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I recently read that a violin soundhole should not be too thin edged or tapered as that will reduce the resonance.I applied that idea to a classical guitar and it ran away with me.I made a bigger rim of 11mm ---.Better!   Then I went to 18mm ---Better again.All with thin plastic and card.Next stop 65mm .Really good sound.Better high and low notes.
      This is all attached with masking tape and it really works.I saw that Domingo Esteso made a guitar with air resonance about 75Hz.That`s D below bottom E.It works like a dream. One "failure" with a full bodied Spanish guitar seemed dull.Then I realised you cannot just measure and fit.You need to work out what the air resonance note will be.I trimmed off one centimetre of card at the bottom and Bingo it was spot on with really warm resonance on all strings.You can fit it from the outside as a small reduction of hole width will not reduce the air resonance nearly as much as the depth of the tornavoz.
  The Tornavoz will reduce air resonance frequency,give great deep bass notes and makes the high notes very sweet.Once the sizes and sounds are worked out the card or plastic can be replaced with thin balsa wood.Maintenance is not a problem as balsa wood is easily replaced.That`s usually the main worry with fitting them.  Well worth a try.
     The air in the "tube " acts like the plug of air in the neck of a bottle.       That gives it the ------------------Resonance.))))))))))))))))))))))).

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There is a site for this topic.Hago guitars. Look on FAQs under contrabass " sleeve".There are some photos and a bit of technical explanation.
Paul Major progress to report. I have a Mexican Requinto guitar with a deep tubby body.I fitted a card "tube" in the sound hole same as before.It was nothing special to listen to so I repeated the shortening proces by cutting off one centimetre from the bottom edge. Suddenly I got a very sweet sound.Very pure musical tones.I carried on with the experiment and removed another centimetre.Back to a really ordinary sound.By leaving the "tube "where it was I could use that as a guide for a sliding tube which could be adjusted up or down and held by two pieces of tape.
By sliding the new internal tube down into the "good " spot I was able to find the ---------magical tones---------- I heard before. (Just to emphasise the effect it has ) That clearly demonstrates that the "tube " can act just like a sound post does in a violin.An ordinary sounding guitar can be brought on line to make some good music. Use this method and your ears rather than worrying about exact body resonance frequencies. That`s all old hat after this test.
There could well be other positions to get even better tones than I have found. I have only made large movements so there is a lot of room for fine tuning.
Many new makers will not know what the final body resonance will be.It is very hard to know exactly what notes the top and back will produce and there is no mathematical formula to combine all these features together so you are down to judging the final result with your ears. In that situation a "tube tone tuner " is a get out of jail free card.
I have started to use the word tube to avoid scaring people away.
I just need to add that the current depth of tube is 6 cms. ( In this particular guitar ! ) 5 cms and 7 cms did not work. Each guitar will have it`s own individual measurements.
Also the character of the sound for each instrument is enhanced.There is no alteration to the character itself . Just a large dollop of beauty in the sound. You will have to beat the women off with a stick when they hear this.
I forgot to mention another bonus with the tube fitted.The harmonic notes are very clear and pure now.They were nothing like this without the tube.I always thought it was because it was a Requinto. There`s a good objective thing for you to compare.What do the harmonics sound like?
Paul You had me going there .I looked up Joshua Alexander .But you put french with a small f.French is the Spanish maker`s surname. He deserves some credit here.
I have heard of a shape that turns towards the bottom of the guitar.That might confuse some people.The straight "tube" shape is the one to concentrate on. The most important point is that it is Tuneable". If you change the distance from the back the resonance changes.I think a graph would be very useful here.If you want a resonance round F sharp or G you might need a smaller body volume.Something to work on ,I think.
Latest report on my tube experiment.The sliding adjustable inner tube has been very simple to use and very effective.I started with a guess in each case, round about 7 cms deep Then I played for a while with the same pieces.Made a note on tone quality and then moved the tube in one centimetre jumps after each test ,all the while keeping a written record.It was easy to hear the difference and easy to guess which direction I should try. When I got within one centimetre I reduced the size of the jumps. It settled on the best tone.Good harmonics and very responsive when playing softly.The final position was not the same for each guitar but the best position was right down to a millimetre.You could hear the difference if you moved away from the sweet spot. So I wrote each result on the card tubes. This really works and it is the cheapest job you will ever do on a guitar.
YOU MENTION CARD TUBS WHAT MATERIAL ARE YOU USING?
Tubes Paul. When you buy this it`s flat card.About 0.2 mms thick.Same as a cardboard box you buy your toothpaste in. You can get it anywhere. A cornflake packet will do the same thing.It has no resonant properties itself.That`s not it`s job. It`s good to hear the sudden improvement when the right spot is found.Very like tuning in a radio.
Latest rough and ready measurements. Full size guitar weighs in at round 8cms deep. Smaller boxes eg Requinto or Thin body guitars work best around 6 cms deep.
SO John how are you attaching this card to the inside of the sound hole how can i experiment with this my self ?
Haven`t you done it yet???? Here`s how. It`s easier than people think.
Get some thin card.Make a strip 5cms wide and long enough to go round the soundhole with a 2 cm overlap.Draw a line along the strip one cm from the edge. Cut into the pencil line every centimetre all along the line.( cut on the short side of the line ) . Bend the cut bits at right angles .Now when you curve that round the hole you can tape the bent bits to hold it in place.Tape the bent bits to the top surface. Put some tape on the join inside the hole.The second "sleeve" is just a tube , about 6 cms deep ,that will fit snugly inside the first one.Fix two pieces of tape ,one each side ,to control the depth.Bend those two tape "controllers" onto the top when you try a new position.The sticky side should be covered so only the bit above the hole will grip.The depth changes can then be made without moving the strings out of the way each time you make a change.When you move the inner tube DOWN ( excuse capitals ) use a piece of plastic just to nudge the tube down.I used a long plastic rawlplug.No chance of damaging the strings.
The tape is used to RAISE the tube as you search for the sweet spot. ( excuse capitals once again ) You only have to let the strings down once for fitting the tubes.Don`t try squeezing the tubes in between the strings as they will get out all of shape.Try to get a nice even round shape as it works better like that.
Measure each change in depth from the top surface only.( don`t measure from the back ) . For a full size guitar it will probably be near 8 cms. Only you can tell the exact position by playing and listening.(Give each position a 5 minute play time and check the quality of the harmonic notes when you do that ). Come back and say how you got on.
For us Americans I have converted the dims to inches which are approximate. I am hoping some of you will experiment with this as it sounds very interesting.

Get some thin card.Make a strip 2" wide and long enough to go round the soundhole with a .75" overlap.Draw a line along the strip .39" from the edge. Cut into the pencil line every .39" all along the line.( cut on the short side of the line ) . Bend the cut bits at right angles .Now when you curve that round the hole you can tape the bent bits to hold it in place.Tape the bent bits to the top surface. Put some tape on the join inside the hole.The second "sleeve" is just a tube , about 2.35" deep ,that will fit snugly inside the first one.Fix two pieces of tape ,one each side ,to control the depth.Bend those two tape "controllers" onto the top when you try a new position.The sticky side should be covered so only the bit above the hole will grip.The depth changes can then be made without moving the strings out of the way each time you make a change.When you move the inner tube DOWN ( excuse capitals ) use a piece of plastic just to nudge the tube down.I used a long plastic rawlplug.No chance of damaging the strings.
The tape is used to RAISE the tube as you search for the sweet spot. ( excuse capitals once again ) You only have to let the strings down once for fitting the tubes.Don`t try squeezing the tubes in between the strings as they will get out all of shape.Try to get a nice even round shape as it works better like that.
Measure each change in depth from the top surface only.( don`t measure from the back ) . For a full size guitar it will probably be near 3+". Only you can tell the exact position by playing and listening.(Give each position a 5 minute play time and check the quality of the harmonic notes when you do that ). Come back and say how you got on.
John,
Your use of caps for emphasis is appropriate. They only become a problem when EVERYTHING is written in caps. Not only is it hard to read, it is also forum convention that emphasizing every word is equal to yelling at the reader.

Ned

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