question about acoustic properties of semi-hollow bodies

Discussion in 'Tele Home Depot' started by Dimitree, Jun 30, 2020 at 2:32 PM.

  1. Dimitree

    Dimitree Tele-Meister

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    I was looking at how the Rickenbacker 330 and 360 guitars were built during the years.
    I noticed that during the '60, the body were made with 1/4" thick top and bottom and then reinforced with x bracing (first picture).
    Then some years later the top/bottom became thicker and the bracing disappeared, but the area around the bridge was not totally carved (second picture).
    Many says that the first example sounds more acoustic and more resonant (unplugged).
    I'm wondering if there is any technical reason to explain that.
    Afterall, both examples are semi-hollow constructions.

    Also, I'm wondering, for this kind of construction, would it be better to have a 2-pieces body (the two pieces joins only through the thin top/bottom area) or a 3-pieces body (when the pieces joins togheter though the mortise shoulders, so more glue area)?
     

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  2. Ronkirn

    Ronkirn Doctor of Teleocity Vendor Member

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    Nope.. the explanation has more to do with economics...

    r
     
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  3. Dimitree

    Dimitree Tele-Meister

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    why economics? please notice that those bodies are carved from the bottom, so you don't really save any wood by keeping the top thicker
     
  4. guitarbuilder

    guitarbuilder Telefied Ad Free Member

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    I never held a Ric in my hands, but the appearance of the two pictures, the top and back braced should have more of an acoustic sound, because that is kind of like how acoustic guitars are built. The second looks like it it cnc'd out. Solid maple is very heavy, so I'm sure that was one reason to make it hollow. As far as top thickness....I can only guess that they did that for a mechanical reason. I'm also guessing that they were getting hard maple from the US east coast and that may have had some impact too. These are all just guesses though.
     
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  5. Freeman Keller

    Freeman Keller Friend of Leo's

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    What you are seeing is bracing - it is there to resist the shear and rotational loading on the top, and to support the pickups. While the top might vibrate (a little) and might produce some acoustic sound (ie move some air), the effect is going to be very minor. Every semi hollow or hollow body electric guitar that I've ever touched has had a lousy acoustic sound.

    However, the fact that the top (and for that matter, the reset of the body) does move a little bit will color the strings vibrations which you will hear thru the pickups. I'm having some fun with a FFT program right now and one of the things I want to do is compare a chambered LP with a solid one - if its interesting I might report back.
     
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  6. Dimitree

    Dimitree Tele-Meister

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    that's exactly what I was wondering..I mean, what is the element that enhances those characteristics? the bracing? the thinner top? Afterall the two guitars are almost the same..it shouldn't make much difference (like for example when comparing a semi-hollow to a solid body).
     
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  7. guitarbuilder

    guitarbuilder Telefied Ad Free Member

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    I think you have to think in terms of "acoustic" on a sliding scale. Obviously a braced spruce top that is .100 thick is going to sound completely different from a 1/4" thick maple top. I think the " difference" is that the other parts of the solid maple are still attached to the top and that would impede vibrations more. As far as acoustic sounding, the ric is probably on par with a Tele thinline or Gretsch Jet type guitar more than what we think of as a steel string acoustic guitar.
     
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  8. PhredE

    PhredE Tele-Holic

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    Those familiar with the classical guitar world spend lots of time designing/considering/debating this sort of thing.

    For those interested, lookup Chladni patterns. To summarize in my own words, these are the [relatively] scientific studies done into the vibrational patterns of a surface -- in this context, applied to surfaces of an acoustic (classical) guitar (the top mostly).

    Basically, to recap, every surface vibrates differentially and vibrates uniquely at a specified frequency.

    Throw these search words into google to get a flavor of the idea:
    'chladni patterns guitar images'

    The old school way of getting a feel of this is to sprinkle sugar or flour (or similar) substance on the top (or back) of your guitar and play the same note repeatedly. You'll see the sugar or flour move around until it settles into a specific 'pattern' on the top (or back).

    This link gives a good visual:
    https://www.classicalguitardelcamp.com/viewtopic.php?t=27819

    This is nothing new to Freeman, guitarbuilder and Ronkirn most likely, but for the rest of us it's something to be aware of and can be interesting.

    Fast Fourier Transform?
    Cool. You're bringing back memories of a former working life I once had (image processing stuff mostly).
     
    Last edited: Jun 30, 2020 at 4:28 PM
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  9. LOSTVENTURE

    LOSTVENTURE Tele-Afflicted

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    As far as Gibson's chambering goes, all you will experience is a slight weight difference. The amount of chambering is so minimal, with either the "swiss cheese" style or the more recent "fan" style, that the tone is not affected. I have versions of both, and to be honest, the lightest one of the group is not one of the chambered models.
    And my 1963 Ric 360/12 has dismal acoustic properties. It's pretty though.
     
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  10. Ronkirn

    Ronkirn Doctor of Teleocity Vendor Member

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    it's not the cost of the woo. it's the cost of the craftsman that has to fit those cross braces... ever try to buy one of those guys? :p

    r
     
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  11. Freeman Keller

    Freeman Keller Friend of Leo's

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    By definition and acoustic guitar produces its sound by moving air. Top, back, air inside the guitar - something moves and makes the air move. Those of us who attempt to build acoustic guitars spend a life time trying to figure out how all of that works and how to make it work the best we can. An electric guitar will also move some air and thus has an "acoustic" sound but there is nothing done to optimize it, to make it what Trevor Gore called "alluring" (whatever that means).

    There has been a lot of research done on how guitars make their sounds and many "good" guitars (and violins and other instruments) have been studied try to understand this. Chladni patterns tell us which part of a guitar "wants" to vibrate and what frequencies it "wants" do to this at. Its pretty easy to do that with a finished guitar, and we say "oh look, it has its monopole mode at 105.3 hz..... But its really hard to design and build a guitar that has that mode.

    One of the other things we can look at is when the guitar is plucked or struck, what frequencies are present in the note. We all know that the string vibrates at more than one frequency simultaneously - if we pick a fifth string A it will vibrate at its fundamental as well as a number of higher multiples - that is what makes the A 110 on a guitar sound different from A 110 on an oboe. One of those notes happens to fall on one of the frequencies that the guitar "wants" to vibrate at, ie, one of the Chladni modes then that note can do weird things - it can be really loud, it can beat against itself, it can form what is known as a wolf note. Again, lots of cool theory, might not be easy to apply.

    I have a beta version of some lutherie software that if you pick a note shows the waveform in the time domain (Audacity and others will do that) but also give a snap shot in the frequency domain, so it shows all of those partials. Its very interesting to pick the same string at different places and watch how the partials change (which also relates to placement of pickups).

    I've spent the last 15 years trying to understand what the great guitar builders are hearing when the tap and flex and shave their braces. Its a work in progress and will be as long as I'm building guitars.

    However, with the guitars in the original pictures, any "acoustic" sounds are coincidental.
     
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  12. crazydave911

    crazydave911 Poster Extraordinaire

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    Coincidental perhaps but not inconsequential.i built my tenor electric this so as to practice quietly at night. Is it a Martin D-35? Why hell no but you CAN hear it

    Dave
     
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  13. Dimitree

    Dimitree Tele-Meister

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    thank you everyone!
    so basically if I understood correctly, the differences are not given by the x-bracing themselves but by the fact that the guitar using the bracing has thinner top and bottom, and so it vibrates more.
    Another thing I read online, that x-bracing guitar is quite fragile since it often requires a neck-reset..probably because of the glue used too
     
  14. guitarbuilder

    guitarbuilder Telefied Ad Free Member

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  15. Freeman Keller

    Freeman Keller Friend of Leo's

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    Lets look at it this way (remember that volumes have been written bout acoustic guitars). Here is your first image

    [​IMG]

    Here is a typical well braced acoustic guitar. The fact that it is one of mine is coincidental.

    IMG_4971.JPG

    Lets make some observations. Depending on gauge and scale length an electric guitar will have about 100 pounds of string tension, the acoustic about 165. Since the energy to drive the top comes from string tension you will have about 2/3 the energy to start with.

    If the tops are the same material (lets say spruce) they will have approximately the same Youngs modulus (a measure of stiffness), however wood varies dramatically. Lets say they do, however. The top of my guitar is a bit less than 1/8 inch thick, the top of the electric is about 1/4. Since stiffness of the top varies with the cube of thickness the electric is 8 times stiffer than the acoustic.

    Those braces are massive (and some of the worst worksmanship I've ever seen). Again, brace strength and stiffness vary as the cube of height - those braces are going to be incredibly stiff (and sloppy)

    The bracing in a steel string acoustic is a mastery of engineering. The weakest part of the top is between the sound hole and bridge, string tension is trying to rotate that area down into the guitar and pull the top up behind the bridge. The strongest part of the X is right where the weakest part of the top is and it lets the big area of the top move to pump the air. The two angled braces in the lower bout are called "tone bars" - they both brace the top from splitting along the grain but a skillful builder uses them to taylor the sound - they stiffen the treble side more than the bass.

    A couple more braces should be mentioned. Part of the rotation of the neck extension into the top under string tension is counter acted by the two braces above the sound hole. The big one is the Upper Transverse Brace - its function is to keep the top from cracking near the sound hole. The little flat guy is the infamous "popsicle" brace - early Martins didn't have them, later ones do.

    The patch between the legs of the X is the bridge plate - a harder wood like maple that protects the tops from the ball ends of the strings (sort of like ferrules).

    The whole idea of all that bracing is to create a top that wants to move and can move easily, but doesn't self destruct under all that force.

    Another thing to compare is the volume of air in each of the boxes. I have no way to really judge but it looks like the electric is maybe half as deep and not nearly as big a hollowed area - lets say the volume is 1/3 that of the acoustic. That is actually pretty important, the volume of air in the box determines its most fundamental resonance (the lowest bass note it can play). There is a good reason cellos are big and violins are small.

    So, while everything about the acoustic is optimized for turning string energy into air movement, nothing about the electric is. It might have an sound when strummed unplugged - I'm quite sure it does. But I doubt that it will be loud and I doubt that any of us would say it sounded "good".

    What ever that means.
     
  16. Freeman Keller

    Freeman Keller Friend of Leo's

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    The second picture is simply a hollowed out solid body, what we would call a chambered guitar. It doesn't matter if that was done by a cnc router in a production facility or someone with a hand router and some templates. The idea is to reduce weight. Yes, it will change the properties of the body (less mass for one) and yes it will affect the sound (both plugged and unplugged). I've built two pretty identical les pauls, one chambered, one not, most important with the same materials and pick ups, there is a difference both plugged and unplugged. Its subtle.
     
  17. Freeman Keller

    Freeman Keller Friend of Leo's

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    X bracing is both fragile and very strong. Building an acoustic guitar is walking a very fine line between having enough strength and structure to withstand the 165 (or more) pounds of tension and being light and stiff and strong enough to freely vibrate. Its a compromise.

    I mentioned before that the way an acoustic guitar with a fixed (pinned) bridge works is that the bridge rotates, pulling the top up behind it and pushing it down towards the sound hole. The neck block is rotated, the fretboard extension is driven down into the upper bout. Over years the wood and the entire structure deforms, the guitar gains a bit of a belly, the neck angle goes down. Nothing is "broken" (usually), nothing has failed. However the guitar becomes increasingly unplayable and at some point the neck is taken off and "reset" to the correct geometry.

    Its a fact of life for an acoustic guitar. Nylon string guitars seldom need resets but their tension is less than a typical electric and they have fairly strong neck joints. My guitars mostly use bolted neck joints which you can see in the above picture - that makes resetting much easier but as long as the builder prepare for the eventual reset it can be done with other neck joints.

    The "glue used" has nothing to do with it - the common feeling is that lutherie glues do not "creep" and test indicate that almost any well done glue joint is stronger than the parent wood.
     
  18. Dimitree

    Dimitree Tele-Meister

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    many thanks! :) now I'm studing all of that
     
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