Main Air is at 110 Hz, aka the A note

[klooker]

Looking at the attached plot, you can see that my main air response is right on 110 Hz, or the A note if you prefer.

This is an X-braced guitar.

If I add side mass my main top frequency will drop from 210 but I need to get it below below 207.65 Hz so that it doesn't land on G#. If I attempt that, and add mass, will it have any effect on getting the main air off 110 Hz?

Next question, anyone tried adding side mass loading blocks to a finished build? Asking for a friend.

Thanks,
Kevin Looker

[Trevor Gore]

First, I need a little more information:

1) Is this guitar complete and if not what stage of build?

2) Do you know what mode the 239Hz peak is?

[klooker]

The guitar is complete.

I was attempting a live back, I assumed that 239 was the back.

I have a frequency generator, speaker, etc. so I can test it.

Thanks

[klooker]

Did another round of taps, results are a little different.

230 Hz is the back. I don't know what 241 is, I couldn't get any visible pattern at that frequency.
Here's the back at 230 Hz.

BackMode.JPG (121.06 KiB) Viewed 10308 times

Kevin Looker

[Trevor Gore]

Kevin,

Adding side mass to reduce the frequency of the T(1,1)2 mode has very little effect on on the T(1,1)1. Check out Fig. 2.4-2 in the black book.

Shaving the back braces can reduce the T(1, 1)1 (and the T(1,1)3 of course), but you don't want to reduce the T(1,1)3 any lower than it is now. The "anonymous" 241 Hz resonance is likely a shorted out air mode (the T(1,1)1 but with the upper bout air volume "missing", so a Helmholtz mode minus the upper bout air volume).

To me, both the T(1,1)1 and the T(1,1) 2 are high and imply the guitar may not be optimally responsive. Is the open A actually giving a problem? If it isn't, don't worry about it. Assuming it is, and also assuming there isn't a sound port, dropping the T(1,1)2 (top bracing shaving) is something to consider, as that will also reduce the T(1,1)1. If, for whatever reason, you don't want to do that, reducing the sound hole diameter is an option. In the attached pic a piece of rosewood binding has been inserted in the soundhole. It is barely noticeable, but reduces the T(1,1)1 frequency by ~3 Hz.

P1010394s.jpg (138.53 KiB) Viewed 10284 times

I have glued in side mass supports for a customer as a retro fit. I did it using epoxy and a lot of magnets, which gets everything lined up and holds the mass support block in position whilst the epoxy cures.

[klooker]

Update:
I did some brace shaving and sanded the X.

Here's where things went:

FigSapeleOM-Nov22-2018.jpg (60.87 KiB) Viewed 10200 times

T(1,1)1 is off the A note and the other modes dropped too.

It looks like I could add a realistic amount of side mass to drop T(1,1)2 by 5 Hz which would get me 2 semi-tones of separation, not the ideal 3+ but better than where I am now.

Kevin Looker

[Trevor Gore]

Heading in the right direction!

Try the side mass and see how it sounds. If you need to stiffen up the back, there are ways of doing that. Let us know if you want to do that and I'll explain a method how.