Grumpy - I can see why you are confused. In another post I mentioned to Kirby that I am designing a ladder braced double necked acoustic lap slide and Kirby was interested to explore the design process of double necked instruments on the forum. I suggested he started a separate post and see how it went. If no one else was interested then we will do it via pm/e-mail. This is a (very poor) picture of the embryonic design (see the bottom of the post):
K.O. wrote:
Dave the first thing that stuck in my mind about using ladder bracing for this application is that because of the high amount of string tension the most difficult part of this build will be to get the bracing just right? The window between to stiff and to light has been considerably narrowed and is much narrower than if you had gone with some sort of modified X-bracing?
I don't think this is a problem as the ladder braced harp guitar I made has the same string tension and is working fine.
K.O. wrote:
The second thought was that in this instance the ladder bracing and the use of a one piece bridge might create a strong couple between the string courses. Since open D and open G share three D notes it seems the course not being played could act as sympathetic strings possibly creating an imbalance in string to string response and sustain? Because of ladder bracing effect on the cross dipole it would be strongest on the high D?
Part of the attraction of the double neck is to have the strings linked sympathetically. I know that a lot of people use sperate bridges on the harp guitars and Lance McCollum advised me to do this but I made the harp guitar with one bridge and like the resulting sound. Also with the high arch I use for the top (13' radius) one bridge would seem to me to drive the overall top better and give a better structural effect. If I don't like the coupling I can always take it off and replace it with two seperate bridges.
K.O. wrote:
The third thought was in Grumpy's field of expertise. Intuitively it seems to me that one of the points of high stress is where the necks meet the head block and I wondered if that influenced you for its shape & would curving it into the necks very slightly help widen the area that is taking the load? This is assuming that I saw things correctly and the inside of the necks are one curved piece??
Maybe you meant Rick Turner rather than Grumpy.
My design has two seperate hollow Weissenborn necks and in the middle is a "neck block" that will have cf flying buttress braces tied in there and to the sides. I decided to have one central sound hole rather than two at each upper bout edge as this lets me use my soundhole re-enforcement patch with two A type braces on each side that will tie into the neck-block. The lower arms of the A braces but against the ladder brace just below the soundhole. This gives good support to the weak area of the Weiss - end of soundboards and soundhole. The two seperate hollow box necks are a strong construction, plus I will do a zig-zag lattice of small cf struts just under the soundboard on the necks as I do on my single neck Weisses. The necks don't really tie into the "neck-block" as such.
K.O. wrote:
The offset of the source of the long dipole and cross dipole from the center of the lower bout has to be doing some weird things to them and their relationship to each other? What is the neck/top mode relation ship for a Weiss style instrument? is-determined mostly by the fingerboards function as a brace and its weight?
I'll use Grumpys quote on this one. Over to Al C maybe?
K.O. wrote:
If two people play it at once would the competing long and cross dipoles cause some weird cancellation/reinforcement effects?
It's meant for one person to play but they could well be playing in D on the lower neck and throw in a suitable strummed chord from the G neck and vice-versa . I really want the two to interact and until it's built and tried I don't know if this will be good, bad or indifferent. Paddy Burgin seemed to be experiencing positive interplays with his double necked Weiss.
K.O. wrote:
Not that this should be done but am curious if the effect of V-ing the necks and changing the vectors of the string tension would allow lighter bracing and help achieve a broader mellower sound?
Offsetting bridges to put them slightly out of phase ?
See my answer to your first and third questions.
_________________
Dave White
De Faoite Stringed Instruments". . . the one thing a machine just can't do is give you character and personalities and sometimes that comes with flaws, but it always comes with humanity" Monty Don talking about hand weaving, "Mastercrafts", Weaving, BBC March 2010