WBO 23 - Certandan Deep Strings

[Bruce Johnson]

That’s cool. My first thought when you said you didn’t want to make a round “soundboard” section was what about an oval? I suppose that’s good for one of us and scary for the other…




Jury’s out on who’s who…

You know, there's actually a good engineering reason to make the soundboard oval shaped. If the soundboard (diaphragm) is circular and symmetrical, it will tend to have very focused and loud harmonic notes. Boom notes, like on a bass drum. Some notes that are really loud, and harmonics above them. Banjozilla has very distinct Boom notes, and there's not much can be done about it. I can raise and lower the pitch of the boom notes by changing the tension on the head, but that's about it. It's an inherent feature of a drum head.

When you change the shape of the diaphragm away from circular, such as to oval, it subdues the Boom notes. They are still there, but there are more of them and they are not as loud. A big part of the development of any "wooden box" type acoustic instrument is making the shape of the top non-symmetrical to minimize the Boom notes and make the volume level of notes up and down the scale as even as possible. Location of sound holes, bracing under the top, bass bars, sound posts all figure into that same formula: evening out the response of the top to minimize loud and soft spikes. The standard designs of violins, acoustic guitars, mandolins, etc., all evolved with that purpose.

Note: I'm talking about the mechanical resonance of the top itself. How the top itself vibrates, even out in free air. But there's also acoustic resonance happening in a wooden box instrument. That's the vibration of the air inside the wooden box under the top. It will also have Boom frequencies determined by the volume and shape of the box. That's a whole 'nother problem to deal with in acoustic instrument design. The Wolf Note problem in upright basses.

But regarding top resonance alone, an oval top is easier to deal with than a round top. That's a good start. Making one end of the oval smaller than the other is even better.

 

[Flamingo21]

Making one end of the oval smaller than the other is even better.

That would be kind of an egg shape, I guess. I'll experiment a bit with that idea.

 

[Bruce Johnson]

That would be kind of an egg shape, I guess. I'll experiment a bit with that idea.

Yes egg-shaped is even better than oval, to minimize mechanical resonance Boom notes. And it doesn't have to be dramatic, just make the top end radius a little smaller than the bottom. That helps.

 

[Flamingo21]

With all that in mind, I went to work trying to design ellipses and eggses. I was given extra time, because today's band rehearsal was cancelled as the singer's dog is very ill. (May be cancer, we'll know in a few days.) So off I went: looking at images, taping sheets of paper together, hunting for pencils. For reference I looked up the rim diameter of Banjozilla, which is an impressive 25.5".

Then I realised that I cannot exceed the size of the leather I have, and I'd better measure it! It's a large piece, but since it was once a couch, and couches have complicated shapes, it has all kinds of seams running through it. I found that the largest seamless (ha!) piece is a rectangle of 69x56 cm, or about 27"x22". The rim will of course need to be smaller than that, because you need room to fasten the leather somehow.

So I designed an egg-ish shape that's still quite close to an ellipse, that's 56x46 cm, or about 22"x18". This is how I did it: first I drew a rectangle of 56x23 cm, to make half an egg, which will be mirrored to obtain the other half. On one of the long edges I set a "mid-point" that's not quite in the middle, it divides that edge into about 25.5 cm and 31.5 cm, which looked about right. From that mid-point, I drew many lines outward, and marked a point on each at a distance to the mid-point, going from 25.5 cm to 23 cm (at 90 degrees) and back out to 31.5 cm. Then I sketched a long curve through all those markings. Then I cut the paper along that curve.

Here's the leather rectangle, with the paper template:

 

[daveman50]

Bowing bass is addictive. I'm (finally after how many decades?) studying with a classical teacher. All the work is with the bow, to improve intonation and stamina. Playing scales and studies slowly. So now I'm bowing as often as I can in jazz settings, working towards the sound of Slam Stewart, Major Holley, etc. Anyhoo...I recommend it!

 

[KidAmnesia]

I like the egg shape idea. You can look at lutes for inspiration, the barroque theorbo in particular might give some "deep strings" vibes.

 

[Flamingo21]

For the back of the sound box, I dimensioned a sheet of plywood to 84x47 cm. I'm sorry, I don't remember what wood this is.

Then I rough cut the egg shape on the band saw.

I will refine this by hand, without a template. This will probably yield a slightly irregular shape, which is totally appropriate.

Question for the hive mind: I think I will cut a large hole in this, reducing it to an approx. 8cm (3") rim, because I think the sound box needs a port. Your thoughts, please?

 

[Reedt2000]

For the back of the sound box, I dimensioned a sheet of plywood to 84x47 cm. I'm sorry, I don't remember what wood this is.
View attachment 4923724


Then I rough cut the egg shape on the band saw.
View attachment 4923726


I will refine this by hand, without a template. This will probably yield a slightly irregular shape, which is totally appropriate.

Question for the hive mind: I think I will cut a large hole in this, reducing it to an approx. 8cm (3") rim, because I think the sound box needs a port. Your thoughts, please?

What's the final design gonna be? Are you gonna build it like an acoustic where we're looking at the top and you'll be putting bent sides and a back on it? If so you could port the side(s)

 

[Reedt2000]

Could look cool to port the sides, and have "horns" that flair out from the ports, kinda like the backs of these fiberglass drums

 

[Flamingo21]

I imagine it more like a rain barrel: short planks, fitting snugly together, that are attached to this back panel at a 90 degree angle.

 

[Haroldo]

What's the final design gonna be? Are you gonna build it like an acoustic where we're looking at the top and you'll be putting bent sides and a back on it? If so you could port the side(s)

Exactly this.

 

[Flamingo21]

But the horns are an interesting idea. I'll think about this.

 

[Matt Liebenau]

I imagine it more like a rain barrel: short planks, fitting snugly together, that are attached to this back panel at a 90 degree angle.

Coopering. (I think that’s the right word) Sounds like fun!

 

[Flamingo21]

Coopering. (I think that’s the right word) Sounds like fun!

Now that you mention it: yes, that's the word!

So I'm going to cooper a soundbox, then.

 

[Matt Liebenau]

Now that you mention it: yes, that's the word!

So I'm going to cooper a soundbox, then.

Our 20lb psycho cat is named Cooper. He likes shredding things. I can send him over if you need help rough cutting some lumber. He’s not great with the detail work though.



Note: I’m fairly convinced the first picture is him praying for “one of the big ones” to die so he can eat us.

 

[Flamingo21]

Today I found some time to be in the workshop. I also took my first climbing lesson, and climbed to a height of 17 metres - yay!

Now I have this "egg" shape which will be the back of the sound box. It was actually a rather rough shape, so I took a rotary sander to it. Grits from 40 to 320.

It's still a bit rough and irregular, fitting the Certandan theme, but there are no pointy bits now.

The egg will have a large sound hole. So I drilled holes with the drill press, up to 13mm.

The next step will be linking these holes with the jig saw.

 

[Flamingo21]

Also, I decided to cooper the rim from the same wood. So here's the necessary wood pile pic:

 

[TFM94]

So I drilled holes with the bandsaw, up to 13mm.

Oh my, that must be some magic bandsaw. I want one of those!

Ok, I'll see myself out.

 

[Flamingo21]

Oh my, that must be some magic bandsaw. I want one of those!

Post edited. Thanks for pointing it out.

 

[Flamingo21]

Linking the holes with the jig saw....

... and here's the cutout. If you found this without knowing the back story, what would you think it is?

Then I decided I want the hole to be bigger. And since cutting this material with the jig saw felt easy and controllable, I decided to free-hand this cut.

That came out well. The width varies from 85mm to 90mm. Like I said before, a bit of irregularity is fitting.

Cue toilet jokes...


Now for the rim. I want it to be 20cm wide. I measured the wood I have left, and it should be juuust enough. Work carefully, Flamingo!