A a guitar builder and teacher of the art of Lutherie I am often asked what makes acoustic guitars sound so different from each other and why is it so hard to find a good sounding acoustic? It is not at all uncommon to pick up two of the same guitar models with the same everything and hear two very different sounds, and then when you finally find "the one" you reach in your pocket and come up empty.....if you are lucky it will still be in the store when you have the cash but the odds are not in your favor. Why is it that one can go into a music store, play a few guitars and leave wondering why some of the cheaper guitars played and sounded better than some of the really expensive Martin or Taylor or Gibson guitars. Even in the Luthier community I have observed some seeking good sound as if they are on some kind of elusive cosmic mystery egg hunt. In the real world, sound is the result of many things so let me see if I can break this down in a comprehensive manner.
Acoustic Guitar Wood
If you haven't read my article on Choosing The Right Guitar Wood then please read it now. Wood choice is where it all begins. This process should be done with a seasoned Luthier or entrusted to one. If your wood choice is poorly done then you may want to go back to the "Cosmic Egg Hunt" method, find something cheap and save your money.
Sine 80-90% of the sound of an acoustic guitar comes from the top plate then it would make sense to carefully choose the right one. Having cut thousands of guitar sets I feel confident in saying that guitar tops are like snowflakes......no two are the same. I can cut several book matched sets from the same billet and if I am lucky one or two of them will have the tap tone that falls within the range of premium sound that I seek. From there I look at the strength of the chosen book matched set, as both of these factors will impact exactly how thick I will sand the top plate. The thickness of the top plate should be individual to the top within certain standards. Most major Manufacturers can not afford this luxury as production is the name of the game. As a result the consumer pays thousands for the next top on the shelf (that has been sanded to a standard thickness) with little consideration for sound. This is the equivalent of putting on a blindfold and then just grabbing any speaker off the shelf to hook up to your stereo........the odds of it sounding great are going to be determined by the quality of the speaker (and system) and how well it matches the rest of the system. Guitars are similar, the top has to be a quality top and a good match for the rest of the instrument. This is why (over the past decade plus) I have cherry picked and stashed hundreds of the best guitar tops to be found anywhere.
Glue vs. Sound
Glue is no friend to sound. Poorly fitted glue joints will impact sound and the more there are the worse it will sound. When wood is glued to wood then the joint should have absolutely no gaps what-so-ever and very little glue should be required. Poorly glued joints can make the best guitar top in the world sound poor and will also weaken the joint. As for sound....I think every seasoned builder has certain glues that he or she likes to build with, some will claim that certain glues have better sound properties then others. This may be true, but the impact of glue in a perfectly fitted joint is going to be very minimal as the glue will be only thin enough to soak into the wood fibers creating almost an interlocking effect. This allows the sound to travel throughout the guitar body better. When you looks inside the sound hole of a guitar you should see few to no signs of glue.
The two neck joints that are common are the dovetail joint and bolt on necks. Those who do dovetail joints feel that it is a critical part of the sound puzzle and those who prefer bolt on necks like the versatility of being able to make easier adjustments to the angle of the neck, and also that there is little to no glue often used in the bolt on system. Ultimately the objective should be just like any other joint on the guitar.....as much wood as possible making contact at the joint with as little glue as necessary. Personally, I have developed my own system that allows for the best of both worlds as my guitar necks bolt on with no air gaps where the neck meets the guitar body. Along the same lines, it is critical to sound that the nut and the saddle are both perfectly seated at all points or sound will be negatively impacted.
Back and Sides
The back and sides of a guitar impact the sound in more of an indirect way and should be a match tonally to the top plate set. The back and side set has more of a tendency to color the sound of the top (much like a sub woofer) rather than be the main speaker. For instance, Mahogany will tend to give the guitar a warmer tone while Rosewoods lean toward the brighter side. From there it simply comes down to looks and building practices.
Good acoustic guitar sound is not the mystery that many believe it to be. It is the result of proper wood choice, precision building, understanding how sound travels and years of experience. Over time I have seen many would be guitar builders try and get around these principles (especially wood choice) only to spend 100 hours building a piece of furniture. These principles are the foundation to building a premium instrument. As a side note; If you have a guitar that just isn't putting out the sound you want then it may be a good idea to have a Luthier tone shape the guitar.