I’m playing around with a suspension for my new plinth and did some rough measurements of the two options that others might find interesting. In the plots below Z is up and down X& Y are horizontal. “springs” are a set of metal springs from a VPI HW19 “With Damping” means with the original VPI foam inserts in the springs. “Foam” is mixed cell polyurethane designed for building or machine foundation isolation. “Constrained” means that I built a solid wrap in the XY dimensions from plywood about 80% of the height of the foam blocks. I calculated the size of the foam blocks from the manufactures literature and should be getting a resonance close to what I get with the springs but I think the manufacturer’s data is based on wide thin sheets and the shape I am using (essentially a cube) is changing things a bit. One thing you can’t see in the roughly 1 second of data I posted for each is that the undamped springs rang for an incredibly long time, minutes in fact. The vpi foam inserts while they did raise resonance a bit cut the ringing time considerably.