Wood blocks underneath components?? snake oil?

The last time I replied to a similar post it never made it past the moderators for some reason. Placing different materials like wood or brass under a cd player or tube equipment will alter the sound slightly at times for better or worse but are not very effective in isolating the component from the mechanical vibrations via the rack. If you look up isolation or vibration damping devices you will find that materials like polyurethane are used often. Sorbothane is just ultra soft polyurethane. As a mechanical engineer with access to vibration analysers it is easy for me to see which materials dampen vibrations and which don't but I doubt I can convince any of the serious audiophiles to give up their high dollar cones and ball bearings for something that cost a few bucks.

883dave, I am going to try and explain what an isolator really is and what it does refering to my textbook and some catalogs I use at work. Firstly, I did skim through the stereophile article mentioned above and it is somewhat confusing since it mentions the best you can do is 100% transmission and then the last few pages are about proper isolation/damping. Their conclusion then was getting a rigid, sturdy rack and then isolating the source components individually so even if I didn't know better, I wouldn't just put cones under everything. Can you imagine being rigidly coupled to the wheels of your car without springs and shocks? I cannot comment on the stuff about effect of vibration on passive components and phase shifts and such.
To put it simply, every material has a natural frequency at which it will vibrate when subjected to a disturbing force. Natural frequency of a system depends on several variables including stiffness of the material used, mass, shape etc. In general, lesser the stiffness, lower is the natural frequency.
Vibration is a force and establishing an opposing force can effectively reduce it's transmission. This is the function of an isolator. Simply put it is a spring. It can be a steel spring, air spring or an elastomeric spring. A damper is a device by which vibrational energy can be dissipated, typically by converting it to heat. A steel spring is only an isolator whereas an elastomer can be an isolator and a damper. An isolator must deflect under load to oppose vibration. NO PROTECTION WITHOUT DEFLECTION! The fundamental property of an isolator is it must be resilient, ie, it should be able to return t it's original state without load.
An isolator is defined by 2 properties, it's own natural frequency and how much it can deflect (in our case compress) under the maximum allowable load which is dictated by the material and structure and is called static deflection. In general, the greater the difference between it's natural frequency and the disturbing frequency, the better the efficiency of isolation. The greater the static deflection, the more effective the isolation. Deflection however should be limited to the direction of travel of the force, usually vertical thru the isolator. An isolator must be constrained to limit horizontal movement although there are devices that isolate in shear also.
Pneumatic springs have the lowest natural frequency, can be as low as 2Hz and are excellent for audio purposes. However by using only an inner tube or soft feet you get plenty of sway. If you look at typical isolators in Lord, Mcmaster etc, the elstomers are bonded to some steel to add rigidity to prevent this. They are inexpensive and can be mounted between a shelf and a platform for the source. I don't know what a vibraplane mentioned in the review is but I am sure it is a platform with constrained air springs. Bright star may has one also I think. Usually precision lab equipment is isolated using some type of spring. As the article says, "draining vibration" with fancy hard substances underneath your component is all bunk, sorry.

Dan_ed, I never said you would end up liking an isolated component sound. Having balls made of exotic material under your tube amp may amplify certain frequencies which could provide addtional mid-bass warmth for example. My point is people who want to reduce the effects of vibration should know that stuff like cones and ball bearings are not isolation devices or "drains" even though they are marketed as such.

Rotarius,

Fair warning! Nasty wars have already been fought in this forum about the efficacy of the devises which are purported to drain vibrations from audio equipment. Been there, done that. When it was all over there was no agreement about anything, just a lot of PO'd folks. :-)

Oh crap! We don't need more PO'd folks. Our hobby is all about perception anyway. Just thought some of you might be interested in how it's done in the *real* world :)

cdc, yes the air borne vibrations is another component that has to be addressed. This is where there are plenty of products being marketed as "mechanical drains". I had never heard of such a thing until I visited audiogon. This concept (or rather misconception) only exists in the audiophile world. If you put brass or some ball bearing under the chassis, it will obviously transmit the vibration. So what? The chassis still vibrates doesn't it? Why not try to minimize the problem by first isolating the chassis from the rack and then dampen the chassis? I have also seen people trust the numerous "white" papers out there. They are marketing tools nothing more. Unlike a technical or scientific research paper, they are not verified by peers in the industry and are not backed up by experimental data. If you actually see vibration analysis data which shows the effectiveness of a product in controlling vibration, buy it!

cdc,
1) Even if you rigidly couple all components in your rack, each component will still have a different resonant frequency due to differing mass, volume, materials which result in different natural frequencies. This based on scientific fact. A spring or an elastomer is typically used to reduce the amplitude of the wave that produces vibration in a medium btw. Rubber is used to isolate a jet engine from an aircraft frame. For the record, unless the component is bolted down to the shelf, it is not a "rigidly coupled" system to which you can apply the formulae from rigid body mechanics. A component on brass cones can very easily be disturbed compared to the rack it sits on.
2) Please read my earlier post on this subject. The only way to reduce magnitude of the vibrational force transferred from the shelf to the component is to have an isolator and damper in between. Increasing the mass of the platform helps in theory but for practical reasons can't be the only way to dissipate vibration. If the offending force can make it's way through concrete slabs of your floor, you will need a lot of mass to dissipate it if you do not want to use a real "isolator". BTW, what makes you so sure that by rigid coupling everything you have raised the natural frequency of all the components beyond the audible range? Glass ringing/resonance occurs at higher frequencies, why are glass shelves frowned upon by audiophiles in that case? Lastly, anyone in the isolation business ought to have measuring devices (that are readily available) and back up their claims with numbers. How many of them do?

cdc: The ideal stand as I gather would be:
1) completely rigid and operate as a single unit. Bad vibes article "Minimize the relative motion between different elements that comprise a system"
2) resonant frequencies would be pushed as HIGH as possible so vibration's amplitudes are as LOW as possible. Bad Vibes article:
"lowest natural frequency will be the most dominant". Minimum resonant frequency = maximum amplitude".
"Reduction in frequency leads to an increase in dispalcement...resulting in a "noisier" less stable
platform".
"The lower the resonant frequency of a platform, the less desirable-the associated increase in amplitude will cause more serious ringing that damping can only partially reduce." So using sorbothane actually INCREASES ringing which damping can only partially reduce.
"Enough damping should be applied" BUT ITS PURPOSE IS TO "further lower the displacement of resonances" not increase it.

As I understand it, sorbothane has
1)high amplitude as in actual dimensional dispalcement
2) soft material with numerous, complex. vibration modes.

cdc, I just wanted to add one more thing, those conclusions you arrive at from reading that article about resonant frequencies are not accurate. "Minimum resonant frequency = maximum amplitude". Does amplitude of vibration in a medium only depend on the frequency of vibration? Can you provide a reference showing that relationship or an equation or something? What about the magnitude of force that causes the vibration in the first place? I just want you to know that some of the best and most effective isolators I have seen have a natural frequency of less than 2 HZ and achieve 99.8% efficiency of vibration isolation above 10 hz.