Very interesting. One question: Is it not always the case that the coupling between top, internal air and back/sides are always coupled such that all the resonances are really only relevant in the assembled guitar? Or is there enough of a correlation for tuning of the top alone to be meaningful?Alan Carruth wrote: ↑Mon Jan 15, 2018 6:05 pm
... I will also note that in theory you will not see the same mode at more than one pitch; if you do it's a sign that it's 'coupling' with other parts or resonances. In the case shown, the three 'monopole' modes are evidence of coupling between the top, the internal air, and the monopole mode of the back. In a case like that it's not strictly correct to speak of any of the observed patterns as a resonant mode of the top.
Well Santos Hernandez was certainly "secretive", as he did not want to train apprentices...oops, I mean competition. Had he done so, "Hernandez" might have become as widely known as "Ramirez".amezcua wrote: ↑Mon Jan 15, 2018 6:57 pmGood question prawnheed and in the guitar diagrams I assume all those odd shapes will happen even if a top is just fed through the (mincer) er sander . (My little joke ). In the violin forum the mention of area tuning brought a wary comment in case it was treading on toes and revealing trade secrets . That gave me an uneasy queazy feeling . Do guitar makers have that secretive side as well ?
That makes a lot of sense. Is it not possible to produce the same kind of Chladni patterns for the higher modes and frequencies? One may still not be able to tell what the guitar would actually sound like, but a comparison between the free plate and assembled guitar patterns would maybe reveal something. Or do the "patterns" just dissolve into a undiscernable mess at the interesting frequencies?Alan Carruth wrote: ↑Tue Jan 16, 2018 7:16 pmThere is not a lot of secretiveness among American makers, but I'm told the Europeans do tend to be more so.
There are two things to keep in mind about 'free' plate tuning. The first is that 'free' plate mode frequencies are only of very limited utility in predicting the assembled modes of the guitar. If you make two instruments that are 'the same': use the same wood, patterns, thicknesses, and so on, and the 'free' plates end up having the same mode frequencies, then the assembled instruments will have the same frequencies in the lower order modes as well. If you make any changes, such as using rosewood for the B&S of one guitar and, say, walnut, for the other, the modes will be different. This depends on how different the woods themselves are: if you match the density and stiffness the results will be more similar.
Second: the low order mode frequencies of the assembled guitar are very far from being the whole story. They establish the basic character of the tone, but may not tell you much about the quality. In other words, you can't tell just from those low order modes whether you have a good guitar or not. Most of the difference in quality seems to have to do with what goes on at higher frequencies, and that is both harder to control and harder to measure. My argument is that the high frequency behavior may be related to the free plate mode shapes, and that the utility of 'free' plate or 'tap tone' tuning has to do with that. But, again, it's hard to look at what's going on in that range, so it's difficult to demonstrate any links.
Very interesting. I imagine a speckle pattern interferometer is beyond the budget of most guitar makers and, as you've confirmed, would probably only yield what we already knew i.e., it's complicated.Alan Carruth wrote: ↑Wed Jan 17, 2018 6:51 pmprawnhead:
There are several reasons why it's hard to get useful information on the higher order modes. The most basic is simply the power required to generate Chladni patterns at higher frequencies. As you go up the plates tend to break up into smaller and smaller vibrating areas. In order to generate a pattern you need to get enough amplitude so that the glitter (or whatever) is actually bounced off the plate. Getting that kind of acceleration on a small area takes a lot of power. This is especially true since we use a loudspeaker to drive the plates, avoiding adding any sort of load from a more direct driver. The coupling between the speaker and the plate is not all that efficient, so there's a lot of wasted power. In other words, IT GETS LOUD.
Another issue is that these resonances don't just show up at one frequency, but can be driven over a more or less broad band. When we speak of, say, the 'main top' resonance as happening 'at' 200 Hz, in fact it can usually be driven fairly effectively between, say, 195 and 205 Hz: 200Hz is just the peak. If there is another part of the guitar that has a resonance that's active anywhere within that band it will 'couple' with the 'main top' mode, sharing energy. This actually modifies the frequencies of both modes. As you go up in frequency there get to be a larger number of vibration modes of the different parts of the guitar. Somewhere around 600-800Hz or so there get to be so many that they overlap; you hit a 'resonance continuum'. At that point it is impossible to say what causes the shape you see: it's simply the whole instrument cooperating (or not).
There are ways to look at what's going on in the higher frequency regime, but they are tricky. Martin Schleske in Germany holds a PhD. in physics and a Master's rating in the violin maker's guild. He uses a laser setup to help him in making 'tonal copies' of fine violins. The laser is first used as a tool to measure the exact shape of the violin, so that he can copy that. It can also be used as a 'Doppler interferometer' to see how each point on the violin vibrates at different frequencies. A computer program is used to sort out the 'operational deflection shapes' into likely resonant modes of the parts, and he tunes these to match as closely as possible to the model instrument. He says that the output of the computer depends strongly on what you tell it to expect: if you think there will be a strong resonance at, say, 2410Hz, it will give you one. If you decide it's more likely to be at 2250Hz, it will change everything else and give you that. His copies are said to sound remarkably similar to the originals, but not exactly the same. It's a difficult problem...