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Modes: other sites give lots of information on how to see the patterns that arise when a plate of any shape is vibrated at one frequency. Have a look at the Really Useful Links page.
These were originally discovered by a Mr. Chladni. His patterns of vibrating plates are well documented: have a look for instance at the USW site, and look at Chladni patterns for Violin plates ..... or look on on YouTube. This Ernst Chladni(1756-1827) was a German physicist, and is known as the ‘father of acoustics’.
What they tell us
The Modes, particularly Modes 1, 2 and 5, tell us about the important characteristics of the wood :
- its bendability (speed of sound) across the grain is ‘measured’ by Mode 2, and
- the speed of sound or ‘bendability’ along the grain is ‘measured’ by Mode 5.
If we get these under control (i.e. known), then we are ‘copying’ what Sr. Stradivari did hundreds of years ago, and he knew that if he made a plate a particular way then an excellent violin would result. We may find that a tap tone of say 350 Hz on the free plate produces a resonance of say 440 Hz in the final violin and not 430 or 450 Hz. It reminds me of aiming an arrow when shooting a bow. We calibrate the sights on the bow because gravity, the arrow and how it flies are almost the same every time. We set the tap tones as suggested because a violin is, as far as we can make it, a standard shape, and the ~1.5 times increase (a fifth) in the tap tone to become the main body resonance it what it does when we glue the body together.
What we are missing however is the Italian Special Magic of scraping here, and scraping there that allowed Sr. Stradivari to tune the plates after the violin was assembled (‘in the white’) to turn a good instrument into one of the best violins ever made. I do think Herr Schleske has come closer to reproducing this magic consistently without having to resort to sacrificing frogs legs or nubile virgins.
Again, good violinmaking practice is the foundation. I have found that I need to convert the violin I’m working on to be as near a ‘standard’ good-practice violin as possible. It needs good blocks: you may need to add corner blocks if they are missing, as so often the case with old, cheap factory fiddles, and you may need to reduce the bouts (sides) to 1.0 to 1.4 mm thick by carefully grinding then down with a Dremel hand-router fitted with a sanding cylinder. Some fiddles have bouts over 2 mm thick. By adding good, standard blocks and thinning the bouts I remove most of the variations in sides (bouts) and the structure so that a tap tone of say 350 Hz on top or back plate really does become 440 Hz in the complete violin.
Historically speaking
The first reference I can find to these ‘Modes’ and ‘nodal lines’ on violin fronts and backs is in a Victorian book on violin making of 123 years ago! “Violin - Making: as it was and is” by Ed. Heron-Allen, published 1885-6. The author studied under (one of the) Georges Chanots, probably in London or Manchester.
There are some contradictions and “mistakes” especially on belly tap tone frequencies, but amazingly, re-reading it the other day, I found a reference to Modes 2 and 5 tap tones visualised using sand, using a rosined bow drawn across the edge of the plate to excite the modes.
Chladni’s book on plate acoustics was in circulation a full 70 years (1787) before Mr. Heron-Allen was writing. It’s fascinating to think that he may himself have seen patterns in sand, as shown right, as he drew a bow across the edge of a plate he was holding. This is on pages 132-133 of his book. He refers to Mode 2 as the “normal tone”, and believe it or not, refers to “nodal lines” in the tech. footnote on page 133. As a reality check, this is from 123 years ago. What’s new under the sun?
What are the Modes?
What is Mode 2 or the X-Mode? and what is Mode 5 or the Ring-Mode? Here are the ‘tea leaf’ patterns shown up when the plates are vibrated using a loudspeaker underneath at a particular frequency - the ‘tap tone’. It is a resonant frequency of the plate as a whole.
The pictures above show that when a plate, either the front or back of an instrument is vibrated at one constant frequency then fine sand, tea leaves, sawdust or glitter placed on it will move to areas of the plate that are not moving i.e. not vibrating. This only happens at particular resonant frequencies, and finding those resonant frequencies is what this is all about. These lines of no motion are called the ‘nodes’, or in this case the ‘nodal lines’. Anti-nodes are where the motion is at a maximum.
Understanding these Modes and their shapes is fundamental to understanding tap tones. You will need to know how to get these Modes vibrating. Originally Carleen used a speaker placed under the plate at an anti-node, and fed the ‘speaker at various frequencies until a pattern shows up. These patterns are well illustrated by other website pages to be found in the “Really Useful Links” page.
We can also get the plates vibrating by holding at a point on a nodal line, and tapping at an ‘anti-node’, or a point where the vibration is or will be a maximum: it works well, but there is information lost about the detailed shape of the nodal lines: Carleeen thinks that is important.
What tap tones don’t tell us
Well you wont make a violin that sounds like a £15,000 violin using tap tones like this unless you are very, very, very, very lucky and at least prepared to let an expert choose the (costly) wood to start with.
There is another and much higher level of the art (and science) that the master craftsman has that us mere mortals can only dream of. Others for instance have tried area tuning ( see this fascinating site of Keith Hill’s) and ‘Vigdorchik strip’ tap tuning (see David Langsather’s example here) which I think has great possibilities, but you will need a lot of faith, patience, and a very, very good ear to do it. And much of a lifetime.
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