Chladni and VA tests ???

Talk about musical instrument construction, setup and repair.

Moderators: kiwigeo, Jeremy D

blackalex1952
Blackwood
Posts: 776
Joined: Tue Aug 27, 2013 6:36 pm
Location: North East Victoria

Chladni and VA tests ???

Post by blackalex1952 » Sat Nov 28, 2015 1:00 am

Here are tests I did on an OM I am building. I need advice, things don't seem to add up here.I think I have made the back rather stiff, but my intention is not to build a live back instrument. I am wondering however if I should be aiming at 185Hz for the back resonance...The chladni results don't seem to accurately correlate with the VA results.There is also a top monopole around 124Hz in the Chladni results but the file won't seem to load.
Attachments
IMG_5460.JPG
IMG_5468.JPG
IMG_5459.JPG
IMG_5456.JPG
IMG_5454.JPG
IMG_5452.JPG
Uncoupled back no bridge.JPG
uncoupled back no bridge
Uncoupled top.JPG
Uncoupled top no bridge
OM kf tap.JPG
Body assembled no bridge
"Everything I say on the topic is based solely upon inexperience and assumption!"

johnparchem
Blackwood
Posts: 546
Joined: Mon Jan 21, 2013 2:59 am
Location: Seattle
Contact:

Re: Chladni and VA tests ???

Post by johnparchem » Sat Nov 28, 2015 3:46 am

I might be wrong but looking at the body assembled no bridge It looks like your mic gain is too high on the VA tests. If the input signal clips you get also sorts of peaks.

User avatar
kiwigeo
Admin
Posts: 10582
Joined: Sat Sep 29, 2007 5:57 pm
Location: Adelaide, Sth Australia

Re: Chladni and VA tests ???

Post by kiwigeo » Sat Nov 28, 2015 8:30 am

Hi Alex,

Good to see you coming over to the dark side :mrgreen:

As John mentions your gain might be a bit high...the plots look a bit lumpy.

Can you give us a run down on your equipment and your method? ie VA settings, mic, computer, where youre holding mic, how your holding the guitar etc. Ditto for your chladni testing.

One thing you'll find with the Chladni testing....as you get into the higher frequencies you get some modes that appear complex and heavily influenced by the bracing.
Martin

blackalex1952
Blackwood
Posts: 776
Joined: Tue Aug 27, 2013 6:36 pm
Location: North East Victoria

Re: Chladni and VA tests ???

Post by blackalex1952 » Sat Nov 28, 2015 3:13 pm

Here are plots with a much lower gain structure. I dropped the mic gain by about 15dB. The mic is an AKG C414 condensor switched to omni pattern ( no proximity effect in the bass, so essentially a flat response). The soundcard is an M Box2 usb into a HP computer running Win 7 Professional. Perhaps a large diameter condensor is too sensitive? I'll experiment with a dynamic mic later. I am wondering if lowering the back stiffness will also reduce the Top monopole to target? I am also wondering why I could only get the back to rattle the tea leaves at 337Hz, what back mode it actually is and why I am unable to get the back monopole to vibrate? I have a sine wave generator, a 5 1/2 inch speaker mounted in a poly tube baffle suspended over the guitar which is sitting on foam blocks.
Attachments
KF OM uncoupled top.JPG
Uncoupled top no bridge
KF OM uncoupled back.JPG
Uncoupled back no bridge
KF OM no bridge.JPG
Fully coupled guitar no bridge
"Everything I say on the topic is based solely upon inexperience and assumption!"

User avatar
kiwigeo
Admin
Posts: 10582
Joined: Sat Sep 29, 2007 5:57 pm
Location: Adelaide, Sth Australia

Re: Chladni and VA tests ???

Post by kiwigeo » Sat Nov 28, 2015 3:44 pm

blackalex1952 wrote:Here are plots with a much lower gain structure. I dropped the mic gain by about 15dB. The mic is an AKG C414 condensor switched to omni pattern ( no proximity effect in the bass, so essentially a flat response). The soundcard is an M Box2 usb into a HP computer running Win 7 Professional. Perhaps a large diameter condensor is too sensitive? I'll experiment with a dynamic mic later. I am wondering if lowering the back stiffness will also reduce the Top monopole to target? I am also wondering why I could only get the back to rattle the tea leaves at 337Hz, what back mode it actually is and why I am unable to get the back monopole to vibrate? I have a sine wave generator, a 5 1/2 inch speaker mounted in a poly tube baffle suspended over the guitar which is sitting on foam blocks.
Alex,

1. I'm using a Shure PG57....into mic input on a windows laptop. I also have option of going in through USB port via a mic plug/usb adapter. I also use a pre-amp to boost signal a bit.
2. The mode at 337Hz. Funnily enough my current build is giving to strong peaks at around 240Hz and also a second peak at around 350Hz. I haven't done any chladni tests but your test at 337Hz looks like a simple monopole. If this is your main back resonance then it's very high.
Martin

blackalex1952
Blackwood
Posts: 776
Joined: Tue Aug 27, 2013 6:36 pm
Location: North East Victoria

Re: Chladni and VA tests ???

Post by blackalex1952 » Sat Nov 28, 2015 4:56 pm

Yes. It does seem like the back main is at 337Hz..I aimed high...if it was a rifle on a long range target shoot I would have missed admirably! I'll take the plunge and start shaving braces. I want to put the bridge on first, then test again, brace mods and side mass etc after that. I am wondering what the ideal targets are for an OM without a live back?
"Everything I say on the topic is based solely upon inexperience and assumption!"

User avatar
kiwigeo
Admin
Posts: 10582
Joined: Sat Sep 29, 2007 5:57 pm
Location: Adelaide, Sth Australia

Re: Chladni and VA tests ???

Post by kiwigeo » Sat Nov 28, 2015 6:39 pm

Alex,

Im seeing T(1,1)1 at just over 100Hz and T(1,1)2 at 214Hz.

Reducing stiffness of the lower back brace will drop back frequency and also top resonances. Trevor will chime in with advice on this one but by the time you trim the top to desired frequencies your back is still going to be fairly high.

Does the guitar have a finish on it? If it doesn't then thinning the periphery of the lower bout is a better way of trimming main top frequency.

You need to make allowance for effect the bridge will have on final main top frequency....for a cross braced steel string expect a drop of up to 0.3Hz/gram of bridge weight as per Fig 4.6.3 in The Books (Design).
Martin

blackalex1952
Blackwood
Posts: 776
Joined: Tue Aug 27, 2013 6:36 pm
Location: North East Victoria

Re: Chladni and VA tests ???

Post by blackalex1952 » Sat Nov 28, 2015 6:45 pm

My name is Ross.Alexander is my middle name.
It wouldn't be too much drama to thin the edges of the back, if that would help. The top has already been edge trimmed from the inside-a 10mm curved sole violin makers plane to reduce to the flattened edge where it mates with the liners- the top is 1.9mm at the edges, 2.8 mm everywhere else. Sitka Spruce. The finish is Nitro over shellac grain fill on claro walnut back and sides. I am not really interested in the fear involved if I were to tackle a re finish of the top! So the nitro has the same advantage of shellac...you can blend, even pad it on, with suitable fume control of course.I'll wait for Trevor's input, hopefully he will have something to say. I have a 30g ebony bridge I have made, which is theoretically too heavy and a rosewood blank standing by, side masses are a strong contender as well.
"Everything I say on the topic is based solely upon inexperience and assumption!"

blackalex1952
Blackwood
Posts: 776
Joined: Tue Aug 27, 2013 6:36 pm
Location: North East Victoria

Re: Chladni and VA tests ???

Post by blackalex1952 » Sat Nov 28, 2015 7:16 pm

It's incredible how questions lead to further questions a lot of the time!
So what is going on in my uncoupled back plots? Twin peaks, one around the 337Hz mark and the other which looks like 288.68Hz?
"Everything I say on the topic is based solely upon inexperience and assumption!"

jeffhigh
Blackwood
Posts: 1536
Joined: Thu Feb 07, 2008 5:50 am
Location: Caves Beach, NSW
Contact:

Re: Chladni and VA tests ???

Post by jeffhigh » Sat Nov 28, 2015 7:28 pm

Hey Ross,
IMHO
-don't be too concerned about the results at this stage or start modifying anything, it will change when you get the bridge on.
-the only tap test that is really meaningful as far as target resonances,is the coupled top, again with the bridge on. The Uncoupled(soundhole plugged) is only used for input into monopole mobility calcs

blackalex1952
Blackwood
Posts: 776
Joined: Tue Aug 27, 2013 6:36 pm
Location: North East Victoria

Re: Chladni and VA tests ???

Post by blackalex1952 » Sat Nov 28, 2015 7:44 pm

More info here: My question is, given that the 32g ebony bridge I have double sided taped on to the box has moved things in the right direction, are there refinements I can make, and given that the ebony bridge is not very light, ( I understand that 25g would be better ) and is there a good way to get those resonances to a similar target, utilising a lighter rosewood bridge?
I think that the two back peaks on the uncoupled back are the bottom and middle back braces. If so is this an indication that their resonances are standing out because the back itself is too stiff anyway, rather than functioning as a single plate? I was tapping on the bottom brace and listening for the two resonances evenly so as to see them clearly on the graph. Tapping seems like an art in itself! I have mastered the plectrum!
Also, Jeff, since you have chimed in, I am heading towards a Selmeroid next, being a GJ player among other things. Have you been tap testing and chladning on those? I would love some reports on your thoughts re a responsive good sounding Selmer...
Attachments
KF OM with 32g ebony bridge and saddle.JPG
KF OM with 32g ebony bridge and saddle
KF OM uncoupled back 32g ebony bridge.JPG
KF OM uncoupled back 32g ebony bridge
"Everything I say on the topic is based solely upon inexperience and assumption!"

User avatar
kiwigeo
Admin
Posts: 10582
Joined: Sat Sep 29, 2007 5:57 pm
Location: Adelaide, Sth Australia

Re: Chladni and VA tests ???

Post by kiwigeo » Sat Nov 28, 2015 8:01 pm

Alex,

How is the back braced? Gore style with radial bracing over lower bout or conventional with three transverse braces?

Even with conventional transverse bracing for the main back frequency I'd expect to see a monopole chladni pattern centred over lower bout. If the tea leaves are gathering along braces then it looks to me like some more complex mode....one Id expect to see at higher frequencies.
Martin

User avatar
kiwigeo
Admin
Posts: 10582
Joined: Sat Sep 29, 2007 5:57 pm
Location: Adelaide, Sth Australia

Re: Chladni and VA tests ???

Post by kiwigeo » Sat Nov 28, 2015 8:32 pm

blackalex1952 wrote:The chladni results don't seem to accurately correlate with the VA results.There is also a top monopole around 124Hz in the Chladni results but the file won't seem to load.
When doing Chladni testing I pick peak frequencies from the VA spectrum plot and then dial that frequency into my Bradley frequency generator. Often the frequency at which I get best activation of the mode will be a few Hz either side of the pick from the VA plot....obviously most of the error is from visually picking the peak on the plot.

For the record my Chladni set up consists of a Bradley 50 watt frequency generator, a car stereo speaker and I use poppy seed as a visual indicator. I usually find the higher f modes need more volume to drive them.
Martin

blackalex1952
Blackwood
Posts: 776
Joined: Tue Aug 27, 2013 6:36 pm
Location: North East Victoria

Re: Chladni and VA tests ???

Post by blackalex1952 » Sun Nov 29, 2015 12:15 am

Four transverse back braces. I modified the body depth-shallower than standard, made the soundhole smaller to bring the body resonance back down and built the bracing in the lower bout very light, the aim being to use light guage strings. Probably not strictly an OM. The back bracing is probably overkill, in hindsight. But with that many braces, I guess the original plan of carving the braces after assembly is still the logical way to proceed.
"Everything I say on the topic is based solely upon inexperience and assumption!"

jeffhigh
Blackwood
Posts: 1536
Joined: Thu Feb 07, 2008 5:50 am
Location: Caves Beach, NSW
Contact:

Re: Chladni and VA tests ???

Post by jeffhigh » Sun Nov 29, 2015 6:38 am

X braced top?

My take on this-

You wanted a non live back, you have got it, no further tuning of back braces is required.
The results you get by tapping the back or Chladni testing of the back are academic, they will not be produced by string energy because the back is too stiff to effectively couple with the soundboard. This is NOT a problem.

Go ahead install the bridge, saddle and strings and then tap test on the bridge into VA

I have only built one Selmac guitar, it ended up with a main top of about 230Hz from memory. one of these days I'll make up a tool to reach inside and shave down the ladder braces a little.

User avatar
kiwigeo
Admin
Posts: 10582
Joined: Sat Sep 29, 2007 5:57 pm
Location: Adelaide, Sth Australia

Re: Chladni and VA tests ???

Post by kiwigeo » Sun Nov 29, 2015 8:48 am

Top bracing is conventional so you have the option of brace shaving if you want to play with top frequency.
Martin

blackalex1952
Blackwood
Posts: 776
Joined: Tue Aug 27, 2013 6:36 pm
Location: North East Victoria

Re: Chladni and VA tests ???

Post by blackalex1952 » Sun Nov 29, 2015 9:13 am

Thanks for your input guys! Much appreciated...all logical and great to access peoples shared experience here! Jeff, I would like to see that tool! My thoughts so far on the Selmer petit bouche is to attach the top first, maybe then the back with weak HH glue or HHG and paper so it can be removed, then do all the testing, remove the back and fine tune the braces. I like the idea of CF ladder bracing on a Selmer, which is where I want to head. Loud and responsive as possible...I have made a pliage bending jig which gives repeatable accurate results.The pliage angle can be changed as well for experimentation, but I have it set for now at 6 deg. I have also experimented with the neck angle adjustments and feel confident that the Selmer build will now be pretty straight forward. I also have some ideas re the Busato design, and will probably explore that afterwards.
As for the OM, the reason I built with a stiff back was to get some projection out of the guitar, I also used a 25ft top radius, but pretty light bracing...I'm not so sure that further top brace carving is a good idea. It's pretty much on the edge as it is.The upper bout has been left nice and strong, with A frame bracing into the neck block as well, and both CF and a dual action truss rod in the neck.
So what do you guys think of the 32g ebony bridge vs a lighter rosewood one, given that with the ebony bridge the resonances are in a reasonable place?
"Everything I say on the topic is based solely upon inexperience and assumption!"

jeffhigh
Blackwood
Posts: 1536
Joined: Thu Feb 07, 2008 5:50 am
Location: Caves Beach, NSW
Contact:

Re: Chladni and VA tests ???

Post by jeffhigh » Sun Nov 29, 2015 9:28 am

Personally, I'd go with a lighter bridge.
You will be adding the mass of the saddle, pins and strings
Easy enough to add mass later

User avatar
kiwigeo
Admin
Posts: 10582
Joined: Sat Sep 29, 2007 5:57 pm
Location: Adelaide, Sth Australia

Re: Chladni and VA tests ???

Post by kiwigeo » Sun Nov 29, 2015 12:11 pm

jeffhigh wrote:Personally, I'd go with a lighter bridge.
You will be adding the mass of the saddle, pins and strings
Easy enough to add mass later
Ive gone completely off ebony for bridges. if you like black then you can ebonize lighter woods such as walnut.
Martin

blackalex1952
Blackwood
Posts: 776
Joined: Tue Aug 27, 2013 6:36 pm
Location: North East Victoria

Re: Chladni and VA tests ???

Post by blackalex1952 » Sun Nov 29, 2015 1:52 pm

Walnut sounds great, given that I have lots of it and the B&S and the headstock veneer is Claro. Also have rosewood, and the rosette has ebonised pearwood that I ebonised myself, so I know that trick...
"Everything I say on the topic is based solely upon inexperience and assumption!"

blackalex1952
Blackwood
Posts: 776
Joined: Tue Aug 27, 2013 6:36 pm
Location: North East Victoria

Re: Chladni and VA tests ???

Post by blackalex1952 » Mon Nov 30, 2015 9:34 pm

Update on my VA issues. I have worked out why my VA tests have been inconsistent and lumpy. It is important with the version of VA that I have to set the soundcard in the devices menu to the external soundcard I have been using each time I boot it up, or the computer's internal soundcard and mic default. All of the VA versions I have tried on this particular computer refuse to save settings or screenshots. So I have to follow the following protocol each time I load the program.
Under the ‘main’ menu
1. set the FFT rate to 16384
2. set the frequency sampling to 11025
3. click confirm and the spectra line resolution should change to 0.67Hz
4.on the "Spectrum" tab, uncheck "automatic" and set the Freq Range upper 1000, lower-I use 50.
5.on the Device tab select my external device then check mic level.
On my VA version, perhaps it is this computer, the capture tab settings don't do much at all, so I do a series of taps looking for consistence and hit the off tab to capture the one that is the clearest. I then use a freebie called "snipping tool" to take a screenshot and save.
One thing I would like to see in VA is a function where one could cursor click multiple peaks and have each peak's frequency displayed above the peak.At the moment you can only show one at a time, so the screenshot reflects the one you target and not all the ones of interest.
"Everything I say on the topic is based solely upon inexperience and assumption!"

User avatar
Trevor Gore
Blackwood
Posts: 1605
Joined: Mon Jun 20, 2011 8:11 pm

Re: Chladni and VA tests ???

Post by Trevor Gore » Tue Dec 01, 2015 7:10 pm

Ross, which version of VA have you tried and which operating system?

(FYI, I still use VA 9.0, which works well in XP and nearly as well in Win7)

blackalex1952
Blackwood
Posts: 776
Joined: Tue Aug 27, 2013 6:36 pm
Location: North East Victoria

Re: Chladni and VA tests ???

Post by blackalex1952 » Tue Dec 01, 2015 8:00 pm

Have both 2012 and 2011...I'll try to find version 9 and try that. My latest direction is to work on bridge and side mass...ebony 25g brought the top mono down to around the 185Hz mark, a black walnut bridge 20g brought me to around 190Hz. The air was 95 and 97.57 respectively. The bridges were only fixed with double sided tape, no saddle and bridge pins. I am wondering if 95Hz is too low for an OM? Also 190 for the top mono? (Non live back) Should I be experimenting with side mass? I appreciate your input Trevor...I showed you that box at the Luthiers meet at Abbotsford, if you remember-Roscoe
Attachments
KF OM 20g walnut bridge.JPG
20g walnut bridge
"Everything I say on the topic is based solely upon inexperience and assumption!"

User avatar
Trevor Gore
Blackwood
Posts: 1605
Joined: Mon Jun 20, 2011 8:11 pm

Re: Chladni and VA tests ???

Post by Trevor Gore » Wed Dec 02, 2015 8:26 am

That plot still looks a bit weird, Ross. Maybe too much mic gain?

With respect to modal frequencies, 100/180 or 95/180 work fine. Best to stay away from 95/190 if you can.

blackalex1952
Blackwood
Posts: 776
Joined: Tue Aug 27, 2013 6:36 pm
Location: North East Victoria

Re: Chladni and VA tests ???

Post by blackalex1952 » Wed Dec 02, 2015 8:23 pm

I have tried installing and reinstalling and removing all the VA versions I could find. Version 9 wouldn't recognise my external soundcard, but was the only one that saved my settings. None of them save screenshots.Back to version 11 (I think it came out in 2014 ) the only one that I can get results with. Dropped the mic gain and I am getting results, with a lot of mouse clicks to open and set all the parameters each time, including the soundcard settings.Had to re install three times to get the frequency info to show up when clicking the cursor over the peak...MOVING ON!
So here are my latest tap tests with bridges double sided taped in position with a saddle blank for good measure.
My questions: I am leaning towards the lighter 20g walnut bridge and side mass to adjust. Or will the heavier 35g ebony bridge do the trick without too much mucking around, allowing for some further falls after the guitar is strung up and played in? I have the option of enlarging the soundhole a little to tune the helmholtz, as I deliberately made it smaller to give me some room for adjustment because I had also made the body a little shallower than the standard OM size. The top liners are solid beechwood, and I am not so keen to add side mass-hoping to get somewhere with the least amount of further messing around. Also I have found some poppy seeds for chladni, instead of tea leaves hoping to get some better correlation with the VA taps. I am also keen to work out exactly what the peak around 350Hz is...looks like the non live back to me.
Attachments
OM 20g walnut saddle.JPG
20g walnut bridge and saddle
KF OM 32g ebony and sadle.JPG
32g ebony bridge and saddle
"Everything I say on the topic is based solely upon inexperience and assumption!"

Post Reply

Who is online

Users browsing this forum: No registered users and 105 guests