Filling of seams in body cavity

[James Sanford]

I am currently working on #3 and am curious about the pluses or minus' of filling the seams inside the body cavity in order to make all as smooth as possible.

By seams I means the glue lines around the kerfs and even the slots in the kerfs. I know that we all spend a great deal of time making the top braces fit as correct as possible with no voids or gaps.

I had thought of wood filler or some other filler to do this with.

Has anyone had experience with this subject? I know that there are commercial processes that work on doing this after the mandolin is finished. Why not do it before closing the body?

[sunburst]

Am I missing something? Why would we want to fill the kerfs and edges of the linings?

[Antlurz]

In fact, now that i think about it, those kerf cuts would have a deadening/soundproofing aspect to them. How big is anyones guess, but I can see the thinking behind it.

Ron

[Bernie Daniel]

Antlurz: In fact, now that i think about it, those kerf cuts would have a deadening/soundproofing aspect to them. How big is anyones guess, but I can see the thinking behind itIn fact, now that i think about it, those kerf cuts would have a deadening/soundproofing aspect to them. How big is anyones guess, but I can see the thinking behind it.

Assuming that is true and therefore you did want to smooth over these kerf cuts maybe the way to do it without adding a lot of weight would be to glue a strip of aluminum foil over the slots around the edge? -- smooth surface almost no increased mass?

[Dave Cohen]

So 'splain to me how the kerf cuts have a "deadening/soundproofing effect to them"?! What are the wavelengths involved? And for that matter, wavelengths of what? And what does this "deadening/soundproofing effect" do to those wave motions???

[Rob Grant]

FWIW...

If it really bothers you, don't use a kerfed linings. Heat bend strips of whatever timber you happen to be using for linings and don't resort to saw cuts. Frankly, I can't see a problem with the normal kerfed linings.

[John Saxon]

Aluminum strips?

I like it - smooth and less filling.

I'll have another. http://www.mandolincafe.net/iB_html/non-cgi/emoticons/tounge.gif

[Bill Snyder]

...#I know that we all spend a great deal of time making the top braces fit as correct as possible with no voids or gaps.
You want all of the glue joints in an instrument to fit really well. It makes for a lot stronger joint.

[markishandsome]

I know that there are commercial processes that work on doing this after the mandolin is finished.

There are? I've never seen a commercially produced mandolin with "filled in" linings. If you have gaps in your JOINTS, you either need to take the plates off and try again or cross your fingers. Wood putty isn't going to provide a structural joint.

Making the inside of the body "as smooth as possible" is one of those half-baked ideas a lot of beginning builders entertain early on, at least I know I did. There are just too many great sounding instruments out there with kerfed linings for me to believe it would make any difference.

Ron, by soundproofing are you making an analogy to the ridged walls of movie theaters and music studios? I would think you'd need the same size ridges inside the mandolin as you would in the theater, and those are way bigger than the ridges formed by the kerfs.

[billhay4]

Jim,
I am one of the novice luthiers who made this mistake in thinking early on and was quickly corrected.
The purpose of having the tone bars fit exactly on the top is so they will transmit vibrations exactly.
But the top vibrates the air in the body cavity creating sound. Apparently, the smoothness of the cavity does not affect its vibrations at all.
It is the volume of the cavity that affects its sound.
Hope I've got that right.
Bill
Edit: That being said, violin makers regularly use smooth linings made of willow wood which bends easily. You can also bend solid spruce linings the same way to bend the sides.

[James Sanford]

#I know that there are commercial processes that work on doing this after the mandolin is finished. #

I am referring to a process of cleaning up the inside of the mandolin as an aftermarket luthier supplied service.

I do not have any ax to grind with this process just wondered if by doing as good a job as possible of smoothing the inside of the body cavity during construction had any measureable effect. #Filling would be one way but as suggested by one of the replies perhaps using smooth kerfs would be the easiest way to do what I had in mind. #No additional weight to speak of. I guess that the changes made would be so small as to not be noticeable.

I guess there is nothing new under the sun but appreciate all who responded to this beginner.

[sunburst]

So, you were thinking there might be a sound difference if the inside surfaces are smooth? Is that suggested by those with the "commercial processes" that you referred to? (Sorry, I don't know anything about that.)
If so, it sounds to me like they're trying to sell snake oil to those suckers who are reportedly born every minute, and want to "hot rod" their mandolins.

BTW, having the tone bars fir exactly isn't so much for transfer of vibrations (that's not really what they do) it's simply creating a good glue joint. Any glue joint is best if the parts fit as exactly as possible, and good glue joints don't fail as often as poor glue joints.

[Antlurz]

Ceiling tile drilled full of small holes has been sold for years as a sound deadner. Silencers on guns are merely a series of small baffles. The baffles in a muffler on a car work the same way. As noted, theaters have baffled surfaces.

I don't believe the size of those baffles, in whatever form they take need to be "matched" with a particular wavelength to have an effect. True, specific sizes may have an enhanced effect over a randomly sized series, but they will have an effect as I see it, nontheless. Notice I didn't say they would have a particular quantifiable effect, but they will have an effect.

I seriously doubt it would amount to anything meaningful, and likely not worth the effort, but on some scale, detectable by ear or not, there will be a difference.

[sunburst]

I'm no expert on sound, baffles, and so forth, and in fact I don't really know anything about them at all in a technical sense, but it seems to me the baffles in a muffler are a way to prevent standing waves in the exhaust pipe and reduce sound levels. Silencers use "wipers" and baffles to release pressure more slowly and less directly into the air and lessen the sudden pressure wave that comes from muzzle blast. The sound baffles in a theater are to help control the sound inside the theater. I don't see how any of that relates directly to the inside of a mandolin. Furthermore, I suspect the little spaces in the kerfed linings would be considered "infinite baffles", something I've been told the makers of speaker enclosures don't want because they don't really do anything (if they would be considered baffles at all).

[sheppard686]

The following is part of a question and answer session with Michael Heiden:
See the entire session @:
http://www.mandozine.com/resources/CGOW/heiden.php
"Sound is round".... I keep this in mind when I'm carving and sanding all the parts as want no sharp edges anywhere including the tone bars. I discovered years ago that even sanding the edge of the binding to a round corner makes a significant difference. Graduating the top and back are the next most important aspects to getting the 'diaphram' working correctly ... leaving enough mass in the center of the top to produce smooth, clear treble notes, thin enough in the recurve to 'pump' the air giving a big woof on the bass.

Food for thought,
sheppardmandolins.com

[Dave Cohen]

So many wrong things here that it's hard to know where to start.

Iirc, an infinite baffle is an extremely large enclosure, like f'rinstance, outdoors.

In concert halls, the dimensions of the room are large compared to the wavelengths of the sounds. Meaning that sound absorbing panels or walls are used to deal with undesireable reflections of traveling waves. In the cavity of an instrument, the motions ("normal modes") of air in the cavity are three dimensional standing waves having wavelengths that are large compared to the dimensions of parts like linings, braces, tailblocks, etc. The point being that the kerf cuts are practically invisible to the air motions in the body cavity of the instrument. That is why I posed my earlier post as a question and gave a hint about wavelengths. Rather than just blurting out something like "You're wrong", I tried to get people to think along the lines that would cause them to question their assumptions.

Michael Heiden might have had some very good intuitive and/or empirical reasons for wanting rounded surfaces on all the parts in the interior of a mandolin body, but his physical reasoning was not based on actual physics. Additionally, I am not so sure that the effects he was claiming were actually due to the "smooth" surfaces. A number of guitar luthiers have been finding that there does not seem to be any basis for thinking that rounded braces are any better than braces left either square or triangular in profile.

[Hans]

I'm with Dave. Take a look in any old Gibson...machine marks, rough, triangular transverse braces, etc. Some of them are pretty raw inside.

[Fretbear]

BTW, having the tone bars fit exactly isn't so much for transfer of vibrations (that's not really what they do)
What do they actually do?

[Rick Lindstrom]

This reminds me of the "finishing the inside of an instrument" debate. Intuitively a good idea, but doesn't seem to matter much in reality.

What would Lloyd do? http://www.mandolincafe.net/iB_html/non-cgi/emoticons/smile.gif

Rick

[sunburst]

What do they actually do?
Boy, I wish there was an easy answer to that one.

Tone bars do two main things:
1. Statically, (statically meaning what they do structurally when the mandolin is at rest, not being played, though they still do it when you do play) they act as beams to distribute the load from the center of the top from the pressure of the bridge to more of the rest of the top. In that way, they are acting as small braces.
2. Dynamically, (dynamically meaning what they do when you play the mandolin) they modify the plate modes of the top through their stiffness, mass, and positioning. Because of the tone bars there are differences in the frequencies and amplitudes of the top modes compared to what they would be like in the same top without tone bars.

[Phil Goodson]

There was a big debate on one of the guitar boards a couple of years ago (Martin?) about the effect of curved surfaces on tone.
One of the debaters had some evidence that even rounding off the corners of the glued-on bridge on the flat guitar top changed the standing wave vibrations in some demonstrable way. (I can't remember whether he sprinkled stuff on the top and watched where it collected or whether he had a more sophisticated method.) And I don't know if any of that would translate to a carved top mando.

Does any of this discussion relate to any of the mando-voodoo processes that we regularly speak of on this board?

Phil