How can you change the resonant frequency of a solid guitar body? I am trying to figure out if it would be possible to change the resonant natural frequency of a solid-body guitar. For example, let's say it resonates at 100 Hz and I want it to resonate at 200 Hz. Is there an existing electronic device(s) that could be used to somehow change the resonant natural frequency? What about adding different density metals into the body? I am just brainstorming possibilities.
 A: Yes, the resonant frequency of a solid guitar body can be changed by adding or removing material but it provides no benefit to the sound that can't be accomplished by less destructive means such as the choice and placement of guitar pickups, choice of amplifier, etc. For the purposes of sustaining notes, a guitar that resonates equally for every fretted note is ideal. I know of no solid body design that accomplishes that but electric guitar strings are slightly magnetic, so resonant frequencies on individual guitar strings can be enhanced by changing the distance between the string and the surface of the pickups.
A: Solid-body electric guitar does indeed have resonant frequencies. They work through a transduction process generating an electric signal that is later radiated to the air using a loudspeaker. This is different from a classical guitar where the radiation process happens through the top plate. There, the first two vibration modes are really important for the mechanical behavior of the guitar (see this answer for more details). So, it is probable that you don't want to change the resonant frequency of a solid-body electric guitar since it does not play a role that is that fundamental in its behavior.
After mentioning that, let's consider a single-degree-of-freedom vibrator. The resonant frequency is given by
$$f = \frac{1}{2\pi}\sqrt{\frac{k}{m}}\, ,$$
where $k$ is the stiffness and $m$ is the mass. So, you need to increase the stiffness-to-mass ratio. You could carve some holes, for example, making it something like a foam.
