Replies to This Discussion

Permalink Reply by Doc Oakroot on July 20, 2009 at 5:37am

The original resonators had a biscuit bridge (i.e., center of cone higher than the edges). When John Dopyera broke away from National to form Dobro, he switched to a spider bridge (i.e., center of cone lower than the edges) to avoid infringing National's patent (which is long expired now, so we don't need to worry about it). (BTW, I may have it backwards which style was first).

The difference is just a legal manipulation and both work fine.

For a cigarbox body, the most important consideration is maximizing the volume inside the box for Helmholtz resonance purposes. When you mount the cone inside the box, the volume above the cone is excluded from the resonance volume. You can alleviate this by using a flat "cone" mounted on top of the box (with a big hole under it). But that may involve using something other than a real spun cone and that might hurt your sound - experimentation is the answer here.

The cover plate is entirely ornamental, so it doesn't matter what it's made out of - just needs to have enough holes for the air to move freely through it.

For the cone, the heavier the material, the higher string tension you need to move it. For anything heavier than thin aluminum, the necessary string tension will probably be prohibitive.

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Permalink Reply by kelly pratt on July 20, 2009 at 7:04am

Wow... alot of good info there, a little more than a newby like me can wrap my brain around. I "assume"...( yes I know...)
that the sound "bowl" or cone is facing out-towards the strings... I got lost at mounting the cone ONTOP of the box w/ a hole under it... does this mean there is no cone INSIDE the body? And is the cone mounted ONTOP basically acting as the decorative outter cone as well?
Yall have to remember- alot of us are just starting at this- some of the terminology doesn't mean much to folks who arent luthiers full-time.
Thanks for your patience and assistance....
kp

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Permalink Reply by Doc Oakroot on July 20, 2009 at 7:20am

And example of mounting the "cone" on top would be using a paint can lid as a resonator. I put "cone" in quotes because, of course, it's not cone shaped but flat.

You cut a hole in the box just a bit smaller than the paint can lid and set the lid down over the hole. The bridge sits on top of the paint can lid. This works - although maybe not as well as a spun aluminum cone - and preserves maximum (spacial) volume in the box.

You can also unroll the side of an aluminum Coke can, cut to shape and glue it over the hole - sort of banjo meets dobro - for a thinner and more responsive resonator.

Using a cone-shaped cone on top of the box would result in a very high bridge and you'd have to deal with that in the neck design, but it would work (but look pretty ungainly IMO).


kelly pratt said:

Wow... alot of good info there, a little more than a newby like me can wrap my brain around. I "assume"...( yes I know...)
that the sound "bowl" or cone is facing out-towards the strings... I got lost at mounting the cone ONTOP of the box w/ a hole under it... does this mean there is no cone INSIDE the body? And is the cone mounted ONTOP basically acting as the decorative outter cone as well?
Yall have to remember- alot of us are just starting at this- some of the terminology doesn't mean much to folks who arent luthiers full-time.
Thanks for your patience and assistance....
kp

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Permalink Reply by kelly pratt on July 20, 2009 at 11:24am

Ah...I saw the may examples of this- but wasn't sure if it could be that simple...Bueno!
I guess the old saying....K.I.S.S. applies eh? lol
Thanks again!
kp

Doc Oakroot said:

And example of mounting the "cone" on top would be using a paint can lid as a resonator. I put "cone" in quotes because, of course, it's not cone shaped but flat.

You cut a hole in the box just a bit smaller than the paint can lid and set the lid down over the hole. The bridge sits on top of the paint can lid. This works - although maybe not as well as a spun aluminum cone - and preserves maximum (spacial) volume in the box.

You can also unroll the side of an aluminum Coke can, cut to shape and glue it over the hole - sort of banjo meets dobro - for a thinner and more responsive resonator.

Using a cone-shaped cone on top of the box would result in a very high bridge and you'd have to deal with that in the neck design, but it would work (but look pretty ungainly IMO).


kelly pratt said:

Wow... alot of good info there, a little more than a newby like me can wrap my brain around. I "assume"...( yes I know...)
that the sound "bowl" or cone is facing out-towards the strings... I got lost at mounting the cone ONTOP of the box w/ a hole under it... does this mean there is no cone INSIDE the body? And is the cone mounted ONTOP basically acting as the decorative outter cone as well?
Yall have to remember- alot of us are just starting at this- some of the terminology doesn't mean much to folks who arent luthiers full-time.
Thanks for your patience and assistance....
kp

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Permalink Reply by Mark Werner on July 21, 2009 at 3:20pm

The resonator, whatever it is that you use, must be driven by the energy of the strings. Essentially, it's a mechanical speaker, rather than an electric one.
That means it has to be of sufficiently light material to vibrate under the vibration of the strings. That's why materials like spun aluminum are used in commercial models; very light, very stiff, and easily picks up the energy of the strings.

Then, the sound waves produced by the resonator have to get out of the box. You can nest the resonator inside another cone facing the other way, with the soundholes arranged to project these sound waves out. The guitar body will, of course, vibrate to some degree as well.
A simpler design is to have the cone of the resonator facing "down", but spaced away from the back of the guitar by some sort of spacers. The sound waves bounce off the back, and then out through the cover and soundholes.
(see my pics for a fairly typical design)

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Permalink Reply by kelly pratt on July 21, 2009 at 5:04pm

Hey- Thanks Doc- Great pics on your page! Again, alot of good info there-
Will keep everyone updated as to progress on my cbg's...
Thanks!
kp


Mark Werner said:

The resonator, whatever it is that you use, must be driven by the energy of the strings. Essentially, it's a mechanical speaker, rather than an electric one.
That means it has to be of sufficiently light material to vibrate under the vibration of the strings. That's why materials like spun aluminum are used in commercial models; very light, very stiff, and easily picks up the energy of the strings.

Then, the sound waves produced by the resonator have to get out of the box. You can nest the resonator inside another cone facing the other way, with the soundholes arranged to project these sound waves out. The guitar body will, of course, vibrate to some degree as well.
A simpler design is to have the cone of the resonator facing "down", but spaced away from the back of the guitar by some sort of spacers. The sound waves bounce off the back, and then out through the cover and soundholes.
(see my pics for a fairly typical design)

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