Double Bass Good vibrations???

[arnoldschnitzer]

That may very well be true. However, proponents of plate tuning are looking for a methodology to achieve consistently good instruments, not necessarily making a master class instrument every time (if such a thing is even possible). IMO,this is particular beneficial for beginning bass makers who may lack the experience to know by feel what is desirable resistance and/or twisting of the plates in their hands.

Well stated, Bob. I think one of the reasons for the development of the electronic plate-tuning methods is the breakdown of the master/apprentice system. Without someone to guide the aspiring luthier, he is pretty much on his own to figure things out.

Matthew, I did not mean to rain on your parade. Anytime you want to come to NY with a partially-carved plate I will be happy to share my meager experience with you regarding tapping, flexing and rubbing. (That sounds naughty)

 

[Matthew Tucker]

Anytime you want to come to NY with a partially-carved plate I will be happy to share my meager experience with you

Thanks. And maybe I'll bring my riving knife and loud speakers and we can borrow the Storioni for some chladni analysis as well ... I bet it has a GREAT klingon mode

 

[forester]

wanted in on the smartass part...am i on the right track?

 

[KSB - Ken Smith]

Thanks. And maybe I'll bring my riving knife and loud speakers and we can borrow the Storioni for some chladni analysis as well ... I bet it has a GREAT klingon mode

Matt, are you talking about pulling a Knife out on an Italian in NY?:scowl:

You will be sent back down under with a new pair of cement shoes!

By the way, it's at Jeff's for restoration, not Arnold's. On the other hand, Arnold has 3 other projects of mine to complete. Two Italians and one Yankee/German (A.Gemunder Sr.)

Still, even if you get up here by Plane with a Knife on board, count your blessings but don't try anything with any Italians in NY.. lol

 

[ctregan]

This will help make sense of it all.

 

[Mike Arnopol]

Pardon my lack of knowledge in advance, but are you talking about the kind of tuning that Carleen Hutchins was working on?
I know that the wonderful repairman Bob Daugherty was doing that sort of thing. He took a factory contemporary carved German bass and redid the top and back. Marc Johnson used to own it. Steve Rodby does now. This bass sounds as good or better than most top hand made basses. Not that I reccomend it, but Marc let him regraduate the top of his Camilli ( or whatever it really is--a great 18th century Italian ). It was weird, I'm looking at the underside of his top with a mirror and it's all white wood!!! But Marc said that his bass sounded better in all aspects. Chip Jackson has Bob regrad the top on his bass and said the results were fabulous. Just thought that I'd throw that out there for what it's worth.

 

[Matthew Tucker]

Well, firstly I just want to say that my riving knife is thin and blunt, and used only for removing the tops of basses ... not intended to be used on any person unless they need their top removed as well

Why do this sort of analysis, well, I just see it as another way of measuring the properties of a piece of wood carved a certain way.

If you are just making a favourite cake you might change the recipe each time a bit at whim or to suit your taste or the variability of the ingredients. However for communicating that recipe to someone else, or for consistency of results, its usually possible to write down the recipe. It might not communicate everything, and doesn't explain why grandma's version always tastes better than the one made according to the recipe ... but at least it provides the basis for the same cake. And of course, an expert chef won't follow his own recipe anyway, at least not a written one. But he might copy someone elses recipe, before adapting it to his own ...

I suppose I see the value in Chladni patterns the same way. It could be one way of measuring the flex properties of a bass top. If those patterns mean something - and I presume they do - then that's yet another way of communicating or recording that piece of wood/work. As well as arching height, outline, graduations, wood type.

If someone like Carleen Hutchins or Link Removed can interpret these patterns in such a way that they can guide their work on tuning top, and come up with good results, then great. I'm not necessarily looking for that type of use yet though, I'm just having fun.

By the way, Chladni used a bow to excite the plates he was studying. Perhaps he was inspired by watching a luthier in some dusty workshop testing his violin plates?? So the italian masters would have had that technology as well, although whether they used this or not, is pure speculation.

Harry Vatiliotis and other makers weigh their tops religiously, and graduate to a certain target weight. I thought "why on earth do this?" but he explained that in fact it was a very good way of compensating for the variability in density of different pieces of wood. Kind of makes sense.

 

[Matthew Tucker]

Quoting Alan Carruth, for no other reason that it is interesting what he claims, although I have of course no evidence either way. He makes guitars but the same principles probably apply:

"The lowest several vibration modes of the top and back of the guitar are always pretty symmetrical, even when the bracing is not. All guitars tend to have similar modes, but each type of guitar (steel string, classical and archtop) tends to have the modes in a particular order and it seems to be the order of modes that gives them some of their characteristic timbre.

The lowest mode of a top or back plate will almost always involve a single large area moving in and out like a piston or loudspeaker and it is this mode of the top that accounts for most of the radiated sound of the guitar. It is thus important that this mode be as “free as possible. To do this, a luthier will try to make the top as light as possible and only as stiff as it needs to be to withstand the pull of the strings and give the desired timbre.

The main back mode, if it occurs at a frequency close enough to the main top mode, can reinforce it and it is thus helpful to tune it as well. The back is usually heavier than the top and moves less, so this is a secondary contributor to the sound, but it can make a noticeable difference, particularly in the bass range."

I find this interesting, don't you? It might be quackery, but it's interesting nonetheless and food for thought. It might be very useful stuff to know.

 

[Bob Branstetter]

.

(I had the pleasure of working with Professor Irwin Corey when I was in the house showroom band at the old Kansas City Playboy Club in the 1960's. At age 93 he is still pretty incredible.)

 

[ctregan]

This is turning into a youtube thread.
It looks easier with violin plates, the trick is learning how to interpret the patterns.

 

[Matthew Tucker]

What I did discover - from another forum - was that the mode patterns don't just magically appear, you have to hunt for them, and supporting the edges of the plate at the points where you think the antinode lines will be, is part of it.

If I get better results with this knowledge, I'll keep you posted.

 

[Ken McKay]

Arnold I agree with what you're saying but I'm experimenting nonetheless.

Ken McKay has suggested that rather than using it to slavishly guide plate carving, it is maybe one way to "document" the plate flexibility before gluing on the bassbar and fixiing it to the top, when of course everything changes. Especially if I am using the same or similar wood each time, it may enable me to understand in another way what is going on. Flexing and tapping is one way, but this is another. Both are probably as valid as the other. I'm not looking for symmetry, I'm looking for a way to understand what the wood is doing.

for example, I'm interested to kn ow what is different between the left and right half of the lower bout here. I'll do some more thickness readings and see if that tells me anything. I may not respond by moving more wood, but if for example it reveals that the RHS is 1mm thicker all over ... I might.

The 71Hz shake was such a visual surprise when the plate started oscillating wildly. I think a stroboscope might reveal some interesting things too. But I don't have one of those.

Hi Matt,

I think you are on to a good start with the measuring of your wood stiffness. How far are you from being to your desired thickness for the plate without ff's?

You are looking for 2 patterns. Both patterns divide the plate into thirds. The first is parallel to the grain and the second is probably at least twice the frequency and is across the grain. Because it is not a flat sheet, the patterns are going to be curved and "distorted" by the stiffness that results from the curve and variable thickness. 91 hz looks a little like mode 2 (cross grain) but I can't see anything that looks like the "vibraphone" mode 5 (along the grain). When you sweep your sound, try to identify that pattern in the dance and then stop there (with sound on) and push the sand away from the areas in the hollows where it is dancing. You might not have enough amplitude to get the sand to clear those areas. Then you will see the clear pattern when you turn it off.

So...two things. 1. proceed with your plate thinning. 2. Once you are comfortably where you think you want to be, measure again for the two stiffness frequencies. Stiffness=plate weight X mode frequency squared/100,000. Per Joe Curtain.

And once these patterns emerge, you can also use them to estimate the effect you are having on both along and across the grain stiffness by removing wood from a given area.

I estimate that your cedar is probably lower density and stiffer than spruce. But the ratio of stiffness across to along the grain is highly effected by grain runout or off quarter grain.

As for helping you make a great, resonant, responsive bass. You are going to have to ask Arnold. And once you do, we will know the stiffness of your plates.

And if we could measure some great basses for these two measurements, it might help.

 

[Matthew Tucker]

The "desired thickness" eh?? The desired thickness will be the one that makes a great, resonant, responsive bass!! What's that then? Well the other cedar bass was carved about 1mm thicker all over than the measurements given by Chandler for spruce. So I'm going to do the same, I think. Not far to go now. Maybe go a little thinner. The arching is different. Some areas are already at their minimum thickness (eek!) but I'm still graduating the waist and upper bout. This cedar is longitudinally very stiff, but across the grain quite flexible even at 8mm, especially at the flanks of the lower bout. So, I think it is going to be a nice top.

 

[Ken McKay]

"The lines indicate antinodes, or points of least vibration. Anyone give me an idea of where I remove wood to "move" the antinode? My instinct would be to remove wood from the side of the antinode I want it to travel to ... is that right?"

Generally nodal lines will move away from heavy stiff areas toward less stiff areas.

And..lowering the FREQUENCY of a mode is accomplished by decreasing the stiffness where the maximum bending or torsion strain takes place. But these areas can be common to more than one mode so it is hard to selectively lower one mode without effecting another. Or so I am told by the engineers.


Tap tones and Chladni patterns can be thought of as identical in terms of plate stiffness. That is if you know how to hold the plate and where to tap. Hold on a node near the plate edge and tap at an antinode. You might get slightly different notes at each antinode and the chladni frequency is the average of the two areas. I am talking about mode 2 and mode 5 here. Or so I am told by the engineers.

For best results, suspend the plate by resting it on 4 little foam blocks at the area where the corners would be for mode 5 with the speaker in the center. For mode 2 the plate is suspended on the nodal lines at each end of the plate with either the right or left side of the lower bouts over the speaker.

Personally I am interested in the stiffness of your top plate with the ff and bass bar and its weight. I want to see how we compare.

 

[Felessan]

Ok, so this is bugging the hell out of me. (and you'd think I'd want that hell out of me!... but no.. it's better to leave it inside. )

Anyway. Here's some definitions for you all:

an·ti·node /ˈæntɪˌnoʊd/ [an-ti-nohd]
–noun Physics.
the region of maximum amplitude between two adjacent nodes in a standing wave.
[Origin: 1880–85; anti- + node] (anti- means "opposite")

node /noʊd/ [nohd]
–noun
8. Physics. a point, line, or region in a standing wave at which there is relatively little or no vibration.
[Origin: 1565–75; < L n&#333;dus knot]

mode /mo&#650;d/ [mohd]
–noun
10. Physics. any of the distinct patterns of oscillation that a given periodically varying system can have.
[Origin: 1250–1300; ME mod(e) (< OF) < L modus measured amount, limit, manner, kind, tone]


I hope you enjoy these definitions!
-Trevor

 

[Felessan]

A couple of things before we start:

Sine waves are commonly used for these "glitter tests" (Chladni patterns, Eigenmode analysis, whatever, it's all the same idea.)

Sine waves hurt your ears. Bad. Especially for these tests, it's easy to have them playing too loud and for too long. You and your loved ones should wear ear protection while exposed to sine waves!

It's better to find the right frequency at a lower volume (by being very meticulous with your frequency-finding and bouncy-glitter-observation skills), then crank up the volume just enough so the glitter goes to the right spot, then turn it down again. It's best to turn it all the way down to zero volume when you don't actually need the thing vibrating.

So now for the definitions in use portion:

Nodal lines are where the glitter/iron filings collect.

Nodal points are where you put the foam things, because those points won't be vibrating (or shouldn't be, anyway)

Antinodes are where there's the most vibration between two nodes. These are best described as "areas."

Modes are pattern-states where we say "hey, look, now that's something useful!"

Basically, if you think about vibration as limited folding (like when you make crease lines in your clothes as you fold your laundry) when you do this test, you're finding out where (physically) the violin tends to "fold"--given certain resonant frequencies.

People have been doing this for a long, long time--in the form of tap tones, and in modern times with scientific instruments. It produces excellent, reproducible results.

The difference between tap tones and the glitter method is that one uses your ears and one uses your eyes. Some people have better eyes than ears, and some are the other way around. Which one do you trust? Why not use both?

Here's some websites that are fun:

http://www.phys.unsw.edu.au/jw/chladni.html

Invalid Link Removed

http://www.catgutacoustical.org/research/articles/modetune/modechrt.html


Enjoy!
-Trevor

 

[Ken McKay]

Thanks Trevor, I had to edit my post after reading the definitions closely.

 

[Matthew Tucker]

Thanks Trevor, that was useful and I like that &#650; character ... must use it more often!

I don't have much luck with tap tones. I tap here and I tap there and depending in what I tap with and what direction I tap in, i get completely different results. Or the tap tones are so full over overtones I can't find a pitch centre to identify.

And it all seems to depend where I hold the pate while tapping, and there seem to be no rules about that. May explain why when I suspend the plate from the neck, the whole plate sounds pretty much the same all over when tapped. Or it could be my graduations are perfect, whatever that is.

Hmmm maybe I need to experiment more with foam block placement, find those Nodes first. Then tap

 

[Ken McKay]

Matt, I think mode 5 is going to be around 80 Hz with bar and ff's cut. That makes mode 2 around 40 Hz, or maybe a little higher without ff's. Seems pretty low to measure but maybe you can get it. It seems us bass makers are going to have to figure out how to measure these very low frequency modes if we want to compare plate stiffness.

I am not interested personally in how to do "plate tuning" using the glitter paterns. I am looking at it differently. More along the lines of Joseph Curtains article in Strad magazine on tap tones and plate weights. It is easy to get with a violin plate, but try holding a bass top between your thumb and finger and listen for a note of just around 40 Hz! That is a challenge. I can hear the mode 5 around 80 Hz pretty well. I use a piano hammer that works great to thunk the plate and listen for tones. I also bring the plate into my computer room and use an fft program to see where the frequency peaks are when I tap it.

I will be starting final graduation of my bass top pretty soon and hope to get mode 2 and 5 frequencies. I already cut the ff's so It will be with and without bassbar.

 

[Matthew Tucker]

I'll be cutting my FFs soon too. laid them out last night and decided i want them slightly larger; the bass I have made has turned out 2-3cm longer than the original I was copying, not sure why, and it doesn't matter really.

I tried supporting the plate at the thirds points as you suggested, but that doesn't work very well,as the lower bout is heavy and tends to tip the whole plate up. might have to weight the neck cutout area! And the tap tones seem pretty much the same all over, maybe a semitone difference in places but its hard to tell. I get a more solid thunk just next to the blocks, but that's because i have left those areas thicker, my hunch is that by leaving the thicker centre strip thicker the top will be less likely to collapse longways over time, but allow the top to vibrate laterally.