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Well i was close to finishing my first classical...I left the top thick, and planned the graduating as the last step, the be able to hear the changes.

I was happy with the sound, clear, balanced, strong. This after about 3 days during which i only sanded using fine paper around the waist, behind the bridge, and less between the bridge and hole. There wasn't any noticeable change, and I thought the trebles are kind of thin, and the plan was to keep sanding.  Yesterday i only sanded the upper bout, to make it even with the rosette, remove scratches etc. so i only removed quite a little wood. After a few hours I picked it up to play and noticed the voice changed: lost some volume, and become deeper but muddy.

You can hear the difference by comparing these 2 samples:
http://ursultrubadur.ase.ro/modules/xg2/engine/main.php?g2_v %20iew=core.DownloadItem&g2_itemId=77109
http://ursultrubadur.ase.ro/modules/xg2/engine/main.php?g2_v %20iew=core.DownloadItem&g2_itemId=77124

In the real I think there is more difference than the mp3s suggest.

I am really tearing my hair because it is not making sense, i did not remove any serious wood from the center of the plate while doing the upper bout and certainly did not touch it behind the bridge.  I was thinking to try a new set of strings.  Should i try lower tension or higher tension?   

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Alex,

That is a beautiful Brazilian waltz!! It is really hard to tell the difference from the MP3 recordings. Are you using cedar or spruce for the top?

Yes that is a wonderful piece, as all of Reis's compositions, but I'm a butcher player


The top is Engelmann, AAA from LMI.  It was glued on and binded at about 2.8mm, a bit less at the waist.  Im sure it is still thick, especially on the upper bout and between the hole  and bridge, and nowhere  less than 2.2mm  on the lower waist.  I can estimate that based on the binding height. 

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There is debate as to whether the upper bout contributes much to the sound of the instrument. There is no debate, however, that it is very hard to make drastic changes to the sound by modifying the upper bout. My guess is that the guitar was just about to make that first change in tone that is part of the initial breaking in process and you just happened to be thinning the upper bout when it happened.

That's a pretty big difference. I would suspect that a brace has popped loose.

I'm with Mike on this one. A lot of the sound has to do with the relationships of the various resonant pitches of things, and often these have to match pretty closely in order to influence each other effectively. Small changes in these pitches can make large change in the tone. It's sort of like nudging the guitar toward the edge of the table; nothing happens for the first few nudges, and even when part of it is over the edge it's still on top of the table, but the next nudge..... Put it aside and see if it recovers when the humidity changes or something.

Remember, too, that most finishes will add some stiffness, particularly across the grain on the top. If, for some reason, you have gotten it too thin, it might come back when you finish it. Nitro is pretty stiff, and even shellac adds a fair amount of stiffness. It's usually the first couple of coats that do the most.

I've checked the braces, they seem to be OK, and there are no rattles or buzzes....

I also think she started to sound better this evening.  Worse than a woman...drives me crazy....

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An instrument in the white (unfinished) is highly subject to humidity variations.

     I'm going to venture to say that the simple fact that the guitar is in an
unfinished state is having much more of an effct on it than your sanding did.

    When unfinished, the humidity can heavily and quickly cause changes like
this. That's just what I'm suspecting is more the culprit than anything in this
case.

Regards,
Kevin Gallagher/Omega Guitars

When you move into a new house its a bad idea to immediately rush into renovations and extensions before seeing the existing house through a full four seasons.

I take the same approach to a new guitar....I give it time to "play in" before doing any adjustments. My latest build (a Torres classical) got finished about 2 weeks ago. Since then its been sitting strung up on the lounge room table. I    play it every few hours and listen to it. Over the space of a week the sound of the instrument has changed quite markedly....the mid range sounded a bit dead after string up but with time its starting to sound less dead and the instrument is sounding alot more balanced.

Thanks everyone. She's got back her volume or most of it anyway, while
the tone seems to change a little every day indeed.  It's quite
interesting (for a newbe like me) to compare with my cedar alhambra,
which has a much simpler tone and obvious separation. The spruce one
seems to form a very colored and complex voice. I scheduled a
full month before doing more sanding or starting the French polish.
Till then, I put a coat of shellac on to reduce any humidity
interference. 

Here is some tormenting on another Brasilian  (Baden Powell)
http://ursultrubadur.ase.ro/modules/xg2/index.php?g2_itemId= 77293

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