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distorted chladni mode 5

 
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jethro
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Joined: 07 Apr 2007
Posts: 178

PostPosted: Sun Sep 30, 2007 2:17 am    Post subject: distorted chladni mode 5 Reply with quote

I am doing final graduation on top and bottom plates. My mode 5's are
about 320 and 360. I used strobel's pattern for the back. I have graduated and adjusted to about .003 inches variation across both plates.
When I vibrate the back plate and look at mode 5 tea leaf pattern the
mode 5 looks as expected except that the north-west quadrant (upper
bout- left) part where there should be a nice uniform arch ----- I have
more of a diagonal line. It starts in the corner and rises up and to the
right near where the arch from the other side should meet it at the
center joint. I can't see any difference in the arching, the thicknesses
are very close........
the only things I can figure are a very small hidden crack in the center joint or a large variation in wood density between the left and right half
wood. If there was a hidden crack in the joint--- reason tells me that
the right half would "see" the crack exactly as the left half would and I
would an identical pattern mirrored on both left and right.
I don't.
This seems to just leave some kind of density change problem.
They are the usual book matched tiger maple......
Anyone seen this problem???????
AND ...............
Do you all think this will adversely affect the sound of the finnished
instrument????? ( IE should I be very afraid ?????? )

Any thoughts??????
Tim
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Andres Sender
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Joined: 23 Mar 2007
Posts: 275
Location: N. CA

PostPosted: Sun Sep 30, 2007 2:32 pm    Post subject: Reply with quote

I don't believe in plate tuning, but I've read up on it and dabbled in it. So that's the context of this comment. Smile

It does sound like you have a stiff spot in the area outside the 'wrong' nodal line (although you might have a very weak area near the line). The problem with curing it is that even plate tuners generally agree that forcing things out of spec (i.e. making that area thinner) doesn't necessarily improve the instrument.

I would also check the edge thickness in that area just to eliminate it as a factor.

Who knows, this 'problem' might just save your instrument from being blandly monochromatic. Wink
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mikemolnar
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Joined: 30 Mar 2007
Posts: 57

PostPosted: Sun Sep 30, 2007 4:43 pm    Post subject: Reply with quote

I find plate tuning to be a powerful tool for uncovering the unexpected or hidden things. This is an excellent case in point.

I was wondering whether this is a single piece plate where a density irregularity is distorting the patten. In any case, double check the symmetry of your graduations. If they are as perfect as you think then you have a problem with the wood itself. If its density or integrity is not symmetrical, you must then adjust your graduation to compensate for this. The graduation will be non-symmetrical.

Get yourself a copy of Hutchins' classic article Plate Tuning for the Violin Maker CAS NL #39, May 1983 pp. 25-32.

Every luthier's library should have this.
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jethro
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Joined: 07 Apr 2007
Posts: 178

PostPosted: Mon Oct 01, 2007 1:18 am    Post subject: funky mode 5 Reply with quote

Andres and Mike: thanks for the feedback !
It is a 2 piece book matched back. I'm going to drag out my microscope and look at the upper part of the central glue joing just in case there is anthing there to see. The channel area I think is the only area on the
olate I havn't scrutinised for uniformity-- but they should be fairly similar
--- but I will look closer there too.
Last nite I read an article by Charles Woods called "wood testing".
(SCAVM april 1985)
I think he is looking at only spruce for tops, but he talks about making
measurements on the stiffness in the X and Y direction. He says that
if this ratio is to great -- greater than 30:1 then the mode 5 pattern will
be distorted and mode 2 will be lower than expected. Says 20:1 or even
better 11:1 is great. He claims that beyond 30:1 the finished instrument
will sound horrible !
Not sure what good this does me at this point !!!!!
My bouts are in the range of thickness of about .100-.110 inches.
Not sure I have enough meat there to regraduate to bring the straight
line back to a nice arc. Mike, how much change in thickness do you
think it might take???? A few scraper pulls or a good thick shaving with
the gouge ?????? I don't think I have much to play with !!!!!!!!!!!!!!!!!!!!

Tim
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Jack H.
Super Member


Joined: 24 Mar 2007
Posts: 346
Location: Israel

PostPosted: Tue Oct 02, 2007 12:14 pm    Post subject: Re: funky mode 5 Reply with quote

jethro wrote:
Andres and Mike: thanks for the feedback !
It is a 2 piece book matched back. I'm going to drag out my microscope and look at the upper part of the central glue joing just in case there is anthing there to see. The channel area I think is the only area on the
olate I havn't scrutinised for uniformity-- but they should be fairly similar
--- but I will look closer there too.
Last nite I read an article by Charles Woods called "wood testing".
(SCAVM april 1985)
I think he is looking at only spruce for tops, but he talks about making
measurements on the stiffness in the X and Y direction. He says that
if this ratio is to great -- greater than 30:1 then the mode 5 pattern will
be distorted and mode 2 will be lower than expected. Says 20:1 or even
better 11:1 is great. He claims that beyond 30:1 the finished instrument
will sound horrible !
Not sure what good this does me at this point !!!!!
My bouts are in the range of thickness of about .100-.110 inches.
Not sure I have enough meat there to regraduate to bring the straight
line back to a nice arc. Mike, how much change in thickness do you
think it might take???? A few scraper pulls or a good thick shaving with
the gouge ?????? I don't think I have much to play with !!!!!!!!!!!!!!!!!!!!

Tim


Just a few ponts.
When you say
"The channel area I think is the only area on the
olate I havn't scrutinised for uniformity-- but they should be fairly similar
--- but I will look closer there too"
THere is a huge difference between fairly similar and uniform.

second is
"Mike, how much change in thickness do you
think it might take???? A few scraper pulls or a good thick shaving with
the gouge ?????? I don't think I have much to play with !!!!!!!!!!!!!!!!!!!!

"
THere is also another tool you can use to regraduate that is between a scraper and a gouge.
Try finger planes!

Lastly, sometimes all this modern information on plate tuning can be more confusing than helpful in solving problems.
Go with uniform mearurements and then see what happens.
Good Luck!!
Your posts make me want to start building again!

Jack H.
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jethro
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Joined: 07 Apr 2007
Posts: 178

PostPosted: Wed Oct 03, 2007 2:19 am    Post subject: channel uniformity Reply with quote

Jack : (thanks or the reply !)
I have a dial caliper (.001 inch ticks) that has a verticle plunger mounted
to "feel" the plate thickness. I have measured and uniformed the whole
plate to within about .003 uniformity and about the same to the Strobel
topographical map. I have run the caliper measurements all the way out
to the where the flat lip is which glues down to the edge of the body.
I checked all around the last 5 mm on the upper bouts. One was about .005 heavy for about an inch. I scraped that away so it was uniform
too. Looked at mode 5 again ..... as I expected .... no change !
Looks like to me I'll just have to live with it for better or worse.
So far no one has commented back if the front and back tap tones can be
a full tone appart, or 3 half tones or 2 full tones ......
I graphed all resonances on the front and back. The mode 2 and mode 5
points were reasonably evenly spaced. Some of the really low ones
(75 -100 hz) were kind of bunched. These may not of even been reckegnised modes ..... (though they must have been) one was a mode 1
I think......... The mode 2's and mode 5's were 10 hz from being exactly
evenly staggered. If I lower the top 10 more hz all 4 will be in equal steps.

Very frustrating trying to get top notch work when so little is known
about how to get that outcome ! It must be time for innovation !
Just cutting a bridge seems to be a very smoke and mirrors kind of
activity.

tim.
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Jack H.
Super Member


Joined: 24 Mar 2007
Posts: 346
Location: Israel

PostPosted: Wed Oct 03, 2007 10:24 am    Post subject: Reply with quote

As I do not get into the moder plate tuning techniques I am of no help. I try to keep it down ot the basics.

As for the tap tones... I have done some regraduating work using tone to guide but recently I have found that a nice smooth interior at a symetrical thickness that allows the plate to ring more like like a wooden bell as opposed to a closed door woks just as well as tuning to a specific frequency.
I have a feeling that tuning to a specific frequency has more of an influence on the wolf notes as opposed to the notes we like to hear.
But that is just my opinion, that and 10 sheckels will get ya a 2 lite bottle of Cola around the corner.
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jessupe goldastini
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Joined: 25 Apr 2007
Posts: 169
Location: sana' rafaela'

PostPosted: Mon Dec 03, 2007 2:00 am    Post subject: Reply with quote

from the outsider....

to visually dial in what is going on i suggest shining a very strong light behind the plate, when you hold it to the light it will "show" your graduation....in the thin areas the light will pass through, a perfectly graduted top will show symetry as well as the thicker spot for the sound post and or any areas thicker than others....not very scientific but it does show you what calipers cannot
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