BTW, the "unscientific" comment -- what I mean is that there are more accurate ways of observing resonance and determining this mechanically rather than just knocking and listening. Further, knocking on the guitar in different places produces different tones, just like knocking on a wall where it is hollow vs. where there is a support beam. I'm doubtful this knocking is truly a measure of resonance.
It is a perfectly scientific measure of resonance, have a look at the thread I mentioned: striking something (with a hammer for example) excites ALL resonance frequencies. It's a pretty old trick when designing aircraft. In order for this to work you need to suspend the guitar freely, IE use the strap, but don't let it touch your body.
What you hear by knocking on different parts is a more pronounced excitation of some the higher frequencies, IE the overtones. Unfortunately for us musicians, overtones in structures are rarely, if ever, harmonics. So while a string has resonances at 1:2:3:4 the guitar itself doesn't have these nice harmonic intervals. If you are unlucky at hit it at a node (stationairy point) in one of the lower frequencies which will dissipate the knock. This doesn't mean the guitar doesn't resonate, just that you hit it in the wrong location.
The best place is probably at the headstock, but you might wat to try the neck or the lower bout as well.
Note that I'm talking about the resonance of the structure itself (the box-beam that is the guitar body+neck) not the cavities: we're not talking about acoustic guitars.
Acoustic instruments instruments with a sound chamber (cavitiy) can be built in such a way that one or more of the lower overtones IS a harmonic of the lowest, look at the Stradivarius vs a cheap violin:
But even in this example the 2nd and 3rd resonance frequencies do not exacly correspond to harmonics. (they are at Ab3, Db3 and G4)
Your example of the hollow wall is a combination of effects: the hollow chamber amplifies the sound generated by the wall resonating in front of it, exciting it at midpoint is far more effective than at the edges; compare this to picking very close to the bridge i.s.o. over the pups or even midway on the neck.
Old growth wood is denser with tighter grain and is widely believed to make a superior instrument.
The thing is that denser usually means stiffer as well (see my previous post ash/maple vs basswood/alder), so what your doing is shifting the resonance frequency and possibly adding a bit of sustain. If that is your definition of "superior" then yes, it may be beneficial, but for ash bodies, swamp ash is often preferred: the lightest variant available.
If these LP afficionados truly believe heavy, dense and stiff is better, why not do they not rave about guitars made of oak, glassfibre, concrete, aluminium or brass?
Let's look at a table of stiffness over density (an important ratio for resonance)
Carbon fibre reinforced plastics= >40
Glass fibre reinforced plastics=14
Again, it's no wonder Parker is using Basswood for its neck, structurally it is the best material available, even better if you take the geometry into account (cube root of stiffness over density):
Glass fibre reinforced plastics=1,9