20. Thanks. Thinking more broardly about the whole thing, can it be **just** the top? The instrument is a complete mechanical system where resonances are made up from contributions from all the parts, as it appears to me to be. Also, I suppose, the vibrating air inside the body plays its part, as might the fact that the player must dampen body vibrations by the simple act of playing it (the bassist almost 'wears' the bass!). Interesting.....
     

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