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| chladni http://www.luthiersforum.com/forum/viewtopic.php?f=10101&t=25113 |
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| Author: | fric [ Mon Dec 21, 2009 4:58 am ] |
| Post subject: | chladni |
Hi, Well, this is some chladni of my braced classical top. Several things’ is not clear to me but let me explain how I do that. I put the SB on 4 foam pyramid, position speaker over the bridge (about 7-10mm near SB) area and start increase tone gen. from 75Hz . Things that confuse me are how to make valid data? for example freq. 266 and 267 (two atempts) when I reach 266 or 267Hz I see high jumping of gliter (speaker is over bridge area) but not moving so much, when I move a little speaker to the sound hole gliter starts to move rapidly and form some shape then when I move speaker to let say treble side gliter starts to move again forming another shape so I unfortunatly manage to get two different pattern on the almost same freq and ask what pattern is valid. I find that on all resonant freq. I can, by moving the speaker around, make different patterns for the same frequency sometime similar but sometime rather different. Where I’m wrong -How do you do that? Another question is - 266Hz in my case is it ring mode if yes is it little high then normal? regards Fric |
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| Author: | Alan Carruth [ Mon Dec 21, 2009 2:30 pm ] |
| Post subject: | Re: chladni |
All the patterns are 'valid' if they're done right. The rule is that you can have lots of different patterns at the same frequency, but you won't see the same pattern at different frequencies unless there's something else coupling with the system. For example, you will see the 'ring' mode on the completed top of the guitar at two different frequencies (at least) because the top is coupling with the air in the box (and maybe with thwe back as well). Are you moving the pads to the node lines as you work? If the pads arein an active area they wil distort the modes, and alter the pitches. I think that's what you've got with the 266 and 267 modes there. Also the 199 and 194. |
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| Author: | fric [ Mon Dec 21, 2009 3:19 pm ] |
| Post subject: | Re: chladni |
hi , thank's Alan Foam sometime I move sometime I forget and it’s visible , foam on the 266 stay at the end of 199 nodes (right side of pic.). After more practice I’ll correct this. One more question is 6 inch( 15cm) speaker to big for this application? fric |
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| Author: | Alan Carruth [ Mon Dec 21, 2009 7:37 pm ] |
| Post subject: | Re: chladni |
As usual, with things like the speaker size you end up compromising. On the one hand, a larger speaker is more efficient at producing low frequencies, so you need less power. On the other hand, a smaller speaker is less likely to overlap node lines, and thus drives the plate more effectively. If your 6" speaker works OK for you, then it's fine. |
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| Author: | fric [ Wed Dec 23, 2009 5:06 am ] |
| Post subject: | Re: chladni |
Al, thanks a lot, Yesterday I take new speaker (3’’) and find that was so easy to form pattern from frequency above 130 Hz , below this speaker was unusable. So in future for frequencu up to 130 I’ll use 6’’ and for the rest 3’’. BUT life is life when you solve one problem next is knocking on door, when I try to close ring on 266 (removing or adding ....) ooooooou what a great SUDOKU Fric |
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