SteveL123 wrote: ↑
Mon Sep 11, 2017 2:05 pm
Martin Woodhouse wrote: ↑
Mon Sep 11, 2017 12:10 pm
You can also add a shallow tornavoz to the main soundhole to lower the 'air resonance' frequency - I did a simple experiment to test that a few years ago, and found that, on average (though different guitars respond differently), 2.5mm of tornavoz depth will lower the air resonance by about 1Hz, so you can use that as a rule-of-thumb to tune the resonance to whatever frequency you want (within reason).
What material, how did you make it and how did you attach it to the soundhole?
For the experiment, I just used a cardboard tornavoz held in place with masking tape, which was good enough for the test, but not great for a real guitar...
There are lots of ways you could do it on a new guitar, but my method is to glue a 4mm deep and 2mm wide laminated wooden ring around the inside of the soundhole (at the same time as gluing braces onto the top). Then, after the box is assembled, measure the air resonance frequency, work out how much I want to lower it by, and glue a black paper ring of the correct depth onto the wooden ring inside the soundhole... hopefully that description makes sense.
So, it's a two part tornavoz made from wood and paper. The wooden part would be glued with whatever glue you use for braces, and I use CA glue to stick the paper part on. The advantages of this design are that the paper part can easily be removed if you need to get inside the guitar for repairs, it allows for precise tuning of the air resonance frequency after the soundbox is assembled, and it doesn't add much weight or stiffness to the soundboard, so the higher soundboard modes are mostly unaffected.