My understanding is that, to decouple an object from a surface, ideally we would use something like a needle. That is, the minimum possible contact surface area at each interface.
Of course we can’t use a needle, so we compromise by using something with sufficient strength to hold up the other object.
An adjustable spike has two contact points:
floor: small surface area at the end of the spike
rack: relatively small surface area where the thread of the spike touches a nut attached to the rack.
Assuming the nut is solidly coupled to the stand, isn’t this otherwise a good solution? Why would the size of the spike make a difference.
The size of the spike affects the mechanical compliance of the structure rather than the coupling at the interface between the spike and the substrate. The compliance then affects the resonant patterns of the structure.
For a given length of spike, larger diameter spikes are less compliant, smaller diameter spikes are more compliant. For a given diameter of spike, larger diameter spikes are more compliant, smaller diameter spikes are less compliant.
This won’t directly affect the coupling: that is more affected by the interface area at the tip of the spike (which may in turn be affected by the rigidity of each of the materials at the interface).
I confess I am struggling to figure out whether we want to achieve more or less compliance!
Assuming the latter… have I interpreted this correctly if I assume ‘compliant’ means, in this context, more likely to move around. That is, a fat spike will prevent more sideways movement and therefore hold the point more still on the floor. (A bit like your comment earlier about ensuring the brass puck is rigidly coupled to the wood shelf.)
Thanks, Ian
If so, is a suitable solution to use a large gauge threaded rod? e.g. 19mm or something. It’s easy enough to grind the end to a fine point.
Unfortunately the answer to whether you want more or less compliance is one of the questions that would be need to be resolved by the DFEE (or, more likely, by experimentation).
This is where the confusion arises between coupling at the interface and coupling across larger structures (i.e. taking into account structural compliance of the mechanical structures either side of the interface as well as the interface itself).
Thanks @n-lot and @Xanthe, I do find this topic confusing. On the one hand, in my mind I see a Newton’s Cradle, nearly perfectly transferring kinetic energy across a nearly zero surface area contact point. Sounds pretty coupled to me.
On the other hand, I read about devices such as cup & ball as isolating decouplers, including in the blurb about the Fraim:
A toughened glass sub-shelf rests on minimal-contact ball decouplers, providing even more isolation.
Noting that, in my case, the floor is concrete (so for the sake of this discussion let’s assume the spike sits on it rather than sticking into it as it might into a wooden floor).
Help!
That said, I’m aware that this highlights the benefit of R&D and therefore the cost/benefit of a Fraim. So perhaps it’s just time I got on with building it and ensuring I have a solid rack on which to mount the glass platforms.
No matter how hard the one side of the interface is, if the other side is harder still and there is any significant pressure at the interface, the harder side will deform the softer side to some degree. This deformation will either be elastic or plastic depending on the pressure and the compression strength and compression yield points of the materials.
With steel and concrete, and the sort of weights of a HiFi system, then if steel and concrete are of good quality and the tip of the spike is about 0.3mm or larger diameter, I would expect the deformation to be elastic.
Don’t forget that Newton’s cradle requires that the object are of identical mass, made from materials with identical sound velocity and that there is no other route to dissipate energy at the moment of collision - definitely NOT the same conditions as for a spike on the base of a HiFi rack or Fraim balls.
Strangely enough the best approach to the theoretical analysis for this is based on calculus (calculus is used extensively in the basis from which Finite Element Analysis derives)!
(The DFEE I gave above should of course have been DFEA!)
I think then, that I am best to ensure each spike nut is robustly coupled with the rack (much as the brass pucks will be mounted in the shelf). So that the primary goal of the six rack spikes is to achieve a level rack, adjusting the height of each spike as required. The concrete floor is not precisely level and flat so I will need that variability in spike height to support the rack.
Then focus on doing a good job of the brass pucks and silicon nitride balls to isolate the glass shelves as best I can.
I’ll report back once I’ve got under way on the build. Thanks for all the help and please do chip in if you think of other things I should consider.
A question - are the same materials used in racks as in making speakers? I generally see solid wood or glass in racks and mdf in speakers…
would appreciate any responses as to yes and or no - and why??
There is also much laminated MDF in racks and it has advantages. Eventually, both speakers and racks have similar requirements regarding stability and cost.
While I don’t doubt it has acoustic benefits, MDF is much easier to shape with automated machines than virgin wood and it’s also stable so won’t move around and open up. And it’s cheap.
That said, MDF is awful stuff. A hint of moisture and it’s ruined. It can’t be recycled. The dust from working it is extremely fine so requires very good ventilation and dust masking/collection.
Yes, that’s all true, but the first paragraph are all real advantages, same ones in rack as in speakers. As least as far as vibration goes, its rather amorphous internal structure is probably better than real wood, I would guess
I would say that virtually all racks I’m aware of are made from MDF or other composite materials (Fraim, Isoblue, Hutter etc.) with the exception of that company that buys in kitchen worktops and chops them up to make HiFi racks!
It’s far easier to get consistency and stability from MDF than solid wood, much as I dislike the stuff aesthetically and environmentally.
(In fact I have managed to eliminate MDF entirely from our house, the only exception being the Isoblue rack.)
Yes, I worded it poorly, what I meant to communicate is that a manufacturer’s reasons for selecting MDF presumably include that it’s much cheaper to work with!
I am going to jump straight in, regret it in the not too distant future, and give my £0.02
Isolation and decoupling is only needed for turntables. Turntables are basically vibration detection machines, so reducing the amount of unwanted vibration reaching them is beneficial. Sorbothane converts kinetic energy (vibration) into heat. It is considered pretty much the best material for this, so placing this between a turntable (or its shelf) makes sense if walking around causes vibration to propagate to your rack or table. Beyond that…there is little if no reason to think that the tiny microphonic effects within the electronics of hifi are worth worrying about. Tubes, yes, but solid-state stuff…no.
There are plenty of reasons not to have things in your listening room vibrate audibly though.
