So this is an age-old question with people firmly planted in both camps. One group will adamantly adhere to the idea that guitars are magical machines capable of defying physics as we know it and that each is unique and different wherein only one can make the very special sounds it makes. The other group are staunch adherents to physics and they know that the only sound an electric guitar pickup can make is that which is generated by the change in electrical output as a result of the changing magnetic fields produced by ferrous strings moving in said magnetic field.
I'm here to say that both camps are slightly correct and slightly incorrect and it's all about construction (and physics).
Physics is generally right in this case in that if the pickups don't move and the strings don't move, there is no output. It's just that physics on paper is simplified and much cleaner than reality and, in reality, things are always moving.
As an experiment, you can plug in your guitar, max out the volume on your guitar and headphones on the cleanest setting you can find on your amp with no effects of any kind. Also, don't strum, we don't want to deafen anyone. If you just tap on the guitar with a knuckle while adjusting the volume you might convince yourself that you hear the rapping in the headphones. For some guitars, you might be right. Now, let's deaden the strings by placing a cloth below the strings and on top of the pickups, thick enough that there's a good bit of pressure on the strings and you're pretty confident the strings aren't going to move, and do the same rapping. Are you still convinced you can hear the sound? Again, you might actually be and here's why...
If your guitar was made of a material that did not vibrate at all when struck, you'd never hear a sound at all (except maybe the standard 60-cycle hum), but guitars are made of wood, carbon fiber, or other materials that do have the ability to vibrate, some more than others. Now, also consider how your pickups are mounted. On my strat, they are screwed in place and backed by pretty heavy springs. The wood is a solid Alder or similar wood. When I rap on this guitar with the strings muted, I hear nothing in the headphones. This is because there is (for all intents and purposes) no movement between the strings and the pickups, the vibration is too small to create any perceptible sound. However, on my Ibanez superstrat, I have an HSH configuration and the humbuckers are floating on much softer springs so that when I rap on the guitar body, the pickups do move some. Without the cloth in place, they move a lot more than you'd think and some sound from the vibrating wood is heard from the pickup vibrating, yet the single coil in the middle is just like the strat and doesn't move at all and once again produces no perceptible sound in the headphones.
What does this mean to someone who thinks that a guitar should have it's own character and not just sound like whatever pickups are put in it? Well, the more things vibrate, the more the construction will affect what the pickups pick up. Choose pickups that float more softly instead of being rigidly attached to the body. Choose a floating bridge as well since the springs will allow the strings to vibrate with the body. Also, I've noticed that the more solid the body, the less vibration and the less solid the more vibration there is. My strat has the most solid body of all my guitars and pretty much doesn't have any body vibration coming through the headphones, so it is the cleanest tone and really is only colored by its playability (and my skill, which isn't so great). However, my Jazzmaster has a lot of open cavities inside, with a solid tailpiece, I still get quite a bit of what I can only describe as a kind of reverb in the sound when rapping the guitar. This might explain why it was a popular choice for Surf Guitar. The pickups on the JM are supported by two medium sized springs and foam, so they don't get much vibration on their own, but it is still enough to color the sound a little.
One other popular guitar is the Rickenbacker 360. One might wonder what gives it such a characteristic sound. For once, it is a semi-hollow body, so less wood = more vibration in the strings and pickups. Plus, this guitar has a special set of pickups you won't find on other guitars. The combination is an open and jangly sound that you just don't get from other guitars. That doesn't mean you couldn't take those pickups and put them on something like an ES-335 and get something similar, maybe even better. Go ahead, give it a try :-)
Note that the more things do vibrate, the less sustain you will have. This is because the body frequencies are nowhere near those of the strings and will likely be out of phase causing the stings to deaden much quicker. The less vibration you have, the greater your sustain will be. Combined with humbucking pickups which have the lowest output and weakest magnetic field and sustain increases even further. To maximize sustain, use active electronics instead as they will increase your output without increasing magnetic field strength, thereby allowing the strings to vibrate longer.
Now, with all of this said, I have to stress that the vast majority of your sound is based on your electronics. Everything in your signal chain will alter the sound much more than the construction of the guitar. Quality electronics can make wonders of a cheap guitar. My JM is a Squier, solidly built, but had lousy pots (and the stock pickups weren't that great either), so I upgraded those for a much higher quality of sound. The rest of the guitar is awesome, BTW! My Ibanez super-strat is also cheap and could stand to have better pickups and electronics. When I do upgrade it, I'll make sure the pickups don't float because I want a pure sound out of whatever pickups I choose to put in it. I'll also probably block the Floyd-Rose bridge so that it doesn't move either. With that combination, I might get something closer to a Les Paul in sound quality, all on a cheap body with a neck that at least plays well.
I hope this helps some of you out there in choosing your next axe or modifying an existing one. Rock on!
