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From: Ross
Remote Name: 76.101.10.136
Date: 16 Mar 2007
Time: 08:29:59 -0500
Luthier’s Blog 3/16/07 Allocating personal energy is a constant challenge. The time demands on a typical American working married household head with three teenagers, mortgage, numerous pets and church/extended family obligations (odd to hear oneself described as that) expand to overfill any given time slot. There seems to be two approaches to dealing with time demand. One can make plans for the expected demands in advance, during quiet periods of reasoned reflection, allowing to prioritize and maximize effectiveness. Or. One can plunge willy nilly into the day with no plan, realizing that the number of variables that the previously described family can and will present, coupled with work exigencies and the unreliability of mechanical friends such as automobiles, plumbing, electronics, yard machinery, and the general entropy of things one owns, will render a cogent schedule laughable. Guess which approach I subscribe to. Being self employed usually means that others assume your time to be not only flexible but free! HA! Returning to the design of my robot luthier’s guitar, I see that he is looking around at other guitars to discover what limits lower order harmonics in their designs. For one thing, the standard dreadnought design places a large sound hole just 3” away from the bridge. And not only that, the gaping hole is in the direct string path that exerts a continuing 175 lbs. of pull. He reasons that extending the distance to the sound hole will expand the plate’s area of sound propagation, disproportionately affecting the lower register partials (note: partials and harmonics are the same thing). Since the sound hole cannot be moved further in a straight line toward the headstock, being limited by the fingerboard, he chooses to relocate to one side of the fingerboard, reasoning that this upper area is less acoustically live than any area lower. But which side of the fingerboard? To the bass side would be closer to the player’s ear, and would allow a larger sound hole than the treble side, which is truncated by the cutaway. Other considerations factor in, however. The builder here is reasoning that the soundbox is really an air pump. He believes that , to be really effective, a pump, any pump, must have not only an outlet, but an inlet, much like the air conditioner in a house. The sound hole of a guitar allows for the movement of a column of air in and out of the box. The limiting factors as to how much air moves include: degree of compliance of the box walls; amount of energy driving the pump; and differential atmospheric pressure between compression and expulsion phases (breathe in, breathe out). The builder plans to address the first factor by building as lightly as he can while prudently assuring a reasonable service life. The second factor is out of his hands, as the input energy is provided by the player and not the guitar. But the third factor, he reasons, is susceptible of possible improvement. If two ports rather than one can be incorporated, careful location of each may provide a greater degree of plate excursion. Placement, however is critical. Naturally, the ports need to be located so that the vibrational movements of the box do not cancel out any benefits. The air movement through the soundbox between the two ports must , after all, be push/pull and not push/ push. Phase coherence is a real problem. Our luthier then thinks that if one port can be located on the top, the other can best be located to a side, which is nearly acoustically inert compared to the very active top. This will isolate the second port from participating in counter-productive push/push or pull/pull movement. Which side then? Well, the one facing the player, the bass side, would obviously be the choice. Where along the side? A point closest to a direct line to the player’s ear would result in the broadest frequency range perceivable by the player. And preferably where air movement would not be inhibited by the player’s right arm. Our builder chooses to install the port on the upper portion of the rib, between the neck joint and body’s center point (he has not yet dealt with the question of a bass side “waist”). What else? Oh yes, the luthier decides that the aggregate area of the two sound holes needs to be roughly equal to the area of a single sound hole equipped guitar of equal box volume. Box volume has also not been dealt with yet. But he’s getting closer. All this thinking has tired out our poor luthier, and he’s going to do some bench work to recover. Ross Teigen 8:57 am