Experimental composite double-top & other questions

[Luke Hospadaruk]

Hi!

I have some soundboard questions about an experimental for-fun-for-myself instrument project I'm working on.

I'm starting a little tenor guitar/weird-banjo-like-thing project. Think tenor guitar with a banjo fifth string and no frets. The characteristics I'm looking for from this instrument are:
- fun to make
- an opportunity to teach myself more about making composites & get more familiar with vacuum bagging
- fun to twang on
- tuned kinda like a violin (my parter is an off-duty violin player with actual skills, trying to make it kinda tenor-guitar-ish)
- easy to travel with/impervious to bumps and moisture

I'm planning on using some manner of alternative soundboard, maybe aluminum, more likely some kind of composite double-top style panel. (aluminum honeycomb, home-made fiber-nomex-fiber honeycomb, dibond, etc). Probably the soundboard will be brace-less mostly because I don't want to bother and I'm not too concerned about the sound at this point.

For the purposes of this particular instrument, I don't really care _too_ much about how it sounds. If it sounded half as good as a cigar box I'd be perfectly happy, BUT I'd really like to teach myself something about building acoustic instruments in the process because I think at some point in the future I'll attempt some more ambitious projects where I might care more about the sound.

So I'm wondering:
- Are there any good books/resources that go into both practical and theoretical detail about soundboard design for stringed instruments without being super "heres-how-to-build-a-guitar-out-of-wood" specific?
-- I'm particularly interested to educate myself about the interplay between thickness, mass, and stiffness in a soundboard.
-- I'm also interested to learn about good ways to evaluate the quality of a [not wood] soundboard - especially things that can be done without purpose-built testing equipment.
- I know guitars & other instruments like this have been built - can anyone point me at detailed build logs or builder-oriented (not player-oriented) discussions of the process? I've read a lot of articles about how they sound, and a lot of marketing-ey articles, but I've had trouble coming up with more impartial discussions of build considerations.
- Any references to arch-tops being built in this way? It's not really any more difficult for me to mould an arch-top than a flat top, but most composite guitars I've seen like this are wood double tops which are (almost) always flat. Maybe I could go lighter/thinner with an arch-top? Maybe it would sound nicer?

About me:
- I make a lot of stuff
- I have experience working with laminates & composites (boatbuilding mainly)
- I have a pretty solid amount of woodworking experience
- I've made a solid-body electric violin before (it is awesome and weighs a ton. Oops.)

Thanks!

[Rodger Knox]

Hi!
So I'm wondering:
- Are there any good books/resources that go into both practical and theoretical detail about soundboard design for stringed instruments without being super "heres-how-to-build-a-guitar-out-of-wood" specific?
-- I'm particularly interested to educate myself about the interplay between thickness, mass, and stiffness in a soundboard.
Thanks!

It's not cheap, but it's a college level text with all the math and physics.
http://www.goreguitars.com.au/main/page ... _book.html

[Luke Hospadaruk]

Thanks!

[Wout Moerman]

Hi Luke,

I am very interested in your experiment. But it seems there are some contradictions in your questions. You write that sound quality is not importnt on this moment, but you want to make an instrument that is fun to twang on and end with a question if an arch top would sound nicer. I think it would be wise to put sound high on your priority list, unless you want to focus on "impervious to bumps and moisture". And only if sound is important you can learn about "interplay between thickness, mass, and stiffness in a soundboard". Is using plywood or other wood laminates also an option for the soundboard?

I'll try to come up with some ideas and tune in later.

Wout

[Luke Hospadaruk]

Wout - good points, I've been thinking more about the project and I'll probably start building soon.

As to sound quality - I have zero playing skills when it comes to guitars & banjos (and really instruments in general, I took piano lessons when I was younger, played trombone a bit, can basically read sheet music (slowly), have a flute or two, but generally not a musician). I'm not trying to make an instrument to take to lessons or anything, just plunk on at home.

I think you're right that "impervious to bumps and moisture" is really my #1 consideration. Probably a silent solid body instrument would be the easiest approach there, so let's add that I want it to be super durable, small/portable, and also have a decent amount of volume for its size.

I got my hands on some 1/8" nomex honeycomb and currently plan to do the soundboard with a fiberglass (or carbon)/honeycomb composite panel. I'm already all set up for vacuum bagging and composite work so that's a pretty easy choice for me (probably a lot easier for me than using wood). (also, I'll be honest, I think that a composite soundboard is going to look awesome )

All that being said, I'll be sure to post back here with updates on how it turns out. If all goes well I may attempt some more serious instruments in the future with a similar technique (cello? clavichord?)

[Wout Moerman]

Never thought about the benefits of a clavichord which is impervious to bumps and moisture but doesn't sound very well...

[Wout Moerman]

I would think carbon over a 1/8" nomex honeycomb core would look awesome indeed!

Might a banjo or resonator kind of guitar be an option? I think those are the most rugged guitars because they don't need a big fragile soundboard. The problem Always is that the soundboard must be flexible enough to move with the strings but stiff enough to withstand the pull of the strings.

I don't know how the balance between flexability and stifness will be when using laminated nomex. Especially if you want to leave out bracing. BTW, a domed top does not need much bracing although one or two braces parallel to the strings is common.

Maybe a stiff nomex laminate vibrating surface suspended by a flexible rim? Kind of loudspeakercone like in a resonator guitar?

[Wout Moerman]

Luke,

I found this on the internet while doing some research for my own project. I think it is a good read for you: http://maliposamusic.com/PursuingBraceLess.htm

[Luke Hospadaruk]

Some updates:

Started making some sample panels just to test laminating techniques - made one side of a S2-glass/nomex panel and one side of a basalt-fabric/nomex panel. Sample swatches were vacuum bagged against a piece of glass (an old tempered glass tabletop). I did a terrible job - leaky vacuum bag and not enough wax on the bag (oh well, my first try!), BUT it was still enough to be able to asses the looks and check the nomex-fabric adhesion. I didn't bother with a carbon test panel because I already know what carbon looks like (black, cool, opaque).

Looks: cool.
The S2 glass is cloudy/clear. The honeycomb is clearly visible through the top layer (which is very thin, probably 10-20 mils? I'm using 4-5oz cloth).
The basalt-fabric panel is a slightly metallic dark brown color (it has kind of a gold-spaceship-foil sheen before it's wetted out, the darkens to a still nice brownish color). The fabric is translucent enough to still clearly show the honeycomb, though it is less obvious, and will probably be further obscured when the panel has two sides and is mounted in an instrument body.

Honeycomb adhesion: good.
My layup looked like: glass plate->release wax -> (epoxy) resin -> fabric -> honeycomb -> vacuum bagging film (I forgot peel-ply/breather, whoops). Even with a pretty terrible vacuum bag job I still got (what appears to me) to be strong adhesion. The honeycomb is adhered to the fabric firmly enough that it tends to rip instead of separating from the fabric when I yank on it.

@wout - thanks! Looking at that I could imagine maybe adding some strategically placed additional CF strips to firm things up if my panels end up too bendy. Also those instruments look great - I love the look of that fan-planking.

Also to the point of making a resonator or banjo - I was actually thinking to make a resonator for a while, I'm not far from eldery music so getting my hands on a resonator cone would probably not be too hard (and I could also improvise a cone pretty easily I suspect). I ended up deciding not too because I was worried about the relative fragility of resonator [cones] (to be clear, I've never owned a resonator - I was just reading some scary-sounding stories about bent resonator cones that may or may not have been true). Maybe I'll come back to it after I've made this instrument? I'm still generally interested in it. I think I avoided a banjo-top mostly because once I got the idea to try using a laminate I was really excited about it and wanted to give it a shot. Maybe if I get all the way through the project and the laminate top sucks I'll switch the neck to a banjo top? :p

[Luke Hospadaruk]

Hey! I forgot I started this topic!

Some more updates:

• after languising in my woodpile for many years, the banjo project morphed into a cigar-box-ey steel string baritone ukulele project. I made some progress last summer, got a nice osage orange fretboard installed and cut the fret slots.

• Indeed, I decided to bail on the resonator. Honestly less about fragility at this point and more that I just want to acutally finish _any_ instrument instead of stalling myself out on too much complexity

• I haven't installed a soundboard yet and unsure if I'm going to use the almost-finished honeycomb laminate that I still have or just use a piece of nice 1/8" birch ply I cut out of an old wine box (it is kinda supposed to be a cigar box uke after all, I'm not terribly concerned about pristine sound quality at this point)

• I might also throw a lipstick pickup on it so I can make some funky surf sounds?

Hoping to make some time to wrap up the project this summer, I'll post back!

[Alan Carruth]

Back in the 80s some folks from the Catgut Acoustical Society worked out a 'synthetic spruce', using unidirectional CF on a core of card stock. The resulting composite had similar stiffness to spruce long and across the grain, and about the same damping. They made both guitar and violin tops from it that sounded good. They ran into two problems.

One was that the card stock would absorb moisture from the air, and swell and shrink as the humidity changed. This set up shearing stresses in the glue line between the core and the skin, and eventually the CF (which was about .002" thick, iirc) would peel off in ribbons. This would not be an issue with Nomex.

The other was that Kaman had been following along on the research, and supplying them (as did Martin) with spruce to use to get the baseline properties. As they were preparing to publish Ovation took out a patent on the idea. The Catgut folks were assured by a lawyer that they would win a suit in court, but since Kaman is a big aerospace firm with lots of lawyers, it would take a bunch of time and money. Since they only wanted to publish a paper they didn't take it to court.

There may still be patent issues to watch for. The other point is that, to get it to work right, you'd need to preserve the stiffness ratio of real spruce, which can be 10-12 times as stiff along the grain as across on good spruce. They used unidirectional pre-preg tape, which is cured in an oven to harden it.