I made a fuzz on this board some time ago enthusiastically telling about the "amazing things" I've just discovered back then. Now time has passed and I have tried out the theories and methods on many guitars. Made a (loooong!) blog post with the state of my current truth, translated it to English. Here it is, comments are welcome :- )
I have worked a few years trying out and inventing new solutions to improve the tone in old parlor guitars and I've been quick to both spread the word and advertise them. Before I really have had enough experience and been able to understand what is happening. A big step for my understanding was a book called "Classical guitar design" by Guiseppe Cuzzucoli and Mario Garrone. It was both a challenge and a revelation to plow through the text which was not the easiest read! I'd like to make a little summary of the book and the special methods that I started to use restoring and improving old guitars.
An acoustic guitar is an extremely complex system. But it all starts with the strings. If you strike a chord on a guitar, you load up the strings with all the power and all the frequencies that propel the guitar and which transform into the sound you hear. You can call this package of energy and frequencies a "frequency recipe" as in the book. In the guitar, nothing will be added to this "frequency recipe", just attenuated or redistributed. At each transition between different materials the recipe is filtered, some frequencies are passed through with little energy loss, others bounce back like a ball against a wall.
The first filters that the strings come across are the nut and the saddle. On the other side of the filter, the "frequency recipe" is changed on its way to the next filter. The power is reduced overall and some frequencies are attenuated or redistributed. Here I stop and note that if you want to tweak the tone of your guitar you get the biggest effect in the choice of strings and material in the nut and saddle. The closer to the strings, the more important it is. That said, of course, the whole guitar makes the sounds. The bracing, top, bottom, and air volume in the guitar is the "engine" that converts a finely crafted "frequency recipe" from the strings into something you can hear.
My segmented saddle bone with spruce between posts of bone provides better string separation between the strings in a chord as the individual strings are allowed to move more freely than on a solid bone saddle that links the strings harder. A thick solid bone saddle also acts a bit like a compressor, at a certain limit the volume does not increase even if you strum harder. The segmented saddle provides greater dynamics in terms of volume.
My experiments showed that the tone changed if you had spruce or cedar wood between the bone posts. The important first filter in the saddle for the "frequency recipe" of the strings is different depending on the type of wood between the bone posts. Cedar gives the guitar a darker tone than spruce and dampens more trebles. After testing and listening to cedar and spruce in the segmented saddle on a guitar with a spruce top, I opted for spruce in the segmented saddle. Admittedly, it sounded good with cedar, but I thought something was missing. On the contrary, with spruce, I just got more of the guitar's natural sound. "Extra everything". Now I understand that the cedar filter changes the frequency recipe in a way that does not match the spruce top's own filter. The spruce in the saddle filters the "frequency recipe" in the same way as the spruce top and she "sound of spruce" is amplified. The same thing with my bridge plate in spruce as it also gives more of the spruce's natural tone, the filtering of the frequency recipe in a bridge plate made of maple is not in phase with a spruce top.
The saddle is just the first filter, then we have the transition to the bridge, to the top, to the bridge plate and then out to the whole guitar with many more filters. A kind of series connection. The original "frequency recipe" of the string changes at each transition and what disappears is lost for the parts after the filter. Thus, in order to strengthen the "natural" tone of a guitar, one should strive to have the same material in as many transitions as one can. In a guitar with cedar top, I would use cedar between the string posts and a cedar bridge plate. The joints and alignment between the parts of the guitar are also important. Good fit and a dense and thin adhesive joint provide filters that affect the "frequency recipe" less than a loose joint filled with a thick and tough adhesive. Hot hide glue is recommended, there is no glue that can provide such a strong and thin glue joint between two pieces of wood.
Wood of various kinds also has a built-in property that affects the tone, internal damping. It's a measure of how much energy is converted to heat inside the wood itself when vibrations of different frequencies pass. For example, rosewood has lower internal damping than ebony which makes rosewood ring more beautiful when tapped. In a guitar top hardwood is never a good thing unless needed like in the bridge or as a surface for the ball ends.
One phenomenon that is not talked about in the book or among guitar nerds in general is the importance of ensuring that both ends of the strings are firmly mounted and do not yield when strings are plucked. If the string "ducks" at the ends when plucked, the volume will decrease and the "frequency recipe" will loose some high frequencies. The top end of a string is almost always firmly mounted to a metal tuning post. At the other end, however, the strings ball end rest on a much softer surface. In my case, initially on a hardwood button inlaid in the spruce bridge plate to hold for the abrasion of the ball end, but then on 6-7 mm soft spruce in the bridge plate and top under a hardwood bridge and a hard bone saddle.
I thought the guitar with this configuration sounded a bit too nice and lacked attack. The soft spruce in the bridge plate and top is the Achilles heel and acts as a spring that dampens the transient when the string is plucked. The solution became what I call "turbo plugs", a wooden plug between the ball end/hardwood button and the underside of the bridge that replaces the soft spruce with harder wood. Strings that have fixed mountings in both ends respond better to the transient when the string is plucked with higher volume and greater clarity from the added trebles.
In parentheses I now use metal bushings on all string tuner posts in a slotted head (typical on old guitars). Normally they are only bored in the relatively soft wood of the head, but with metal against metal at the contact surface the attachment becomes harder and should make the string respond a bit better to the initial transient.
When I experimented with plugs, I tested without plugs, all plugs in hardwood birch and all plugs in endwood spruce. In one and the same guitar. Without plugs the guitar sounded very nice and had a soft and beautiful tone. But you could not be angry with the guitar! With birch plugs the volume became high and the attack very aggressive, the treble strings cut into my sensitive ears and the tone turned to ugly. With plugs in endwood spruce, which is about 3-4 times harder than spruce in other directions, the volume increased quite a bit, the attack became quite aggressive but retained the same beautiful tone as without plugs. The difference between the three cases was very big!
Later I realized that extra high treble and volume on the bass strings is a good thing and gives those strings a lovely piano sound and better definition, especially the thick E string. Nowadays I use birch plugs for the two bass strings, endwood fir for the D string (a bit harder than spruce), and endwood spruce on the three treble strings.
Everyone knows that a guitar with floating bridge sounds both loud and sharp. The reason for this is not the floating bridge but the fixed attachment of the strings ball end to the hard metal string holder. The problem and the reason why the tone is not beautiful in such a guitar is that the thinnest strings get too much treble and volume.
Finally, I will add that the effect of soft or hard attachment of the string ends is much greater for steel strings than for nylon strings. The nylon strings loses most of the transient force at the ends of the string as the string stretches all the way to the string ball end or tuning post a bit like a rubber band.
Have you read so far you know what is going on in my head right now as I think of new ways to improve the tone of my guitars. I have a bridge made almost solely of spruce that I have not tested yet. Maybe I should ...