The Semi-Hollowbody SR (Spread Resonance) guitars provide the sustain you would expect from a solid instrument, but with the body vibrance much like you would find in a full size Archtop - all in a thin, light (under 5.75lbs) instrument.
Body Resonance in Archtops and Semi-Hollowbodies
Grez SR Series Construction Explained
The Spread Resonance approach allows body resonances to influence the instrument sound over as wide a frequency range as possible. This is not to imply that focused narrow band resonances are not desirable or tonally useful. Your typical semi-hollowbody would fall into this category. If you tap on the top, bass or treble side, you will perceive two things immediately. First the primary note or resonance is fairly high in frequency, around 300Hz or higher and that both halves generate nearly the same note. This highly focused resonance is much of what gives a 335 its distinct sound.
A brief historical look at the archtop guitar will show a transition in some instruments from solid carved wood to laminated materials to lower production cost and to help control acoustic feedback in amplified situations. To further control feedback blocking and eventually full center blocks were added creating the semi-hollowbody we are all familiar with. But, by adding a center block, the unsupported area of both the top and back plates are significantly reduced. The remaining plate area will be significantly stiffer, in turn resonating at a much higher frequency.
These plates can be re-optimized based on their now smaller unsupported dimensions to bring back some of the liveliness and warmth (lower frequency resonance's) of the full hollow archtops. Similarly, large asymmetrys can be designed into all aspects of the body’s internal structure while keeping instruments exterior looking traditional. In doing so, a very pleasing and useful balance can be found between the loud, rich and powerful, but feed-back prone full hollowbodies and the tight but feed-back resistant sound of today’s semi-hollowbodies.