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Double Bass bass top Chladni patterns

D

Don Harris

Feb 21, 2003
273
6
Colorado Springs
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#1
OK, I'm a total geek who loves to experiment. I just finished graduations today, so I borrowed a friend's tone generator and set my graduating cradle and bass top on top of my SWR Goliath Jr cab with some foam under the neck and tail block areas and spread some craft sand on it (trash bag over the cabinet to protect the speaker). My wife did not enjoy the experience and was anxious for me to finish. Here are the results: The fundamental tap tone of the top is 40hz (low E), so I think this is mode 1. The cab lying on its back really didn't reproduce the fundamental at 40hz, so the next active freq was 80hz which should be mode 2 (the first pic). It was interesting to notice asymetry, check the thicknesses and tweak. Sure enough, the pattern changed around my tweaks and after a couple trips to the garage with a finger plane, things looked more symetrical. The other freq that got things excited was 96hz (98hz is a G). I have no idea which mode this would be if it is a mode. I'd like to know what freq mode 5 would be as Traeger says this one is useful for graduating. I think for higher frequencies I should use glitter, because the sand kind of quit responding in any meaningful way after 200hz.

My graduation pattern has a vertical area of strength (8mm) from neck block to tail block which is why I think on mode 2 I don't have a curve all the way across the top like the pics in the Traeger book, and why there's a line all the way up the middle.
 
#4
I would mark the spots where there was an accumulation that didn't fit the symetrical pattern and check thicknesses on and around those spots and compare them to the opposite side. Usually there was a difference in how I had transitioned from one thickness to another compared to the opposite side. I was taking off less than a half mm usually. Feeling the surface with my hand sometimes confirmed that some of the problems were steep transitions. I still want to fix something I see on the first pic. Notice the little accumulation just off the center line on the upper bout (upper left of the picture). There were a lot of those kind of things before I tweaked.
 
#5
Don good on you for doing this. I'm a geek too but there's only so much time in a day ...!

Problems I have with understanding this sort of thing are a bit like trying to understand whether God exists or not:

1. when the bass bar is glued on, that will change the way the top behaves pretty radically
2. when the top is glued on the edges will be fixed, this will change things
3. When the sound post is in place, this will change things HEAPS
4. Who says symmetry is good?

If we could do this with the whole bass fitted up, I would see more value. Perhaps this is where the ide of mode matching the finished bass comes into play.

Bet its fun though.
 
#6
I totally agree with you, Matthew. I'm not sure what practical use it is other than it helped me find some inconsistent spots. But like you said, that's assuming that symetry is good. I guess I'm operating on the assumption that smoothness and symetry are the goal. It is definitely interesting to see how the top is vibrating on different notes and feel the nodal points. The real test will be stringing it up and drawing a bow across it. That's going to be a bit scary after all this work.
 
#8
This is where I ended up, whatever it means. After refining the area around the edge a bit, the tap tone dropped a half step to Eb. If nothing else, it's interesting to have the visual feedback on how the thicknesses affect the vibrations. To me, it looks like it should sound good!:)

This is how I think it works. Determine the frequency of the tap tone by looking at a frequency/note conversion chart on the web. Multiply by 2 for mode 2. Multiply by 5 for mode 5. This way you wouldn't really need to sweep since you know your target frequencies, so a simple shareware tone generator would be all you'd need.

Whereas before I didn't think I would, now I think I'll try to do the A0/B0 (or whatever) matching.
 
#9
Although I have not used Invalid Link Removedto make a bass, it is my understanding that the you must work with the modes and mode frequencies of both the top and back together to get the benefits from this style of plate tuning. I'm not sure that it is worth the investment in time required to do a bass this way, but I've been to some CAS seminars where this technique was demonstrated on violin plates and then later heard the completed violin played and the results were impressive. Traditionalist frequently reject the concept, but in reality it is just another way of doing what experienced makers have always done with their hands by feel, bending the plates to determine stiffness.
 
#10
Don Harris said:
This is how I think it works. Determine the frequency of the tap tone by looking at a frequency/note conversion chart on the web. Multiply by 2 for mode 2. Multiply by 5 for mode 5. This way you wouldn't really need to sweep since you know your target frequencies, so a simple shareware tone generator would be all you'd need.

Whereas before I didn't think I would, now I think I'll try to do the A0/B0 (or whatever) matching.
Click to expand...
Have you actually tried sweeping with your sine wave generator and noting the frequency where the modes actually occur by simply observing the Chladni mode patterns? In my experience experimenting, I did not find that the modes (particularly mode 5) follow nice predictable frequencies multiples.
 
#11
I did try sweeping first thing and the most dramatic patterns were on mode 2 and and octave below mode 5. Nothing else came close. That's when I decided that maybe I wouldn't need to sweep in the future. I'm happy that I did it only because it helped me visually locate areas of the graduation that were not symetrical or consistent. I didn't go further than the plan because of the chlandni patterns, it just helped me correct my inaccuracies, I think. At this point I'm looking for any feedback on response that I can get since I don't have the experience to judge by tapping, rubbing and flexing.

Thanks for the link, by the way. Looks interesting.
 
#12
Some of the problems with getting good eigenmode patterns may be related to where you support the plate and where your speaker is located. There are some articles available on the web about violin plate tuning using eigenmodes and most will show you the best locations to support the plate and where to place your speaker to bring out a particular mode. I found for some modes I could get better defined eigenmode/Chladni patterns by using a hand held speaker above the plate. Link Removed demonstrated this method on guitars plates at a CAS/MVA joint meeting in Dearborn, Michigan about 10 years ago and I found the technique can be used on bass plates when it isn't practical to locate your (underneath) speaker at the optimum location.
 
#14
My opinion on plate tuning and a buck sixty gets you a bad cup of coffee, seeing as I've never delved to far into the theory of it all. But fwiw, in the violin making community, it seems most makers are moving away from it in the sense of the tuning of the free plates. I think most are coming to the conclusion that the assembled instrument acts very differently than it's individual parts, and not in an intuitive or obvious way. The way a free plate moves is not the same as the final inst. because in the final inst. you're plates are basically loaded shells, because of all the pressures.And rigidly supported, around the perimiter. And affected by the modes from the opposite plate. I think Don has the right idea, using the plate tuning method to basically double check his graduations and confirm flexibility. But maybe I wouldn't take it too far, Don, as in trying to get perfectly shaped patterns, because another critisism with free plate tuning is that when they are " perfectly tuned " ,the instruments sound nice but the tone lacks a bit of complexity, richness. But really , I have no right to give an opinion, since I've not done it myself. So at best I'm just passing along some things I've heard and talked about. To see what some makers are playing with now, see Martin Schleske site
 
#15
Don Harris said:
Now having read a bit more about this stuff, as Bob kindly pointed out, my approach was a huge oversimplification of how this whole thing works. So at this point I have no idea which modes I was exciting, but I was excited to see "something" happening!
Click to expand...
Don't feel bad. CAS members have been working on this for over 50 years and are still in the learning process. As you can now see, the eigenmodes of a bass or violin plate are very complicated. It appeared to me that you were initially looking at the eigenmodes as nothing more than simple harmonics (like occur in strings) and were trying to visualize all of this in terms of pitch. Unfortunately, it doesn't work that way. While the concept does appear simple at first glance, violin and acoustics researchers are still working to figure out the how's and the degree the various eigenmodes must interact to produce a great instrument. Right now we are at the point where many makers using this technique can produce good instruments nearly every time. However, we're not yet to the point where we can use the technique to produce great instruments every time.
 
#16
Darren Molnar said:
But fwiw, in the violin making community, it seems most makers are moving away from it in the sense of the tuning of the free plates. I think most are coming to the conclusion that the assembled instrument acts very differently than it's individual parts, and not in an intuitive or obvious way. The way a free plate moves is not the same as the final inst. because in the final inst. you're plates are basically loaded shells, because of all the pressures.And rigidly supported, around the perimiter. And affected by the modes from the opposite plate.
Click to expand...
Of course, this has been the argument against free plate tuning from the very beginning. I once saw a demonstration of fixed plate tuning by Dr. Bill Atwood. He built a clamping device that looked somewhat like an outside mould that held the plate just as it would be attached to the corpus. He used the same techniques as in free plate tuning, and while the mode patterns were changed some, they were not all that different except around the outside edges. I am not defending free plate tuning, but it should be pointed out that many makers who have tried and given up on free plate tuning were following the earliest publications by Carleen Hutchins and did not read or utilize the follow-up research that has been done and published in the CAS Journal in the decades since the initial publication in the American Scientific Journal.
 
#17
While I have your guys' ear, or eyes, what do you think about using vernice bianca (egg whites, gum arabic, etc) on the inside of a bass? I've seen positive tonal comments from violin builders and wondered if the same factors would apply to a larger instrument. I guess I'm always looking for that "little bit extra" (I use to replace op-amps and capacitors in my electric bass gear. It's a sickness, I know).
 
#19
""but it should be pointed out that many makers who have tried and given up on free plate tuning were following the earliest publications by Carleen Hutchins and did not read or utilize the follow-up research that has been done and published in the CAS Journal in the decades since the initial publication in the American Scientific Journal.""
__________________

That is a very good point, Bob. I was unsure if I should even comment on something I know very little of. And I was not trying to say that plate tuning was a bad Idea, either. Even if it turns out to ultimately not be the method used in the future, it has to be given enormous credit for being one of the first methods to integrate science into a craft that has had the elements of a black art, previously. And it has led to a working methodology, and also informs other areas of acoustic research.

Hey Don, that Schleske site also has some of his results of the testing of different materials, such as vernice bianca. He puts the materials on a test piece of wood, and then measures things like damping and speed of sound and the like. The site is huge, but there is a graph and some comments somewhere in it.
 

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