My first frequency spectrum

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gerryt
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My first frequency spectrum

Post by gerryt » Sun Jan 13, 2019 9:31 am

Hi all

Hope you don't mind me asking but all this frequency/plate testing mullarkey is new to me. I wonder if some of you experienced VA users might take a quick look at the spectrum below and tell me if I'm in the right ball park. This was a spectrum taken when I was testing for the twisting mode. I think the peak at 50Hz shows that mode. I generated separate frequency spectrums for the cross grain and long grain modes but the peaks shown on the photograph tally with these results ie 83Hz for the long grain mode and 141Hz for the cross grain mode.

This is for an Adirondack Spruce top.

I'm using an SM58 mic, a Behringer preamp and of course Visual Analyser. It took me quite a while before I could even get anything out of VA. Tried an XLR to USB adaptor with no success and only when I bought the preamp could I get anything happening with the software. These books make me feel like I'm back at school! This time it's fun though!

Image

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Trevor Gore
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Re: My first frequency spectrum

Post by Trevor Gore » Sun Jan 13, 2019 10:41 am

Looks about right, but I don't know the dimensions of the wood.

Regarding the mic etc. issues, what sound card and software are you using?

gerryt
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Re: My first frequency spectrum

Post by gerryt » Sun Jan 13, 2019 11:16 am

Thanks for the reply Trevor. Bedtime here in Scotland (well for oldies like me it is!) and I'm on my phone. I'll crank up the laptop and Excel and give you more details tomorrow. Enjoy your Aussie sun. Its a wet cold night here in Scotia.

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kiwigeo
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Re: My first frequency spectrum

Post by kiwigeo » Sun Jan 13, 2019 6:17 pm

Gerry,

I got mediocre results running via a Shure SM58 mic into a Windows machine running VA. I slapped a preamp between the mic and computer and got better results.
Martin

gerryt
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Re: My first frequency spectrum

Post by gerryt » Mon Jan 14, 2019 5:57 am

Martin - the preamp certainly cured my teething troubles getting VA to work. I guess technically what I'm using is an audio interface. It's a Behringer UMC202HD if anyone's interested and cost about £50 from Amazon. The XLR to USB adaptor cable was to be frank a waste of space.

Trevor I have a bit more info for you now. I'm running VA on an HP Pavilion laptop with Windows 10. Soundcard is a Realtek something or other. Wood dimensions - Thickness 4.4mm, Length - 557mm, Width 197mm, Weight 220g.

Is it normal to get the three modes in one spectrum capture apparently as obviously as in the photo?

gerryt
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Re: My first frequency spectrum

Post by gerryt » Mon Jan 14, 2019 9:13 am

And another quick question if that's ok guys.

I'm getting a density figure of 455 for this top and a long grain Young's Modulus of 14.69. Firstly are my figures right and secondly is this top going to be too dense to use in a 14 fret OM style guitar using falcate bracing? I don't have a predicted thickness for the top yet as I haven't input all the equations into Excel yet! Thanks again.

gerryt
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Re: My first frequency spectrum

Post by gerryt » Mon Jan 14, 2019 10:29 am

Sorry for the stream of posts - I would have edited one of them with more info but I don't seem to be able to.

Anyway...spent the evening inputting the rest of the equations to get me to to a target panel thickness and I've had a pretty terrifying result

Image

2.35mm for a top on a steel string??!! Surely I've done something wrong. Can such a thing be done and last? Also bearing in mind that I have a very understanding customer who wants me to push my comfort zone and build him a Gore/Gilet type falcate braced instrument - but this top cost me/him £200 - about $350 Australian. Maybe the most expensive top I've used on a build. Maybe I should talk him out of it and into using some of the bearclaw Englemann I have in my stash?

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Trevor Gore
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Re: My first frequency spectrum

Post by Trevor Gore » Mon Jan 14, 2019 9:34 pm

Your density and elastic moduli are OK; your thickness calc is wrong. Should be 0.02857 not 0.2857 as the first constant in the denominator. There may be other errors, but that's the one I can see. Correct answer is 2.71mm (on my workings!) so you will know when you're right. Also use the data in Table 4.5-3 to check your calcs.

Regarding your VA/mic problems, I have an HP with DTS Audio Control Panel and a "mainframe" with Realtek HD Audio Manager. Both have a mic boost option; the Realtek up to 30dB boost.

On my system I plug a mic in and Realtek opens. On a laptop, you will likely have to open Realtek manually. With the mic plugged in, I have a tab called Microphone Effects and at the bottom is a mic slider and the boost option. I'm using Windows 7, but at this level there is likely not much difference between Win 7 and Win 10.

There is a very strong likelihood that if you can find the mic boost, you won't need the preamp. All the computers I've had recently have had a mic boost. If it's not in Realtek, hit the speaker icon in your tool tray (or Win 10 equivalent) "wrong" click on it and select Recording Devices. (You'll need to have the mic plugged in). A dialogue box opens; select the recording tab and click on "External Mic" then click on Properties. Then in the new box click on the Levels tab and you may find a mic boost there. Don't put too much boost on or you will get another set of problems. I've always been able to use a dynamic mic straight into a laptop, no preamps etc.

Good luck!

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kiwigeo
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Re: My first frequency spectrum

Post by kiwigeo » Mon Jan 14, 2019 10:03 pm

I ran your figures through my spreadsheet and target thickness comes out the same as Trevors calculations at 2.7mm
Martin

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Re: My first frequency spectrum

Post by gerryt » Mon Jan 14, 2019 10:36 pm

Now you see why I didn't become a mathematician! I added the missing zero and target thickness is now predicted to be 2.71mm. That's a somewhat less scary figure. Actually pretty glad as this top rings like a bell when tapped for the long grain and twisting frequency. In cross grain mode it sustains a lot less but logically that makes sense to me as I'm tapping it only an inch or two from where I'm holding it.

Oh and Trevor I seem to be able to get a mic boost slider for the actual mic input to the laptop (via the headphone jack?). No slider when I'm using a mic straight into a USB port. Happy enough with the preamp. Does the job for me but I guess it's an additional expense I could have done without.

Thanks for the help guys - very much appreciated.

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