Originally Posted by Drew
Sympathetic resonance is more of a drone note thing, though, isn't it? Like those harp guitars, where you have a bunch of tuned strings that resonate with whatever note you happen to be playing. I don't think that applies to guitar bodies. If you can feel a note vibrating, that's killing the sustain of the instrument.
If you play a dead note on the instrument that has 1-2 of them (Its even worse on fretless, since you get a dead area, not just a single note), you'll notice a huge amount of vibration in the body and then the note dying off quickly. Check out the Fender dead note on a P Bass or J Bass. Generally the 6-9th fret on the G string. Not all of them have it, but its pretty common. The note will die off really quickly, and you'll only be left with a really weak harmonic. You'll notice the vibration in the body if you pay attention. Then, if you take the bass and lean the headstock against a desk or wall, or something big and heavy, you'll notice you can hit the note and it'll sound like any other note and the body won't vibrate heavily.
It's one of the first things i check out once I find a bass I'm considering buying. I've bought a handful of basses off of ebay that have a pretty distinct dead note (If you guys remember that 5 string Japanese BTB505 I bought back at the end of 2010 -- I ended up selling it after I realized the Bb on second string died off almost instantly). My carvin I bought has a slight dead spot on the 7th fret of the G string, but I'm going to be changing the tuners, routing the body for a pickup and having the fretboard epoxied, so it shouldn't be a big deal.
Interestingly enough, my chapman stick HAD a dead note like that, but I noticed when I pulled the pickup housing out, the thing went away instantly. So i put some rubber feet under the pickup and re-adjusted the screws that set the pickup height, and now that the pickup is completely isolated from vibration, there's no dead spot in the playable range of the instrument.