Take a piece of string and attach an object with some weight. Hold the string at one end, and push the object to start it swinging. The object will keep swinging for some time, right? Now do that once more, but don't hold the end of the string at the same place, but move your hand in the same direction as the swinging object. The object stops swinging after just a few swings. This is what happens on a so called "dead spot" on the neck of your bass.
The neck partly takes over the swinging of the string, and a complex pattern of back and forth going vibrations travels through the neck. At some places in the neck, going and returning vibrations can add up, causing a resonance, just like 2 waves in the water in opposite direction, causing an extra big wave when they hit each other. If the neck at some point on the fretboard happens to vibrate in the same direction as the string, it kills off the sustain very fast. It can be extreme, down to a sustain of just 1 or 2 seconds!
This phenomenon is the real "dead spot". Other faults can kill sustain as well, like fretbuzz, badly fitted frets, bad string guidance on nut or bridge, or rusty or dirty strings. But that's not what this is about.
Lots of basses have the dreaded dead spot. It may be more or less noticable, but almost every 4 string bass has one. Infamous for their dead spots are Fender basses. Jazz Bass and Precision bass usually have a dead spot around the 5th fret of the G string (C note). And many many basses are variations on the Fender models: about the same neck width, length and mass.
What are the factors that determine the dead spot?
Dimensions and mass of the neck:. Maybe you can feel the floor trembling when a truck drives by, or watch the ripples traveling through the water when you throw a stone in. Just in the same way, the neck takes over the vibrations from the strings. In general, a big, heavy object will absorb more vibrations than a small, light object. So A heavier, wider neck will resonate less than a thinner, lighter neck. The neck fitted with or without headstock also makes a difference.
Stiffness of the neck: If the neck is rigid, it will take over less vibrations from the strings. The use of very dense woods like ebony can make a difference. Also the use of an extra trussrod, carbon fibre rods, or maybe an entire carbon neck. The ideal neck, deadspot wise, would probably be made from solid concrete, but that wouldn't be very practical, would it? Also a neck-through-body contruction can make some difference.
String gauge and tuning: Heavier strings wile give more tension in the neck, that adds to the stifness of the neck. Adjusting the trussrod accordingly also adds to the tension in the neck. That may influence the dead-spot. A different tuning (not often done on a bass) can also be of influence.
Note: "general principles" means that there CAN be exceptions to the rule, don't shoot the mailman..
I have a dead spot, can i do something about it?
There is no guaranteed cure, but you can try some things:
Some hearsay from the internet:
Misconception about the "FatFinger"
The Fender (or Groovetubes) "FatFinger" is a C shaped clamp that you attach to the headstock. Does it help? Yes. The dead spot will not be completely gone, but the sustain will significantly improve.
The misconception about the FatFfinger is that it just adds weight to the headstock, thereby changing the mass of the neck. Well, just adding 100 grams/15 oz of weight is not going to help at all, it probably isn't even going to move the deadspot to another note.
It is all about adding DAMPED weight. Because of the felt strips between the headstock and the FatFinger, there will be a vibration-absorbing effect. For the best effect, don't turn the screw too tight.
On a large scale, big skyscrapers like Taipei 101 have a heavy weight in top (800 tonnes, see picture), acting as a damper against swinging of the building. The same principle applies to the FatFinger. As long as there is some vibration absorbing material between the headstock and the added weight, there will be a postive effect on the dead spot. Materials like Felt, rubber, foam, cork should do the trick.
Oh yeah, and Billy Sheehan has a FF too. So now it's even cool to use one...