Building a CBG resonator

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Permalink Reply by Huntz Meyer on October 30, 2012 at 5:49am

When I built a ring, rather than attaching straight to the box, I cut two circles from 1/4" masonite and glued them onto scrap wood risers.  One ring sat under the flange, the other around for centering.  The cone was floating.

Huntz

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Permalink Reply by Rand Moore on October 30, 2012 at 12:27pm

Hi Huntz,
Is a supporting ring under the cone resonator really necessary? Old Lowe's cones are usually installed to the sound board with 12 screws, and the cone in the photo looks like it could be installed the same way? All I know is what I've read on CBN, so I'm no authority. Maybe other people can comment on the "best way" to install a cone, and what the benefits are. I do recall that older designs called for a wooden circular chamber (there was a name for it but I've forgotten) in which the cone is placed for supposedly better resonance, but it seems that design requirement has fallen out of favor. Seems like an interesting topic to raise.

-Rand.

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Permalink Reply by Huntz Meyer on October 30, 2012 at 1:35pm

Rand,

I had done several screw down reso's so I wanted to try the floating cone.  I couldn't hear any significant difference.  Either way is fun to build and sounds good. 

Huntz

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Permalink Reply by Rand Moore on October 30, 2012 at 9:34pm

You may also enjoy this discussion we had a while back on adding a piezo pup to a resonator cone.

-Rand.

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Permalink Reply by Rand Moore on October 30, 2012 at 9:57pm

Oh, and you too, Vick. I guess I was reading this too late last night and thought Huntz had originated the thread. Sorry about that.

-Rand.

(Let's see, 9 hours ago would have been 1:00 am China time. It's 10:00 am here now.)

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Permalink Reply by Rand Moore on October 30, 2012 at 10:29pm

The word I was looking for is "soundwell", the circular chamber over which the cone is placed for supposedly better resonance. Here's a photo of an instrument built by JUJU with a soundwell:

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Permalink Reply by Rand Moore on October 30, 2012 at 10:47pm

Here's another good photo of how you could build a shallow well to hold your cone. This photo is by ChickenboneJohn:

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Permalink Reply by Rand Moore on October 30, 2012 at 11:02pm

Hi Vick,

As for building a simple ring, you could build a dodecagon (12 sided regular polygon) as shown in the following diagram:

This info and diagram I put together with the idea of building a roundish banjo frame. But if you use, say, 0.5cm by 2cm trim wood, you could build a pretty nice looking ring to mount your resonator cone. Why 12-sided? Well, one side per mounting hole, with the mounting hole drilled in the middle of each segment.

-Rand.

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Permalink Reply by Rand Moore on October 31, 2012 at 12:03am

That leaves one question: How long should each side of our dodecagon be?

To solve this problem, let's focus on the Isosceles triangle that makes up one piece of our dodecadon pie. What we know are the angles of our Isosceles triangle: 30, 75, and 75 degrees.

Now let's assume your cone is 6" in diameter. To cover it all maybe your ring will need to be a tad bit larger in diameter. Let's say 6.25". So, the two sides of our Isosceles triangle will be half the diameter, or 3.125". So, what we need to solve for is the length of the base of our triangle.

Having forgotten most the geometry and trig I had back in high school nearly 50 years ago, I had to look up how to make this calculation on the Internet. Here is a fairly simple way that uses the sin() function.

How to Calculate the Base of an Isosceles Triangle

1.) Multiply one of the base angles by two and subtract this product from 180 degrees to calculate the angle opposite the base if its measurement is not already given. (We've been given this information, so can skip this step.)

2.) Divide the angle opposite the base by two and find the sine of the result with a scientific calculator.

OK, so 30 degrees / 2 = 15 degrees.

And the sin(15) = 0.2588

3.) Multiply the result of step 2 by two times the length of one of the other sides to get the length of the base of the isosceles triangle.

OK, so:  2 * 3.125" = 6.25"

And:  0.2588 * 6.25" = 1.6175"

So, each side of your dodecagon needs to be about 1.62" wide on the outside edge.

I guess we could test this out by cutting 12 pieces of paper to these dimensions then tape them together to see if they fit the resonator cone properly. This would save trying to cut wood only to find we are just a wee bit off.

-Rand.

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Permalink Reply by Thomas "Duck" Petry on October 31, 2012 at 1:32am

Or you can just draw the thing in the diameter you need and measure the parts.

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Permalink Reply by Rand Moore on November 1, 2012 at 6:19pm

Oops... I forgot to include this photo by JuJu who seems to have beat me to the punch for coming up with a dodecagon reso cone ring...

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