HD Sound Board Thickness Ply vs Solid

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HD Sound Board Thickness Ply vs Solid

Postby Gearloose » Wed Jun 03, 2009 11:34 pm

Should there be a difference?

Assuming a Hammered Dulcimer design with a fixed sound board which is structural, should a maker increase the thickness if using solid wood instead of ply? I'm assuming a floating sound board can be thinner since it is not load bearing.

I went to Home Depot today and picked through their 3/4 x 12 x 8 ft cedar until I found a pretty nice piece with few knotholes. It was smooth on one side only. I bought it mainly to play with, but with the idea that I'll use it for the soundboard if it comes out good.

I first sawed the 8 ft length into 3 approximately equal lengths, then started ripping 3-3/4" wide pieces out, tossing aside the pieces with knotholes - although I briefly had the idea of using natural knotholes as sound holes.

Well, I trued up the edges on the jointer, then moved to the planer. I would have resawn the pieces down closer to final thickness, but I need a good ripping blade for the bandsaw. Besides, I need the practice with the jointer.

I stopped when all the pieces were 3/8" thick, even though my plans show a 1/4" thick sound board.

Should I go back and finish planing down to 1/4" thickness or use as is at 3/8"? The HD I'm starting with is a 12/11.
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Postby mrchips » Thu Jun 04, 2009 4:03 am

A plywood top HD will have a decient sound but the exact same HD with a solid wood will sound different. Also over the long term plywood wont age out like solid wood so the sound wont change much. I dont think there has been any good (insterment) plywood around long enough to actually quantify the difference like solid wood insterments have been done. I make most of mine using quater inch birch plywood but I have made a few using various solid woods including cedar. Plywood allows you to make a decient HD at a cheaper price that a solid wood one. A plywood one will actually work fine for a lifetime for most people too. Theyre not nessarly an inferer HD either. Be suprised how many people have placed in the top 3 at the national championship contest at Winfield over the years using a plywood top HD too.

A fixed top hd can be made where the top isnt as mutch a structial part as many are. If the way one is made that requires the top to be a major structial member that top will be under compression between the pinblocks. Solid wood will expand and contract less WITH the gran than cross grain so for the greatest tuning stability you have to run the grain parallel to the strings. Wood has ts greaest stringth with the grain also. Plywood with equal ammounts of wood both ways expands and contracts about equal both directions.

The thickness of a Hd top affects the sound the same way as the top of any insterment does. With the same wood and all else being equal generally the thicker it is the less sound you will get. But due to the extremely high volume relitave to most other string insterments the difference between quarter and 3/8 isnt much of an issue. The thickness also affects the overall tone, generraly works out to thinner being a bit brighter.

The way a wood used in an insterment affects the sound is the same for any insterment. A hard dense wood such as maple will give a brighter sound and will reflect the highs real good. A soft less dense wood such as cedar will give a more mellow less bright sound. It will alsos asorb some of the highs more than maple acting as a reflector. A cedar top with a maple back will produce sort of a mellow sound with a reasonable brightness. One thats tottaly maple will give a real bright sound that is sorta edgy sounding to me. One that all cedar will sound a bit lacking in the highs.

From there on a Hd is nothing like any other insterment other than pianos as far as construction goes. String tensions can be anywhere from to to 3 times higher than say a guitar or MD. The total tension across a 15/14 2 string per course one can range from around 1200 pounds to over a ton! Where a string goes over a bridge or nut on say a md the strings produce a down force on them. On that md it is probally close to 10 pounds or so. The actual figure is a function of the string tensions and the break angle over that bridge or nut. On a typical Hd the down force on a bridge can be anywhere from 150 or so pounds to well over 300 PER BRIDGE! Now you know why its a real trick to make one under say 18 pounds that wont implode. :)

On mine the total tension is right at 1900 pounds. Every string is within 3 pounds of 38 pounds and the down force on the treb bridge is 315 pounds and on the base 345 pounds. The break angle on the base one is greater. Mine weigh 16 pounds give or take a pound depending on wood. I could easily get 2 or 3 more pounds out of one but I loose something I love doing that; all tuning pins that make tuning it soo easy and you dont have to mess with loop end strings either.

I notice you are expermenting with cedar. Ive made HDs with that as the top in thickness from just a hair over 1/8 all the way close to half inch. The way I do one the top dosent act as much of a structial member. The 1/8 inch top one has the best overall sound in my oppion. Cedar has a far higher with grain/cross grain differnatil expansion that most woods. In just about 19 inches, top to bottom the expanision across the grain with cedar is around 1/8 inch while with grain its far less. I havent actually measued it bt from the change in pitch of the strings its less than a 64th. As a result that 1/8 inch top develpes a bit of a concave buckle right under the bottom treb string after a few high humidity days. After a few days at a normal humidity it gets nice and level. :) It probaly wouldnt do that if I didnt have the eges trapped in that trim band. The total thickness of the outside braces is right at an inch. The actual brace is 3/4 wood and that trim band is usally a quarter inch thick. The trim band sticks up high enough to hide the edge of the plywood tops and I dont changg that for solid wood tops. Far easier to do them all the same other than the additional work to get a solid wood top than a peice of plywood.
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Postby Gearloose » Thu Jun 04, 2009 7:44 am

Excellent advice! From what I've seen in all your posts, you should gather that all together and publish a book! :)

I have some Spanish Cedar coming that was intended for flute making. Maybe I should consider using that for a soundboard. Spanish Cedar is a member of the Mahogany family and makes excellent flutes. I would think those same characteristics that make it good for flutes would work well for HD sound boards.
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Re: HD Sound Board Thickness Ply vs Solid

Postby LarryV » Thu Jun 04, 2009 1:06 pm

Gearloose wrote:Should there be a difference?
I went to Home Depot today and picked through their 3/4 x 12 x 8 ft cedar until I found a pretty nice piece with few knotholes. It was smooth on one side only. I bought it mainly to play with, but with the idea that I'll use it for the soundboard if it comes out good.

Did you find a nice quartersawn piece? That's really what you want to use for any kind of soundboard.

I had ordered some nice western red cedar from The Hardwood Store of North Carolina - www.hardwoodstore.com.
Spoke with a fella named Hil there.

Got 4 BF of clear, quartersawn red cedar (Two 4' lengths of 1x6), and had them resaw them for me. With shipping, was around $32. All they need is sanding to 1/4" and jointing, glue up. - Came pretty quick too. Ordered on a Friday and got it by Thursday.
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Re: HD Sound Board Thickness Ply vs Solid

Postby Gearloose » Thu Jun 04, 2009 11:54 pm

LarryV wrote:Did you find a nice quartersawn piece? That's really what you want to use for any kind of soundboard.


I didn't find any true quarter-sawn, but the piece I got has the grain running about 75 or 80 deg to the face.

I finished planing the pieces down to 1/4" tonight (0.249" actually)

I matched up the grain how I wanted it and glued it together with Titebond III, then clamped it up and weighted it down flat. After 40 minutes I removed the clamps, weights and waxed paper and cleaned off all the excess glue on both sides. I laid some Poplar strips anlgled across the board assembly and put 100 lb of weights on it to keep it flat while it dries overnight. the strips of poplar will allow air to circulate across the face of the layup while the joints cure out.
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Re: HD Sound Board Thickness Ply vs Solid

Postby BillB » Fri Jun 05, 2009 2:09 am

LarryV wrote:I had ordered some nice western red cedar from The Hardwood Store of North Carolina - www.hardwoodstore.com.
Spoke with a fella named Hil there.


Your mention of The Hardwood Store of North Carolina surprised me. I live about 6 blocks from the store. I get to go over often and sort through the fancy woods. Nice helpful folks.
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Postby Gearloose » Fri Jun 05, 2009 11:49 am

My first solid wood sound board came out looking pretty good this morning. :D (actually, first sound board of any kind.

It's nice and flat.
I gave it a quick sanding and took this picture. It's sitting on the 12/11 back board. (1/2" Baltic Birch) I will not cut the final shape until rails and pin boards are in place.

Image

I think I'm going to stain it a very dark red. The bridges and pin boards are going to be Hard Maple, so it should make a pleasant contrast.
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Postby mrchips » Fri Jun 05, 2009 2:38 pm

The easy way to get the edges flush ---- use a router with a laminate trimbit. Just cut the wood with a saw leaving around a quarter inch large then route it flush. I even bought one of those elcheapo harbor freight laminate trimmers just for that use. The bit just stays in the router all the time. :lol: :lol: :lol:

I use quarter inch plywood normally for the tops and bottoms of mine. The bottom is mostly under tension and the real stress is taken care of by the internal braces so thats all thats needed. I cant say for other makers or kits so its best to go with the plans. With a solid wood top I still use quarter inch ply on the bottom unless somebody wants something dfferent.

That is one nice looking peice of wood. You will need some sort of markers on the bridges. It can be as simple as a peice of white delrin with the rest black, a decal or even dots like on a guitar. I usally make the bridges out of either maple or the same wood as the trim band. I just cut out a notch in the bridge top and inlay a bit of scrap of the trim wood or maple if the bridge is a dark wood. For something different go look at the markers on the wifes HD on my web site. Those bridges are purpleheart and the markers are a bit of white plastic edgeing inlayed in flush to the bridge. Hard to see but if you look close themarkers have the nore name on them using those rub on letters. The lettering is on top of the finish so theyre easily removable if desired.

Ive even made a few that have a stripe down the center centered on the holes. Did one a while back that had this nearly solid black wood as a trim band, The top was a nearly white birch with some nice grain in it. So I made the bridges out of maple with a 5/8 stripe of that same black wood down the center of the bridges.

Right now I have 2 underway using some mahoginy that is at least 50 years old for the top. That mahogony was the fancy wood at the end of the house under the roof eves on the ends. The logic of replacing it with plastic siding is beyound my understanding. :lol: :lol:
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