SBO 2022, Big ABG scratch

Discussion in 'Luthier's Corner' started by Crawforde, Jun 22, 2022.


  1. Crawforde

    Crawforde

    Dec 30, 2016
    BAEBC78A-57A3-439B-A0E5-6AF4F0FFB522.jpeg
    as of right now the plan is
    Hollow body, single piece black walnut neck with carved scroll and fretless ebony fingerboard, black cherry back and sides, Sitka spruce archtop, floating bridge, piezo pickup, built as large as I can, and if ergonomics allow and I feel like it towards the end, an endpin for more vertical play.
     
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  2. Bruce Johnson

    Bruce Johnson Gold Supporting Member Commercial User

    Feb 4, 2011
    Fillmore, CA
    Professional Luthier
    But, but, but.....A piezo pickup???? Aw, come on, that's admitting defeat from the start! I encourage you to dive in and build a RABG (Real Acoustic Bass Guitar). You know, one that makes beautiful bass sounds out through its soundholes, without any stinkin' electric bass pickup or amp.....A challenging project, sure, but that's the fun of it!

    Of course, you could design the bridge so that you could add some piezos later.....If you really need to.

    Except for that, I like your plan overall. Put some serious thought into how much the top can move. The basic rule is: To make loud acoustic sound in the bass range, you have to physically pump a lot of air. Lots of movement of a big diaphragm is the secret.
     
  3. ARandGenUsrName

    ARandGenUsrName

    May 25, 2022
    Canada
    Looking forward to reading along with this one! I think it'll look great!

    Are loud acoustic basses possible, without being insanely big? I've always wanted one - enjoy playing them but have never played one that could compete with an acoustic guitar
     
  4. Gary_M

    Gary_M Formerly known as SlingBlader Supporting Member

    Oct 19, 2013
    Northern Indiana
    Is making an acoustically loud instrument mutually exclusive to installing a piezo transducer? It would be handy to have when playing live or when recording, even to mix with a microphone. Just food for thought! :)
     
  5. Crawforde

    Crawforde

    Dec 30, 2016
    About the diaphragm size, on the plywood in the back you can see some chalk marks, they are all outlines I’m considering. The largest will be cut first, I’ll put a fake neck on it and a fake body to get the feel, and I’ll only downsize dimensions as ergonomics require.
    This could end up looking unusual, but I want to optimize the ability to make some volume, even if the shape is odd.
    The physics of airhole size and shape are something I’ve read enough about to get plenty confused, but I’ve seen some interesting modeling of the evolution of C and F hole shapes.
     
    Last edited: Jun 22, 2022
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  6. Wow. This is an exciting project. Now I have two questions:

    Bruce, what does the word "diaphragm" mean in this context? The top? The sound hole? The body?

    OP: how big can you go? What size are the sheets of plywood?
     
  7. Crawforde

    Crawforde

    Dec 30, 2016
    I’ll know for sure once I plane and glue. but I’m looking at about 18x 24.5.
     
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  8. Bruce Johnson

    Bruce Johnson Gold Supporting Member Commercial User

    Feb 4, 2011
    Fillmore, CA
    Professional Luthier
    The diaphragm is a sheet of fairly rigid material that you are mechanically pushing forward and backward rapidly at a low frequency. That movement is pushing and pulling on the air, creating pressure waves in the air. You know, sound! Think of a speaker. The cone is the diaphragm. The coil and magnet drive it forward and backward at a frequency, creating sound in the air.

    In an acoustic guitar, the top is the diaphragm. The movement of the strings causes the bridge to bounce up and down, which makes the top bounce up and down. When the top bounces up, it directly pushes the air toward you. When the top bounces down, it squeezes the air in the box and squirts some of it out toward you.

    Same thing with an upright bass. The flexing of that huge top is throwing air right at you. Plus secondary air is getting squirted out the F-holes.

    The sound is coming from the flexing of the diaphragm. And, the lower frequency you want to create, the more volume of air you have to push with the diaphragm. You need a larger diaphragm, with more square inches of area. Or, you need to give the diaphragm more distance of travel. Or both.

    You can get low frequency out of a small size diaphragm. Look at 6" sub-woofer car speakers. The trick is to have really long travel. Those little speakers can kick a half inch.

    Think about how this applies to designing a RABG. To create LOUD LOW sound, put your engineering effort into getting a lot of travel out of the diaphragm, rather than making the volume of air in the box really big. Make a reasonable size box with a top that can really move.

    Something that's often misunderstood: The main reason why the body of an upright double bass is so big isn't to create a box with a large volume of air. It's to create a large diaphragm, in square inches, on the front that can flex a lot. Thin wood has mechanical limitations on how much you can bend it back and forth repeatedly without it breaking. Making the diaphragm larger allows the center to deflect farther without bending the wood as much. The wooden top of an upright bass is big so that it can flex a whole lot without shattering.

    The big volume of air in the box means something too, of course. The natural resonance of the air in the box depends on the volume (cubic inches) of air space. Larger volume equal lower resonance frequency. Tuning that is a part of boosting the sound of a stringed bass instrument.

    But that's a secondary effect to work with. The main thing that creates the sound is the movement of the diaphragm.

    Here's an example of a moderate size diaphragm, with a lot of travel, and no real air box behind it. It makes deep loud sound similar to an upright bass.

    IMG_3677B.jpg
     
    Last edited: Jun 23, 2022
  9. Thanks, Bruce, for all this info.

    That's your bass banjo, isn't it? I remember it having a name... Banjozilla, or something?
     
  10. Bruce Johnson

    Bruce Johnson Gold Supporting Member Commercial User

    Feb 4, 2011
    Fillmore, CA
    Professional Luthier
    Yes, that is the one and only Banjozilla. I built him 30 years ago (!!!) and he's still alive and well here in my music room.
     
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  11. Crawforde

    Crawforde

    Dec 30, 2016
    Along with trying to follow Bruce’s advice, I figured out one more way to help with this, I built the guitar too, and it has a tiny body so should be relatively quiet. Is that cheating?
     
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  12. Making the guitar player quieter is never cheating! :roflmao:
     
  13. NKBassman

    NKBassman Lvl 10 Nerd

    Jun 16, 2009
    Winnipeg, MB, Canada
    Interesting project!
     
  14. Gruff

    Gruff Supporting Member

    May 19, 2021
    @Bruce Johnson may I share this photo on our bluegrass FB group? I play bass, everyone else plays banjo, so will be a talking point :)
     
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  15. Bruce Johnson

    Bruce Johnson Gold Supporting Member Commercial User

    Feb 4, 2011
    Fillmore, CA
    Professional Luthier
    Sure, Banjozilla is world famous already! Well, among a limited community, anyway. I have a page on my own web site with the whole story.

    Bass Banjo
     
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  16. Reedt2000

    Reedt2000 Supporting Member

    Apr 26, 2017
    Central New Jersey
    Very ambitious and VERY cool :cool:

    I'm in, sub'd :thumbsup:
     
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  17. smart51

    smart51

    Dec 11, 2019
    UK
    Yeah, rest the body of an electric bass guitar on a wooden table and pluck a string. You can hear it clearly. The table top is emitting the sound.

    The cavity of a hollow instrument together with the F hole does a similar job to a reflex port on a speaker cabinet. Reflex cabinet design tips can be useful when tuning the size of F hole for a given volume in the body of a bass. Set the resonant frequency too high and you'll lose all the low bass. Go on, ask me how I know!
     
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  18. Crawforde

    Crawforde

    Dec 30, 2016
    I will need to look into tuning the F hole to the cavity after I have the general shape figured out. The shape will be odd, and asymmetrical and the body will probably be slightly wedge shaped in order to maximize the volume without making it difficult to play. I found a little birdseye maple and a small piece of curly cherry out in the garage while searching for my little glue crockpot. That will likely be incorporated in a cool surprise way.
    I’m considering buying some precut Kerfing, unless that’s the kind of thing that can get one tarred and feathered in a scratch build. But since $20 worth of Kerfing can save me probably an entire day of hand cutting it seems worth the expense.
     
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  19. Bruce Johnson

    Bruce Johnson Gold Supporting Member Commercial User

    Feb 4, 2011
    Fillmore, CA
    Professional Luthier
    I'll suggest again: To get loud sound volume down in the bass range, don't worry so much about maximizing the volume of the body. The important thing is to maximize the ability of the top to deflect through a long distance, driven by the bridge. That diaphragm thing. The top needs to be very flexible and able to throw a bunch of air. Without exploding. And you need to come up with a springy balance mechanism to support the underside of the top and balance against the download of the strings over the bridge. That mechanism will transmit the motion of the strings to the top.

    Look closely at the mechanism in Banjozilla. That clear mylar 24" drum head is the diaphragm. It's very flexible and can easily deflect either way 1/8". But it's soft enough that it couldn't possibly withstand the download from the bridge and the strings by itself. So I built a mechanism with two fairly heavy steel coil springs that presses up on the underside of the head, directly under the bridge. The upward force of the coil springs balance against the downward force of the strings bent over the bridge. When you pluck the strings, the bridge and that spring mechanism bounce up and down together, about 1/8" either way. And the head moves along with them. A 24" diameter diaphragm with 1/8" travel. Like a big bass speaker, it can pump a lot of low frequency sound.

    To make a loud acoustic bass guitar, you need to make something similar, except with a wood top. The top has to be flexible enough to have a lot of deflection. And the structure underneath needs to be able to handle the loads, while allowing that deflection. And not exploding. It's a mechanical engineering problem.
     
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  20. Crawforde

    Crawforde

    Dec 30, 2016
    Thanks. I am hoping that with the large Sitka top, the shape arching, and hole pattern I have in mind to maximize travel, and a concept analogous to your banjozilla spring (appropriated from Banjozilla after reading about it on your website) that I’ve been playing with that will hopefully make it into this design. Given a top action calibrated to resist a given static downward string force, a top this size should be capable of producing some volume. Once there is time to start work I will post pictures of the proof of concept trials.
     
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