T(1,1,)3 Double Peak

You can ask questions here about Trevor and Gerard's exciting new book on Luthiery.

Moderators: kiwigeo, Jeremy D

Post Reply
AKEric
Myrtle
Posts: 50
Joined: Fri Sep 28, 2018 1:29 pm

T(1,1,)3 Double Peak

Post by AKEric » Tue May 18, 2021 6:11 am

Hello all,

I would have thought after two successful builds using falcate bracing and spectral analysis, I'd have a better understanding of it all. This plot has me confused though. The T(1,1,)3 has a double peak at ~218hz and ~230hz. This is consistent after many plots, and the amplitude of both peaks is often similar. Do I use the low frequency, the high frequency, or average the two?

I also had, (from my limited experience) a larger frequency drop of T(1,1,)3 from one construction point to another. After the box was finished with binding it was at 230hz. After lacquer it dropped to 226hz. And with bridge, pins and saddle (25g) it went to the plot you see here, ~218. This is a larger drop then I saw in my two other guitars. All are the same OM size.

The T(1,1,)1 & T(1,1,)2 ended up place pretty well at 99hz and 183hz, but the desired four semi-tone separation between T(1,1,)2 & T(1,1,)3 is not there if I use the 218hz. I'm considering gluing on a scab to the backs main transverse brace and reshaping it to stiffen up the back a bit. I've done Monopole testing, but I think I need to rework my spreadsheet because the results didn't seem within reason. The deflection of the back was .22 with 1000g and a freq. of 207hz with the sound hole plugged.

Does anyone have a suggestion as to the cause of the double peak, which one might be most accurate, and what the sonic ramifications will be? Of course, I will string it up soon and find out, but I'd rather not have them on if I have to get into the box to work on the back brace.

Thanks for any help.

Eric

Eric
05-17-2021.png

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

Re: T(1,1,)3 Double Peak

Post by johnparchem » Tue May 18, 2021 9:33 am

One of the double poles may be another top mode like the cross dipole. The first of the double is probably your back.

KreherGuitars
Beefwood
Posts: 26
Joined: Thu Feb 20, 2020 3:33 pm

Re: T(1,1,)3 Double Peak

Post by KreherGuitars » Thu May 20, 2021 3:55 pm

If you look ten topics down I posted the same problem. I had a mode with two peaks. I use chladni patterns and I only get a mode that is somewhere in between the split peak. Maybe we need to use more conditioner on our guitars to repair split ends? Or maybe there’s some better advice. I’m sure Trevor has seen something like this before.

Semmens Guitars
Sassafras
Posts: 46
Joined: Thu Jul 05, 2012 11:14 pm

Re: T(1,1,)3 Double Peak

Post by Semmens Guitars » Fri May 21, 2021 9:04 am

I wouldn't worry about it until you string the guitar up and hear how it sounds. I always tap test as I go as a guide to what's going on but only at the end after strings are on and I have listened to the guitar do I think about fine tuning frequencies only if there is a problem.

AKEric
Myrtle
Posts: 50
Joined: Fri Sep 28, 2018 1:29 pm

Re: T(1,1,)3 Double Peak

Post by AKEric » Sat May 22, 2021 2:48 am

Thanks for the input guys,

Sorry I missed your post on "spit peaks", I guess I forgot about reading it. I searched "double peaks" before posting this and didn't find what I was looking for. I'm still lacking a chladni set-up, next on the long list of things to do.

Anyway, after stringing it up and running through the analyzer I was amazed to see the double peak disappear completely. One solid peak at 218hz. Most importantly, the guitar sounds good, one of my best yet. My spread between the Top's Freq. and the Coupled freq. ended up at 39hz. a bit shy of recommended, but pretty close.

One lesson learned: I overly shaped the back's main transverse brace too early in the process. I did it just after the back went onto the rim (no top yet) because the back seemed stiff and had a low monopole. I'll wait until the end to get more aggressive (as needed) with that brace in the future.

Thanks again, Trevor, for sharing your work with the world.

Eric

Post Reply

Who is online

Users browsing this forum: No registered users and 29 guests