Max - I had never considered that Thiel's weak link was its cabinets. Let's put that idea on the back burner for now.
Regarding Spruce as a cabinet material - that has merit. Spruce is an extraordinary wood, landing it as the soundboard of choice for many musical instruments. My primary business after Thiel Audio has been producing tonewood for high-end instruments. Red Spruce (the holy grail of tonewoods) is my specialty. As a grown material it's hard to beat for stiffness to weight and so forth. But as a cabinet panel, it doesn't measure up all that well. It is a naturally resonant material, which is what you want to avoid in an enclosure. It is highly directional, being 7-10 times as stiff along the grain as across. Its micro-structure encourages transmission of sound through its thickness, another thing we're trying to avoid. And even though it is relatively stable for as a lumber species, it still expands and contracts enough with humidity changes that the cabinet would change characteristics through the year and eventually crack. So, all in, the idea doesn't check my boxes.
Among the materials science aspects of Thiel's cabinet walls, I chose a Canadian MDF made entirely of Spruce fiber, for the reasons you cite. We then laminated veneer on both sides with a structural adhesive, creating an I-beam effect with about double the stiffness of an unlaminated panel and more tha 1.5x that of an exterior-only veneered panel.
Dr. Toole and his proteges at the Canadian Research Institute looked at cabinet material candidates regarding stiffness to weight, stability and damping, and landed on MDF as the closest to ideal. PSB, et al made a case of MDF being "better" than most of the exotics out there. I wouldn't go that far. But putting cost into the equation tilts things significantly toward MDF being a pretty good substrate.
All that said, my present design work uses other materials to augment and upgrade the basic MDF cabinet.
Regarding Spruce as a cabinet material - that has merit. Spruce is an extraordinary wood, landing it as the soundboard of choice for many musical instruments. My primary business after Thiel Audio has been producing tonewood for high-end instruments. Red Spruce (the holy grail of tonewoods) is my specialty. As a grown material it's hard to beat for stiffness to weight and so forth. But as a cabinet panel, it doesn't measure up all that well. It is a naturally resonant material, which is what you want to avoid in an enclosure. It is highly directional, being 7-10 times as stiff along the grain as across. Its micro-structure encourages transmission of sound through its thickness, another thing we're trying to avoid. And even though it is relatively stable for as a lumber species, it still expands and contracts enough with humidity changes that the cabinet would change characteristics through the year and eventually crack. So, all in, the idea doesn't check my boxes.
Among the materials science aspects of Thiel's cabinet walls, I chose a Canadian MDF made entirely of Spruce fiber, for the reasons you cite. We then laminated veneer on both sides with a structural adhesive, creating an I-beam effect with about double the stiffness of an unlaminated panel and more tha 1.5x that of an exterior-only veneered panel.
Dr. Toole and his proteges at the Canadian Research Institute looked at cabinet material candidates regarding stiffness to weight, stability and damping, and landed on MDF as the closest to ideal. PSB, et al made a case of MDF being "better" than most of the exotics out there. I wouldn't go that far. But putting cost into the equation tilts things significantly toward MDF being a pretty good substrate.
All that said, my present design work uses other materials to augment and upgrade the basic MDF cabinet.