not necessarily so. A plinth I am making for a Lenco-based PTP6 consists of two different honeycomb structures, one of which is used in aircraft fuselages. They will be filled or partly filled to provide enough damping and the correct amount of mechanical impedance to match that of the 4mm thick steel parts of the PTP6. It will NOT be cheap, but it WON'T be thick.
Vibrations do not propagate through, at least not through thin plates, but as the thickness increases and frequencies increase, the vibrations will turn into sideways displacements, and shearing will develop. A thin plate does not suffer from this, only thick pates (plinths).Rega are using very low density material for their higher end tables. The reason being, if there is less material for vibrations to propagate through, there will be less energy to transfer etc. I agree, it is a fine application of physics and it apparently works well if the reviews of these products are reliable.
Yes, the displacements are small, but relative to how much the stylus is moved by the record groove, it is in the same ball park. And damping is an intrinsic property, it does not rely on amount of material. Think density (another intrinsic property), it doesn't matter if you measure a 10mm cube of steel, or a whole ship full of steel, the answer is the same. Same with damping.However, mass loading also works from the other point of view. The energy of the vibrations that we are talking about are tiny. If they haven't enough oomph to get a massive body vibrating they won't give too much trouble. Plus a lot of mass has many particles to lose energy as heat.
What you are alluding to is called modal density, that is, the number of resonance frequencies per unit frequency. And yes, it is important. Thin materials will have a lower fundamental resonance frequency, so the related harmonic (and anharmonic) frequencies will be relative to that. Thicker plinths (of the same perimeter as the thin one) will have higher resonance frequencies, and maybe in the region where applied damping is more effective.It is all very well talking about 'plates' having fundamental frequencies that with vibrate in harmony with external sources of vibration. Well that is true, but, and even counting all the harmonics as well, we are talking of a very narrow band of frequencies that will cause trouble.
However, I have learnt over the last dozen or so years I've been working on material properties that everything is a compromise, and one has to decide which of the properties is more important to optimize. So comparing an 18mm thick plywood plinth, compared to a 100mm thick one, modal density is higher, fundamental resonance frequency is much lower, critical frequency much higher, damping factor a touch higher (but still very low!) and flexural rigidity much much lower. But the overriding parameter which is the most informative is the losses for the two plinths. The thin plinth losses are between 14dB and 26dB, from 20Hz to above 1kHz, whereas the thick plinth only manages 6 - 12 dB over the same range. This is mainly down to the much higher stiffness of the thick plinth. If the damping factor of the material was 0.4 instead of 0.04, then losses would be from 30 to 36dB! showing the importance of high damping factors.
Can't agree. As I have already stated (above), if materials are just placed one on top of another (not fixed in position) then mechanical impedance comes into play. If they are glued/screwed together, then a new material is made, which will have just one set of parameters. All the layers will move together, one set of vibrations. I have measured this over and over again! Your analogy doesn't work.Multi layering of dissimilar materials does have an effect on reducing vibrations transferring at the fundamental of one through a junction to a fundamental of another. Just the same as sound moving from air to a wall back to air. If it didn't lose energy at each junction, playing your music in one room would sound just as loud in the next. It clearly does not.
yes, costs have a lot to do with it, as has appearance and making a plinth fit for purpose!There are many ways to skin a cat. #-o But making massy plinths with lovely wood veneer finishes is 10 times more expensive than a polyurethane sandwich with a rather dreadful appearance, as per Rega. Cost has more to do with it than physics.
and just imagine what a thin plinth made of a damping material would have done for you! But if you are satisfied with it, I'm happy for you. :DBy the way. I have just made a massy plinthed table from a Rega Planar 2. I'm using the original AC motor, and it is the most eerily quiet table I have (not) heard. I'm willing to bet my humble Rega is at least as quiet at any higher end Rega with the new motors and neo PS. And it looks 10 times more beautiful, belying its humble origins. It would cost a lot to produce my table, though and not production line semi skilled operative produceable.