?double back - false bottom?

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?double back - false bottom?

Postby Erwin Nistler » Tue Oct 26, 2010 2:56 pm

Double back-false bottom. I don't understand. Do you put the bottom on then some distance above that put in another bottom? Why? What does it do other then make the MD heavier? Do you make the inner bottom thinner as it will not bet bumped? :?
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Re: ?double back - false bottom?

Postby joe sanguinette » Tue Oct 26, 2010 3:04 pm

one advantage to the double back is that it effectively raises the instrument up off the lap and allows the back to resonate more freely. theoreticly this could also increase volume.

the added weight is hardly noticable. it is quite a bit more work.
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Re: ?double back - false bottom?

Postby KenH » Tue Oct 26, 2010 5:32 pm

Basically you build a complete dulcimer. Then make another back to the same dimensions. Make a bunch of "stand-offs" out of wood -any wood - say 3/8" x 3/8". Glue them around the edge of the back on the completed dulcimer. Then put glue on the ends of all those standoffs, and attach the second back to that. Now do your varnish or other finish and string it up.

Why? The false bottom allows the "inner" bottom to freely vibrate instead of burying the sound in your lap. The result is more volume. Recent experiments show that dulcimers generate more sound from their back than they do from the soundboard.

The thickness of the inner bottom can certainly be thinner, but does have to be.
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Re: ?double back - false bottom?

Postby mrchips » Tue Oct 26, 2010 8:26 pm

The one thing Ken didn't cover with that double bottom -- DONT glue the spacer blocks anywhere than right along the outer edge between the bottoms. Gluing a spacer between them say right down the center will have a huge effect on the tone and volume..
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Re: ?double back - false bottom?

Postby Dulcimerbuilder » Tue Oct 26, 2010 10:46 pm

Erwin Nistler wrote:Double back-false bottom. I don't understand. Do you put the bottom on then some distance above that put in another bottom? Why? What does it do other then make the MD heavier? Do you make the inner bottom thinner as it will not bet bumped? :?


False bottom is a term I gave to my double back dulcimers. I had never heard anyone use that term before and it seemed appropriate. I do not glue an extra bottom onto back of my dulcimers. Instead, I install an extra bottom on the inside of the dulcimer, which is intentionally built extra deep to accomodate this. The only way you can tell it has two bottoms is that there are small holes drilled into the lower edge of the sides between the two bottoms. I have had some very good comments about them as they have great volume and you don't see all the extra glued on parts.
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Re: ?double back - false bottom?

Postby rtroughear » Wed Oct 27, 2010 7:42 am

Ken, Ken, Ken - you must have missed the fine print in my experiments,or it could be my foreign Australian accent. Anyhow, dulcimer tops are between 1 1/2 and 4 times LOUDER than the backs (measured directly above or below).

In the one instrument where I added a false back there was very little change in the front/back loudness on the lower fretboard compared with the same instrument without a false back - the top was 1 1/2 times as loud as the back with or without a false back. But above the 8th fret the the addition of the false back increased the relative loudness of the top to three times as loud as the back. So the addition of a false back increased the top loudness in the high frequencies somewhat. These are actual and accurate measurements of sound pressure level.

But listeners are more likely to be to one SIDE of a dulcimer rather than underneath it, and my impression was that the false-backed arrangement did seem louder from the side than a similar shaped and configured instrument without a false back. My unsubstantiated suggestion is that more of the vibration of the inner back, undamped by resting on the knee, is directed out of the edge vents or holes to the listener, rather than substantially downward as in a single-backed dulcimer. I don't think it produces MORE sound, just directs it where it can be better heard.

I've only made one false backed dulcimer, so can't claim much expertise in results, but I did notice that the outer back (which was thin) still vibrated a lot, and hence lost sound downwards and into the knee. To me it makes sense to try to minimise this vibration of the outer back, and I'd do that by making it heavy and stiff. I'd be interested to hear what experienced makers think of that idea.

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Re: ?double back - false bottom?

Postby Erwin Nistler » Wed Oct 27, 2010 4:31 pm

Thanks for the replies. That clears it up a bit. I feel there is a lot more to be said on this subject. I hope more people add there experiences.

I fixed a very old autoharp that had come apart. They had poor glue in the olden days. I noticed it had much more volume when held then when sitting on the table even though it has 3 pads to sit on. The top and back both were softwood. Probably spruce. The bottom vibrated much.

???Do we need a softwood bottom on MD????
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Re: ?double back - false bottom?

Postby Dusty Turtle » Wed Oct 27, 2010 7:31 pm

No fancy equipment or measuring devices are needed for this experiment:

Hold the dulcimer flat against your lap or on top of your bed. Strum it. Then lift it up in the air and strum it again. It will sound louder. Anything pressing against the back (or the top for that matter) of the instrument will dampen the vibration of the wood. But when the wood is free to vibrate, it is louder. The idea of the double or false back is simply to ensure that nothing interferes with the vibration of the back.

Dulcimer players who play with other instruments have the perennial problem of not being loud enough and a double or false back promises to alleviate (but not solve) that problem.

Plus, like scalloped fingerboards, they just look kinda cool! 8)

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