Will
The note regarding the time was only to say that the information above is also two years old and I've learnt a lot in that time, so things have changed.
The boundary thing is simply that there's a reflection coefficient at any boundary between two materials which is given by (Z1-Z2)^2/(Z1 +Z2)^2. This drops to zero if the two impedances are the same and increases as the impedance mismatch increases. An interesting aside is that because of the square law relationship interposing an intermediate impedance reduces the total reflection*.This is one of the reasons for incorporating boron fibre: the acoustic impedance of a boron composite is between those of carbon and stainless.
I basically used stainless because I like it acoustically and I couldn't bond Ti reliably. I think I've worked out a method of bonding Ti so the next prototype will incorporate that.
The loss tangent I'm using is different from, but analogous to, the RF loss tangent: they're both ratios between an energy loss component and an energy storage component, it's just that the components are different in each case.
* Aside for those interested in TT design: I spent a bit of time and effort developing a platter material based on this concept and the use of high modulus and high density fillers in a polymer matrix. The intention was to achieve gradual impedance change through the platter from the impedance of PVC at the top to that of stainless at the bottom. The fun part was getting the particles to settle at the right velocity, eventually solved using the "Reverse Brazil Nut Effect". I'm not joking, that's actually a thing.
