Guys, so i found this from Acoustics: Sound Absorption
These are two diagrams of the cross-section of the opera house wall design. I start to understand why @donpark in this post: Case material? What's the differences? - #2 by donpark said that aluminum (metal in general) is not inherently high-pitched material for keyboard housing and I think we will continue to use it as the housing material for years to come.
The weight-to-stiffness ratio of the metal is larger than the concrete’s. For concrete vs polycarbonate, the concrete has a smaller weight-to-stiffness ratio. So, you need to use denser material but keep the stiffness as low as possible.
You can imagine this part as the PCB (polymer material), which can eventually absorb most of the noise that bounces around in the area between the PCB and the internal surface of the bottom part. The aluminum housing will absorb some of the noise and pass some of the low-frequency sounds to the surroundings or the ‘thocky’ sound as the leftover, I suppose. I think tape mod also adds more power here but inserting a silicone damper or too much foam will make it too muted and deaden your beautiful premium switch character.
Did I miss something?