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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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The first Wide mouth selmer guitars were made with a resonator like the one you're talking about. The physics behind this is described by Helmotz resonator formula :
freq=sound_speed/(2*pi)*square_root(soundhole_area/(guitar_inner_volume*resonator_lenght))
Pierre-Antoine Thanks for a useful formula. It helps to read the basic Helmholtz resonance information to get a handle on this.The weakness in using the formula is that the volume of the guitar body will be an obstacle .The method of doing small adjustments with card will make this more practical.I`m picturing small strips attached to the bottom edge that can be peeled away while all the strings are in place.I was surprised to see my favourite maker (Esteso ) made a guitar with such a low resonance.I don`t know if he had a resonator fitted. Is it OK to use the word Resonator in place of Tornavoz. Tornavoz makes it sound as if it was made of Brass. The method is supposed to work better if the surface is "rough" (ie not smooth or polished).It spreads the resonance over a wider range of notes.I got these tips from the UNSW researcher John McLennan.He was referring to violins and I have slightly adapted his ideas.(without cheating). My next project is to see how close I can get to using the same principle on a violin.What shape would a resonator /(s) be with the f -holes.This is my latest conundrum. Once the air in a violin or guitar is set vibrating the behaviour will be very similar.I would try to avoid restricting the belly and especially the wings.Would a narrow resonator do the job as well.Very many harsh instruments would benefit from some experiments.Start at the bottom end of the market and work up ??
I shall add on any new ideas I get here. What I wanted to know is ------Could I fit a pair of resonators in a violin? The idea of gluing to a violin belly would restrict it.So I tested the guitar to see if a narrow slit near the top of the resonator would mess up the way it works.I took out the plastic one I had fitted in the guitar with masking tape and made 3 long slits near the edge.-----It still worked when I replaced it. It is held by 3 pieces of masking tape now. That means I need very little contact with the violin resonators .Three contacts is enough for each one. I just need to pick my spots carefully.
To "compute" the inner volume of an instrument, you can pour water in it until it's full an then measure the water volume... just joking, ok.
What's interesting in knowing the formula (if you don't want to compute volumes, etc...) is that :
- resonator lenght*4 => freq/2
- soundhole area *4 => freq*2
With that, for a given instrument you can tune more easily.

The result is more a matter of taste. You can get somme boosted frequency with that method, but not everyone like this.
One reason I wanted to bring up this subject is to clear away some of the mystery about what a tornavoz does to the sound.It is so easy to test in practice that many players might enjoy the sound. On my 2 guitars I have low tension Savarez strings . The volume and the "focussing "aspect do not really interest me nearly as much as the sound quality. I like the two formulae for working out the frequencies. That reminds me about violin bodies.----- 14 inches length is said to be the ideal body length.But the longest part of a violin is the angle from one curve of the top bout to the curve of the bottom bout. That "length " defines one of the modes I believe.
I wonder if Torres stopped making tornavozsss because they were just awkward to maintain.Why did he decide to make them from metal? It does not seem to be an obvious thing to do if you make guitars. How are you supposed to keep them shiny with all those strings in the way? You know how proud guitarists are about appearances. What would happen to a guitar with hot expanding metal fitted in it.
When I made the narrow slits near the top edge I feel that the high notes seemed a bit clearer.Something else to check out.
As it stands I enjoy the low chords in one Chopin waltz so much (Op 69-2) I find myself compelled to go back and hear them again .It`s very addictive. Previously the low notes were uninteresting and a bit dull.
Interesting. there's a guitar orchestra somewhere in the home counties that recommends using a simple plastic card tournavoz on baritone guitars (can't locate the website at the moment, but it might be of interest if you can find it). I've been drawn to the idea, but the practicalities of creating an easily removed but durable tournavoz have so far eluded me.
Regarding volume measurement, I believe I have heard that wheat, or some other dry grain is used. This seems a little less radical than water, and is probably easier to extract from a guitar than from a fiddle; my teacher recommended rice grains as a method of cleaning my fiddle. It was very successful but the last grain proved elusive and was reluctantly ejected two weeks later.

Pierre, assuming that the volume is measurable, how is the Helmholtz formula used? Is the first variable, sound speed, the actual speed of sound in m/s? If so, then would the remaining variable be in metres,square metres, cubic metres as appropriate?
Regards

Dave
Dave, the formula, as any mathematical relationship (not sure of this word...), has to be homogeneous ; for exemple :
speed in m/s
areas in m²
volume in m³
lenght in m

Or for your imperial units :
speed in in/s
areas in in²
volume in in³
lenght in in

Please everyone, look in physics books what's all about this formula, you'll get an exact idea of the impact (volume, frequency boost, EQ, etc..) you can achieve with this.
could you please add a photo i am reading more about the inventor and a Spanish guitar builder who do this today and i find it interesting so the cone shape fits around the inside of the sound hole running to the back as to close off the rest of the box am i understanding correctly?look up Joshua Alexander french guitar builder very interesting I could use a Better photo i am interested if you could please
Paul I think that cone idea is slightly different.The ones I have made are simple tube shapes.I have not made it conical at all.The easiest way to fit something is to check just inside the edge of the hole for a narrow flat rim big enough to glue a thin vertical rim.You can then use that to attach various lengths to suit the instrument. One odd thing about this tube fitting is if the tube takes up space why does the resonance frequency drop? You might expect it to rise.
Apart from letting the strings down a few times there is little chance of doing any damage to the guitar. Animal glue will keep it completely reversible.
JOHN DO YOU HAVE A PHOTO ?I am interested in trying this out for fun I am into building some acoustic guitars right now is a great time to experiment
No I haven`t got a photo but you only have to picture a close fitting tube in the soundhole.There is a sight with pictures of test tubes ( in pink ) the sound samples are quite bad on that .Don`t let that discourage you.
WELL I FOUND ONE VERY INFORMATIVE ONLINE THANKS FOR THE HELP I THINK I AM GONA TRY ONE CHEERS PHB

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