Sound Ports?

Construction and repair of Classical Guitar and related instruments
SteveL123
Posts: 423
Joined: Thu Feb 09, 2017 5:05 pm

Re: Sound Ports?

Post by SteveL123 » 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?

Martin Woodhouse
Posts: 8
Joined: Sat Aug 12, 2017 6:05 pm
Location: Cambridge, UK

Re: Sound Ports?

Post by Martin Woodhouse » Mon Sep 11, 2017 2:33 pm

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.

Alan Carruth
Luthier
Posts: 2662
Joined: Mon Dec 24, 2007 6:56 pm

Re: Sound Ports?

Post by Alan Carruth » Mon Sep 11, 2017 5:21 pm

One elegant solution I saw for adding a 'pipe' onto the sound hole involved using cellophane. It's even more invisible than black paper.

UKsteve
Posts: 610
Joined: Fri Jan 04, 2013 2:48 pm
Location: St Albans, UK

Re: Sound Ports?

Post by UKsteve » Tue Sep 12, 2017 4:16 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
My Philip Woodfield GC3 has a very shallow tornavoz.

No idea of its specific contribution but it's the guitar I'd grab from many if the house was burning down...

SteveL123
Posts: 423
Joined: Thu Feb 09, 2017 5:05 pm

Re: Sound Ports?

Post by SteveL123 » Tue Sep 12, 2017 5:18 pm

Martin Woodhouse wrote:
Mon Sep 11, 2017 2:33 pm
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.
Thanks for the info. My guitar has an air resonance of A2. When I have a chance, I would like to experiment with a tornavoz and see how it changes the tone.

SteveL123
Posts: 423
Joined: Thu Feb 09, 2017 5:05 pm

Re: Sound Ports?

Post by SteveL123 » Tue Sep 12, 2017 5:21 pm

Alan Carruth wrote:
Mon Sep 11, 2017 5:21 pm
One elegant solution I saw for adding a 'pipe' onto the sound hole involved using cellophane. It's even more invisible than black paper.
More details please! What thickness cellophane and how to attach etc. for a finished guitar.

Alan Carruth
Luthier
Posts: 2662
Joined: Mon Dec 24, 2007 6:56 pm

Re: Sound Ports?

Post by Alan Carruth » Tue Sep 12, 2017 5:45 pm

A tornavoz alters the air flow. The forces involved are not very large, so it doesn't take much of a sleeve to do the job. It can't be too floppy, so food wrap won't do, but it doesn't have to be metal or carbon fiber either, to work to lower the 'air' mode pitch. Nor is it something that has to be got right the first time. Try something, and see what happens. If it doesn't do what you want, try something else. I'd be willing to bet that's how most of the 'advances' in guitar design have come about.

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