October 16, 2010
Bass response in the guitar is associated with a top membrane that is loose enough, while also sufficiently ‘held together’ with bracing, to move as a single unit. This can be visualized as a sail that is billowing in and out under the wind. A thin, relatively flimsy top that is held together by any interconnected latticework of bracing will be able to billow back and forth, in unison with itself, and at relatively low frequency. In the guitar, this is called monopole movement. As an example toward illustrating the importance of materials’ looseness to the billowing action of the monopole, imagine a ship with sails made of plywood; the billowing action will pretty much cease. While loss of monopole is not disastrous in a sailboat (it merely needs adequate surface area of sail), a guitar needs topwood that will move. In steel string guitars, any specific high-frequency potential or behaviors of the topwood — i.e., of the material itself, independent of an interconnected bracing lattice — are not so relevant to this mode. This is because the metal strings themselves, by virtue of their own mass and stiffness, will bring plenty of high frequency signal into the system. One doesn’t need the wood to bring its own additional high-frequency contribution into the soundbox.
On the other hand, treble response is associated with a top membrane that is stiff enough to allow high-frequency/low amplitude motion, and which is not simultaneously ‘drowned out’ or overshadowed by prominence of monopole movement. The more the monopole is suppressed, and the top is prevented from moving like a sail or undulating like waves (think sailboat with plywood sails), and the more it is enabled to move in rippling fashion in small-to-tiny sections, the better the high end — which is usually identified in the literature as dipole and tripole movement. Or, looked at from another point of view, the more that the top discharges its energy by billowing in and out like a bellows or a sail (monopole), the less energy is left over for the high end (dipole and tripole). And vice-versa. This is mediated by the fact that — as is true of any set of speakers/amplifiers — it takes a lot more energy to generate low frequency sound than it does to make high frequency sound.
Without getting too technical the trick in guitar making, obviously, is to not make the plates so loose that you lose the high frequency end, nor so tight that you lose the low frequency end. You want both, and the luthier’s task essentially becomes one of management-of-energy-budget. This boils down to having a command of top thickness/size, bracing, profiling, wood choice, stringing, bridge torque, soundhole diameter, etc…. and that’s all a subject of endless… well… uh… endlessness.