Good new and something to scratch my head about

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DarwinStrings
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Good new and something to scratch my head about

Post by DarwinStrings » Tue Mar 12, 2013 5:08 pm

It was time to have a bash, so I learned how to use Excel(with my 15 YO sons help) as well as VA. Then I got my fl,fc,flc,h,Ll,Lc and density out to come up with a El, Glc and a Ec using Equ. 4.5-2,3 and 4(it turns out they are numbers). I then took those numbers and the other numbers and showed them to Equ. 4.5-7. As it turns out Equ 4.5-7 seemed to like these numbers and told me my spruce should be 2.45mm thick for my a and b as well as using Trevor's Vibrational Stiffness Value of 75.0. I was very happy to see a figure that looked right and hope it is right.

Before I started this though I thought I should check if the correct order of doing equations was right in my head. I saw that Trevor had simplified Equ. 4.5-1 to Equ. 4.5-2 so I figure I should do the first part of Equ. 4.5-1 to see if I get the "0.94625" that the first part of Equ. 4.5-1 simplifies to in Equ 4.5-2. I did quite a bit of muttering under my breath but even with that I could not get the same answer as Trevor did so I gave up and moved on. I can only assume I was doing something wrong and if anyone would like to show me how Trevor gets "0.94625" in Equ. 4.5-2 it would be a load off my mind. I guess I don't really need to know for sure as I seem to have a reasonable target thickness for my Sitka but it does have me buggered.

I used my open fingered knuckle to tap and a Shure PG58 vocal mic but will borrow a good quality condenser as well as make a mallet and do the whole thing again to see if it changes

Jim
Life is good when you are amongst the wood.
Jim Schofield

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Tod Gilding
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Re: Good new and something to scratch my head about

Post by Tod Gilding » Tue Mar 12, 2013 7:50 pm

But did you ensure that the PG58 had a frequency response between 60 and 15,000 Hz and took into account the Cardioid polar pattern. :D
Tod



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Trevor Gore
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Re: Good new and something to scratch my head about

Post by Trevor Gore » Tue Mar 12, 2013 8:42 pm

Well done, Jim.

Before you scratch a hole in your head, read this which should answer your questions. Bonus points to you for being thorough!

The little rubber mallet is a good idea, but no need for expensive mics (unless you need it for something else). My PG57 works fine, but a PG58 might have a bit less bottom end, but otherwise should be fine (and you won't need the phantom power).

To check your spreadsheet, see if you get the same answers (or very near) as those in Table 4.5-3.

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DarwinStrings
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Re: Good new and something to scratch my head about

Post by DarwinStrings » Wed Mar 13, 2013 2:08 pm

Cheers Trevor and I thank the powers that be for typos. Glad I decided to move on though and not let it stump me. I will take that advice and check my spreadsheet, made a rubber mallet too.

Jim
Life is good when you are amongst the wood.
Jim Schofield

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DarwinStrings
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Re: Good new and something to scratch my head about

Post by DarwinStrings » Sat Mar 16, 2013 2:54 pm

Just to let you know, I checked my spreadsheet and using your first Engelmann example and came up with 2.72mm which is not exactly 2.73mm but looks close enough from what I have gathered reading the threads on the subject. I can't remember ever getting this much pleasure out of maths and never imagined there were so many numbers hidden beneath the surface of my bits of wood.

Jim

life is good when you are amongst the wood
Life is good when you are amongst the wood.
Jim Schofield

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Trevor Gore
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Re: Good new and something to scratch my head about

Post by Trevor Gore » Sun Mar 17, 2013 10:20 am

Well done, Jim. I'm pleased you're enjoying the meths!

(Don't you love those typo's!)

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DarwinStrings
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Re: Good new and something to scratch my head about

Post by DarwinStrings » Sun Mar 17, 2013 12:05 pm

Ha! Now that is ironic typoing. My spread sheet being my first was just a mess of numbers placed higgle de piggle de all over the shop and today I was going to sit down then tidy it up adding labels and using the full equations rather than breaking them up into bits so that when I come back to it I will know what it is all about. After waking up though this morning, I won't be looking at any "meths" as last night I enjoyed a beer lubricated jam with a mate till about 2am (poor neighbours) and am now feeling the opposite of the pleasure I felt last night so I will leave the maths till I can focus my eyes on a spread sheet again.

Jim
Life is good when you are amongst the wood.
Jim Schofield

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