Body size and shape. The Martin dreadnought body is so popular today that there almost seems to be a "dreadnought conspiracy" among acoustic guitar players. However, in my opinion the Martin D body size has certain deficiencies for playing styles that require a strong treble and maximum protection of the notes played at the higher frets. I think that this is due to its width, which is 4-7/8" on 14-fret models. In general, the deeper a guitar is, the more mellow and boomy it will be. Since the Martin dreadnought was designed for playing bass runs, the deep body works very well. But if a strong clear treble is desired, I don't think a guitar should be much deeper than four inches. Fortunately there are some excellent alternatives to the D size body, such as the Martin O, OO, OOO, and M sizes, which I'll describe briefly. These are convenient examples, but the reader should bear in mind that there are other good body sizes and shapes available. Some of these are produced by guitar companies, and others are original designs by independent makers.
The Martin O size guitars are 13-1/2" wide and 41/4" deep. They tend to have good treble and work well for fingerpicking. Occasionally, they're a bit weak in the bass, but the 12-fret models with slightly longer bodies have surprisingly good bass. While they're not as loud as the D's, the O's can still produce considerable volume.
The OO is 14-5/16" wide and 4-1/8" deep and tends to be a bit more balanced than the O. The 12-fret models in particular frequently have very strong bass for their size. These guitars are excellent for fingerpicking and can also work well for some types of flatpicking.
The OOO, which is 15" across and 4-1/8" deep (12-fret models are 4-1/16" deep) is one of the most versatile Martins ever made. This guitar cuts extraordinarily well and is excellent for flatpicking as well as fingerpicking. The bottom E string is a bit weak on some of them, but overall I think that the Martin OOOs--especially those built between 1934 and '38--come closer than anything to being the perfect all-purpose guitar.
The style M (which Martin introduced recently) is a flat-top version or the pre-War Martin style F, an arch-top f-hole guitar. The M has the same shape as the OOO, but is 16" wide. These guitars are also very versatile with strong treble, good bass, and considerable volume.
Wood. There are a number of factors that affect a guitar's tone: the most significant one is the instrument's top. Most steel-strings have tops of spruce; there are several varieties--Adirondack, Appalachian, Canadian, American Sitka, and German alpine spruce. Each has a different type of tone. Martin guitars made prior to 1946 have Appalachian spruce tops; those made afterwards have Sitka spruce tops. I have compared many Martins whose construction is similar except for the wood in the tops, and to my ear, at least, there was a significant difference. Those with Appalachian spruce tops had a crisper tone and better treble response than those with Sitka spruce tops. Martin continued to make a few guitars with Appalachian spruce tops on a random basis through the '50s and on into the '60s. These are easily recognized since Appalachian spruce ages to a yellow color, whereas Sitka spruce has a different grain and ages to a reddish brown or orange color. Guitars with German alpine spruce tops are relatively rare and are frequently made by independent craftsmen. In my opinion, German alpine spruce tends to be similar in tone to Appalachian spruce.
Most flat-top guitars have backs and sides of mahogany or various types of rosewood. The great classical guitar maker Torres once constructed a guitar with a back and sides of papier mache and a top of very fine spruce. This instrument had excellent tone quality, although it lacked the power and projection of an all-wood guitar.
While Torres's classic experiment demonstrated that good tone is primarily the result of the top, there's no doubt that a guitar derives some of its tonal character (as well as volume and power) from the back and sides. Rosewood and mahogany guitars certainly sound very different. Of course, tone quality is almost entirely subjective, but to my ear a hard, dense wood like Brazilian rosewood works best for rhythm guitar, while mahogany seems to work better for lead playing, where strong treble and maximum clarity are required. Due to trade restrictions, Martin and other large manufacturers have used Indian rather than Brazilian rosewood since the late '60s. However, most older rosewood guitars are of the Brazilian variety, and many people feel that it is prettier than the Indian; some prefer the sound also. Brazilian rosewood is still used today by some independent builders who make small numbers of guitars.
Braces and bracing patterns. Martins made prior to 1944 have scalloped braces, which provide structural strength with a minimal amount of mass. These guitars seem to be more responsive than those with the heavier, conventional struts, and in recent years many people have had the braces in their instruments shaved to achieve this effect--with varying degrees of success. Martin took notice of the public's interest in the pre-War scalloped braces and reintroduced this feature on their herringbone HD-28 and M-38 models.
In contrast to Martins, many pre-War Gibsons such as the advance jumbos and some of the Nick Lucas models and J35s have triangular cross section braces that look rather crude but are lightweight and achieve a remarkably fine result. Maurer guitars built as early as 1910 have unique laminated braces constructed like a sandwich with ebony or rosewood in the center. These guitars have excellent treble and sustain and are extremely durable. I've rarely seen a Maurer with this type of bracing that had the top pulled up or needed a neck set. This is remarkable when you consider that Martins were not even braced for steel strings until the late 20s and rarely hold up as well as steel-string Maurers made many years earlier.
Obviously, the bracing pattern is significant in determining the sound and structural stability of a steel-string guitar, but there has been little experimentation in this area. Most steel-string flat-tops have the familiar X-bracing pattern, although the transverse or simple straight-across type is seen on some cheaper guitars.
Over the years, many tests have been conducted to determine how much difference listeners can detect in guitars of various sizes, shapes, woods. etc.; these experiments have shown that the ear tends to be very adaptable and have a short memory. Guitars may have astoundingly great differences in tone, but the members of an audience won't necessarily hear them. This is particularly true today, when very few people use the guitar in purely acoustical applications onstage. Played through a microphone and a modern PA system with EQ, almost any guitar can sound good for all practical purposes. A guitar's tone and other sound qualities really don't carry over 30 feet and may be irrelevant to an audience; still, they can be highly evident to the player.
In my opinion, the most important feature of a guitar is what it does for the guitarist who plays it. Some models have definite personalities that are stirring, and certain guitars have even inspired pieces of music or playing styles that would never have come into being if the instruments hadn't existed. In my view, it should be the goal of every guitar maker to produce instruments that will be inspirational to those who play them. Ideally, the musicians will in turn convey some of these qualities to their audiences.