Adjusting T(1,1)1

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Kevin Manuele
Wandoo
Posts: 11
Joined: Tue Sep 18, 2018 4:38 am

Adjusting T(1,1)1

Post by Kevin Manuele » Sun Jun 06, 2021 3:55 am

I am doing final testing on an OM-style guitar. X-braced top, ladder braced back.
Back braces are 10x20mm, trapezoidal. That is, non-live.
I am seeing T(1,1)1 at 104 Hz, and T(1,1)2 at 190 Hz. No T(1,1)3 that I can see.
I would like to move that 104 Hz away from G#. Sound hole diameter could be increased by 2mm, max.
But I am wondering about shaving (scalloping) the two lower back braces, maybe getting some coupling and lowering T(1,1)1.
Am willing to do some experimenting here.
Any comments appreciated.

thanks
Kevin

johnparchem
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Re: Adjusting T(1,1)1

Post by johnparchem » Sun Jun 06, 2021 4:14 am

Where are you in the build? My advise is only relevant if the numbers you listed are on a strung up guitar.

My first thought is that unless you are chasing a wolf note on G#, you are in the range to just leave it as it is. With regard to the back braces you would have to really lower them before it couples. It sounds like you built it not to couple. So no back peak is good. Also dropping T(1,1)3 enough to to lower the T(1,1)1 would drop the T(1,1)2 more than the T(1,1)1. So you would lose that nice 190 peak. Keep in mind that lowering T(1,1)3 so that it couples will lower T(1,1)1 so you would need to lower T(1,1)1 through G#.

If you do need to change it the 2mm will move you a bit more away maybe 105 - 106 Hz without changing T(1,1)2 much.

Kevin Manuele
Wandoo
Posts: 11
Joined: Tue Sep 18, 2018 4:38 am

Re: Adjusting T(1,1)1

Post by Kevin Manuele » Mon Jun 07, 2021 3:43 am

Ok, and thanks for the comments.

The build is complete. Numbers are with strings installed, and guitar stabilized for a week or so and played.
No wolf tone at G# that I can hear. Moving T(1,1)1 would be for academic/learning purposes -- what happens if?

With strings removed (pins and saddle in place). T(1,1)2 rises to 193 from 190. Other values unchanged. I would be tweaking/testing without strings initially.

My intent would be to lower T(1,1)1 a few Hz to around 101 Hz -- half way between G# and G. Maybe a concern would be moving T(1,1)2 to too close to 185 Hz (F#) ?

Thanks again.
Kevin

johnparchem
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Posts: 546
Joined: Mon Jan 21, 2013 2:59 am
Location: Seattle
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Re: Adjusting T(1,1)1

Post by johnparchem » Mon Jun 07, 2021 4:07 am

Kevin Manuele wrote:
Mon Jun 07, 2021 3:43 am
... Maybe a concern would be moving T(1,1)2 to too close to 185 Hz (F#) ? ...
If you use the back to move T(1,1)1 you certainly will move the top down. If you wanted to experiment changing the size of the sound hole may be your best bet. If you want to reduce T(1,1)1 you need a smaller sound hole. But you may need as much as 4 mm difference, increasing the sound hole diameter will move T(1,1)1 further from the G# so all the change you make is good.

You can experiment to see what would happen when lowering T(1,1)1 by covering part of the sound hole and watch what it does to T(1,1)1 and T(1,1)2

Kevin Manuele
Wandoo
Posts: 11
Joined: Tue Sep 18, 2018 4:38 am

Re: Adjusting T(1,1)1

Post by Kevin Manuele » Sat Jun 12, 2021 3:39 am

I went ahead and shaved (scalloped) the lower two back braces in the center. Watching the T(1,1)1 as I progressed. As you suggested, it required removing a lot of material (they were 20 mm height), before I saw some movement.
Eventually, T(1,1)1 moved down to 101 Hz, half-way between G and G#.
T(1,1)2 barely moved, from 190 to 189. I expected more.
Before and after spectrum plots attached. I lost a bit of response around 550 Hz.
Attachments
T(1,1)1 adjust.png

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