Secrets Of Body Resonance?

Discussion in 'Luthier's Corner' started by freatles, Jan 24, 2014.


  1. freatles

    freatles

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    So my ongoing skunk works inquiry into the black arts of bass building continues...

    Do you think there's truth to my uh "pals" deductions:

    One guitar luthier presented a tale of how - The long side of the bass guitar body has to be at least 1/4 as long as the longest wavelenght of the lowest pitch?

    Strangely the short side didn't need to be as short as the shortest wavelength :p

    Anyway, what do you know about the way sound is resonating (ie. cut off) or conducted in the shapes of the body?
     
  2. HaMMerHeD

    HaMMerHeD

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    That is utter nonsense.

    The E1 string vibrates at 41.2Hz. At sea level, the speed of sound is ~1116 feet per second, so the wavelength of a 41.2Hz note is about 27 feet. A quarter of that length is 6.75 feet, or 81 inches. I've not personally seen a bass quite so big.

    Perhaps he meant the wavelength of the note through the wood rather than the air, but you'd have to test every piece of wood in order to find the right size for any given body, because of the variance of density and elasticity from one specimen to another (not to mention the complications that would arise from adding multiple species, glue, etc.). Even then, I'm pretty sure the distance would be very large, so even a 1/4 wavelength would not be feasible.
     
  3. wmheilma

    wmheilma

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    I think it has more to do with wood density and the joinery of woods of different properties than the size of the body, but it is hard to say. Another odd thing is that the basses I play the most have become more resonant after 25 years of playing them a lot.
    It would be very hard to get enough control to do a good experiment, but this is the place for folks who might actually be able to pull it off and figure out the science of the voodoo.
     
  4. wraub

    wraub

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    I'll have an IPA, please.
    Good wood without voids, clean mating surfaces, and appropriate clamp times for me, thx. All else seems voodoo.
     
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  6. reverendrally

    reverendrally

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    This is a question worth asking, but you have to ask what resonance is and how it's helping the bass by having it...

    http://www.thefreedictionary.com/resonance

    I think what we're asking for is 3 and 5. However, that said there are a few other things that we need to be sure of.
    1. On an electric instrument, what does the body do?
    2. How will said resonance change the sound of an electric instrument?
    3. How can we measure changes in resonance?

    Ans to #1; The body on an electric is primarily there to hold the bass together, have a secure mounting for neck, bridge, pickups, controls, strap, some ergonomics etc. That's it's primary function, which is completely different from an acoustic instrument. It's a stable, rigid structure rather than an acoustic chamber.

    Ans to #2; My feeling is, to change the resonance means to change the stiffness (or density) to some it extent. Changing the stiffness means in physical terms some loss in vibration, hence a slight loss in sustain. It may also mean that you loose the reproduction of treble frequencies which tend to decay the fastest if the stiffness is less. That said, all of these changes might also be caused by old, flatwound or tapewound strings, changes in EQ, player technique or the like.

    Ans to #3; Right so here is the doozy. Everything on an instrument effects it's sound. You'd have to build a number identical basses. Some with a differing amounts of body stiffness and measure them acoustically and electrically. Expensive, highly difficult and could be compromised by one bad batch of timber.

    I've come to the conclusion that it's pretty hard to measurably accomplish this. It's intuitive at best. So I've got a different approach. Acoustic instruments are known for being both empty and light. In fact, the lighter body, the better the instrument in many ways. Now of course that's a generalization, but basically none of us want to play really heavy basses. So wherever possible, I chamber the heck out of the basses I build. I also use light timbers to begin with. It's my feeling that this 'lends itself' (because it's hard to measure) to more resonant basses. Roger Sadowsky has said similar things about lightness, chambering and resonance.
     
  7. freatles

    freatles

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    In my physics addled world (read: for me) the definition of "resonance" is very simple:

    it is the frequency at which a body starts to vibrate, absorbing the energy (or the sets of frequencies)

    In this string instrument context, resonance of the instrument body at some frequency means that frequency will be lost or dampened.

    Then again, what do I know anyway! Cause resonance might also cause the body to transmit vibrations.
     
  8. Beej

    Beej

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    I'm not sure if all physicists would agree that resonance is very simple. :)

    Does it matter so much in this context what the resonant frequencies are in a given body? How much would the resonance of the body in this system (acoustic resonance?) actually impact the sound as it's produced by this system? Meaning solid bodied electric guitars and basses? Would knowing or building towards a particular resonant frequency impact the production of sound in a meaningful/predictable/significant/interesting/discernible/perceptible way?

    I fail to grok it at this point...

    :D
     
  9. pilotjones

    pilotjones Supporting Member

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    You've got it pretty much backwards. When a mechanical system is driven at its resonance frequency, the energy is not lost, it is retained and tends to increase. The note you get blowing across a beer bottle is at the resonant frequency of the bottle cavity, and a random input (wind) is enough to excite the vibrations.

    And that quarter-wave of the bass side of the body thing is just ridiculous.
     
  10. freatles

    freatles

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    pilotjones - I agree, the quarter-wave story seems rather mythical.

    It illustrates how mysterious this phenomenon is. Despite the fact that there would be millions of string instruments to study this resonance on - hundreds of clever bassist on this site alone!

    I may be wrong but my view is that energy must be lost if something resonates, and that the amount of energy can not increase. The amount of audible sound vibrations might increase, though.

    Otherwise we would run power plants on resonance.

    Thinking in writing, the bottle example is another property of resonance, transforming vibration frequency.

    I understand the bottle-whistle as air column resonance transforming input energy into one frequency (or perhaps overtone-series). In comparison to plain air - where one hears hissing when blowing, the resonance transforms airpressure into audible waves - filters the sound.

    I always thought that "dead notes" on a neck are because that is the resonant pitch of the neck, the body or their combination. The movement of these parts absorbs the vibration, so strings lose their energy (movement) quickly.

    Perhaps the body vibrates - is it transforming the conducted vibration, I am not certain.

    But this is something I believe is very much possible to model with physics, and possibly someone already has done that in bass guitar context?
     
  11. Jazz Ad

    Jazz Ad Mi la ré sol Gold Supporting Member

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    Whatever people are looking for with these calculations, I don't think it is what they think it is.
    If the wood resonates to a given frequency, you get a dead spot, because it absorbs the energy from the string.
     
  12. freatles

    freatles

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    I think it is a very exciting idea that if there was a simple way to calculate these frequencies for different materials, one could avoid dead spots! :cool:

    If not for wood, at least for carbon fiber, luthite, ebonite, truss rods, etcetc
     
  13. reverendrally

    reverendrally

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    You know what, I was about to shoot the whole "I want resonance thing down", but that's silly.

    So I say, get in the shed. Build some stuff with your ideas and theories and let's test them out. You never know, you might hold the key to fixing a swag of stuff we never thought of! So go to work. :)

    BTW, if you want to avoid dead spots, I've found the best way is by avoiding fender style 1 piece necks. ;)
     
  14. pfox14

    pfox14

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    I defy anyone to measure the resonance or wave length or note or whatever that a 1 3/4" thick piece of hardwood has. Can't be done and doesn't need to be done. If you could measure all of a bass' components and what they contribute to the overall tone & volume, I would bet that the wood the body is made of constitutes less than 5% of that equation (if that).
     
  15. freatles

    freatles

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    pfox14 - it is trivial to measure the resonance (for a single, actual piece of wood). It has been done many times. Also, needs to be done sometimes. And yes, you can measure the components. What is hard is making a model out of those resonance graphs.

    See the graph on page 5 http://courses.physics.illinois.edu...ill_Drew_Beeson_P199pom_Fa07_Final_Report.pdf

    What is much more desirable is a way to make some simple equations, so you will know before building those components. A simple model for the resonance.

    The problem with wood is the varying conditions it grows in leads to each piece of wood being slightly different. So it is harder to model than luthite or air, for example.

    Then again, again, what do I know? :cool: I even think composite and aluminium could be great materials for bass guitars!
     
  16. freatles

    freatles

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    Ok, digging deeper into the dark labyrinth, I learned that a device used to measure the resonance of tone woods is called Lucchi device.

    I get the drift it is super easy to replicate if you have a two channel sound interface *with high impedance input, and two piezo disks wired to 3/4" jacks. Anyone use one?

    http://soundpostonline.com/archives/340/

    Looks like I am 'making progress' here :cool:
     
  17. HaMMerHeD

    HaMMerHeD

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    Reminds me of a Scientology e-meter.
     
  18. pilotjones

    pilotjones Supporting Member

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    Look, resonance isn't dampening. The reason for the dead spot is that the neck resonates, and the note is being fretted at and antinodal point for that frequency which is out of roughly 180° out of phase at that location so it cancels the string energy due to superpositioning. If that frequency were damped, it would not have this effect.
     
  19. pilotjones

    pilotjones Supporting Member

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    Lol, true.
     
  20. freatles

    freatles

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    Oh, that sound very credible!
     
  21. freatles

    freatles

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    Waitaminute, so howcome the fat finger works? The headboard vibrating, resonating to the pitch that drains the energy from the string -theory.

    pilotjones - look at it as a whole system. String gets mechanical pluck causing it to vibrate. The headboard happens also to vibrate - resonate - at these frequencies, dampening the string vibration sustain. That is one example of resonance causing damp(en)ing.
     

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