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Double Bass Good vibrations???

#63
I also use a deep throat coping saw and get the vibration that you do especially around the fragile areas. I just cut real carefully around those areas and leave a couple mm from the line. A real big bugaboo can happen with this Engelmann spruce where the summer wood is so soft and the winter wood is like hardwood. Coming out of the hard winter grain the blade wants to lurch forward and can go too far if I am not careful.

Some makers use a knife for the whole thing. That would test my patience.
 
#65
Ken McKay said:
I the summer wood is so soft and the winter wood is like hardwood. Coming out of the hard winter grain the blade wants to lurch forward and can go too far if I am not careful.
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yeah that's exactly what is happening. I think that a finer blade will help me. And no way am i going to cut the FFs out with just a knife. We're not making fiddles here!

If you tape the pickup to the top you might get something. But actually down around 30Hz the plate is just flapping wildly.


I'll tell what WOULD work though, and this is neat ... and working on your "timing the ripples" idea ... a stroboscope! I noticed this when I was trying to film the edge-flapping with my digital camera. The viewfinder must be operating at about 50Hz, and instead of the plate flapping I could clearly see the edges of the plate in slow motion moving up and down flip ... flop ... flip ... flop. Unfortunately, this wasn't captured on the video; the CCD must have a better refresh rate than the screen! But a stroboscope would certainly show standing waves at these low frequencies better than any other technique.

Look here:

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Now all we need is a way to get one of these CHEAP! I'll see if jaycar has a kit ... :)
 
#66
Matthew Tucker said:
y
I'll tell what WOULD work though, and this is neat ... and working on your "timing the ripples" idea ... a stroboscope! I noticed this when I was trying to film the edge-flapping with my digital camera. The viewfinder must be operating at about 50Hz, and instead of the plate flapping I could clearly see the edges of the plate in slow motion moving up and down flip ... flop ... flip ... flop. Unfortunately, this wasn't captured on the video; the CCD must have a better refresh rate than the screen! But a stroboscope would certainly show standing waves at these low frequencies better than any other technique.
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I use a stroboscope when I'm doing vibration dedamping on instruments to determine the frequency/speed of the vibrator. It's fun to use the strobe to observe the "pumping" of the top (in slow motion) when you set the stroboscope slightly off the actual vibration frequency, The first time I observed a bass top slowly moving up and down over 1/8" amplitude, I was pretty amazed. Of course, I am referring to an assembled bass with strings, but I don't see any reason why it wouldn't work for your application.

The stroboscope I use is a WWII era machine made by the Electronic Brazing Company. It's quite a bit larger and heaver than those made today (about 7" cube), but works perfectly. They show up on eBay frequently and usually sell in the $50-75 range. They were originally made for the US military and seem to last forever.
 
#70
Ken McKay said:
My wish list would include a mic that can measure 30 hz I wonder if using a speaker as a mic would actually work better at these low frequencies? Any engineers out there?
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Sure, you can do that. Some audio engineers like to rig up a speaker as a kick drum mic. It'll give you a wacky frequency response, but it WILL pick up the super-low frequencies. There are some really nice mics out there that would be much better for the job, but they cost some money. Look at Rode mics for something reasonably priced.

-Trevor
 
#73
Always the practical kinda guy ... I can see it now ... 9pm suburban Sydney ... 40Hz sine wave blasting out of my bass amp, car in driveway with engine revved up, flashing lights, cameras, glitter, suspicious patterns and strange BSO shadows on the walls ... next door's babies screaming, epileptic cat on the roof, kids locked in their rooms, wife hammering on the door ...
 
#77
Here are some more pics with the FFs cut and the top thinned more.

Remember, I'm not using any of this to guide my graduating. Just for fun.

First, FFT analysis of tapping the lower bout

You can see distinct peaks at about 29Hz (dominant frequency) and at 58Hz (the first harmonic of 29Hz)

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Next is with the plate suspended at blocks only. It shows a mode 1 pattern (twisting) and works best when the lower flank is excited by the speaker at 29Hz.
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Next is almost the same frequency, but suspended at the outer edges, and responds when the lower block area is excited by the speaker:

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This looks pretty much a Mode 2.

Next is 58Hz, a harmonic of 29Hz. You can see this is a combination of both the previous patterns!

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FFT analysis of tapping the centre bout; there are peaks at about 76Hz and 113Hz.

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These are the corresponding Chladni Patterns. Looks closeish to a mode 5 maybe?

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Well; what do you make of all THAT???????
 
#78
Matthew Tucker said:
Here are some more pics with the FFs cut and the top thinned more.

Remember, I'm not using any of this to guide my graduating. Just for fun.
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Matthew- It sure seems as if you are doing to an awful lot of work (for fun) if all you are doing is documenting what you have already done as far as graduation is concerned. Are you saying that you have not made any attempts to modify your graduation in hope of improving your mode 5 frequency or pattern?
 
#79
Well, I have, but not really guided by the tealeaves. I wouldn't know where to start! I have thinned the upper and lower bout near the blocks more than on the first bass, based on suggestions from a few talkbassers. Also because I want to see what happens when I take cedar thinner than before. Also want to see how tap tone changes with flexibility (fingerflexing) and thin-ness. Its just another tool.
 
#80
Matthew,

Nice Chladni patterns! Those seem to be what is expected. I know you don't want to go too far from your other bass's thicknesses, and that makes sense because if it ain't broke why fix it. The pattern at mode 5 is showing a lot of stiffness and or weight in the lower bout. Remember, the nodal lines move away from weight and stiffness. So the frown in the lower bout at the bottom is proving that it is stiff there. It seems to me that you would want to utilize that vibrating area by making it larger inside of the mode 5 lines. ie push down the frownie. Theoretically to do this you would remove wood between the line and the tail block flat.

If Arnold is still out there, my question is: How thin/flexible would you recommend the lower bout to be? Do you use your fingers to tap tones to make that area flexible. I really don't think that area needs to be as stiff as the areas closer to the bridge and I think there is a lot to gain by making it flexible there. Am I wrong?
 

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