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  3. 1064803157

Why Does A Concrete Floor/Spiked Metal Rack...

suck all the warmth and life out of my system?

I have been thoroughly dissatisfied with my hi-fi system for the good part of a year now and I have been unable to until recently to put my finger on the problem. In a nutshell, almost every CD I would play would sound bright and harsh and bass light. The top end and upper midrange would completely overwhelm the bottom end. I have experimented with all sorts of tweaks and in particular various isolation devices, and although I was able to achieve minor changes to the tone the overall top end brightness and lack of bass was still evident.

I was enjoying (as best as I could given the problem!) a listening session and wracking my brain (for the ten millionth time) for ideas on how to make my system work better, when it suddenly dawned on me that I had these small plastic/hard rubber? cups that might be ideal to place under the rack spikes as a last ditch attempt to solve the brightness issue. With the music still playing I carefully tilted the rack enough to slip the cups under each spike on the four corners of the rack, thus de-coupling the rack from the concrete floor. They were a perfect fit and the effect was both immediate and DRAMATIC. The system was for the first time tonally balanced, the bass response increased, the sound stage widened, the noise floor dropped, there was greater depth, increased clarity, and most importantly the brightness and harshness had completely disappeared!

I was firmly of the belief that audio racks should be coupled to the floor for stability and assist with the reduction of floor vibration eminating from the floor. My rack is a rigid design composed of tubular steel and every cavity is filled with sand in order to reduce any possible ringing. The rack is supported by four large adjustable screw in spikes which penetrate the carpet and couple the rack to the concrete floor beneath. The components are supported on MDF shelving. What I discovered this weekend is that this rack/floor interface was completely sucking the life out of the system. Upper midrange and top end frequencies were being accentuated at the expense of the lower mid range and bottom end, thus producing the fatiguing brightness and harshness.

Can anybody explain to me in laymans terms why this occurs?
unhalfbricking

Showing 9 responses by eldartford

eldartford
4,262 posts
 
Although I am skeptical about the need for exceptional vibration isolation/suppression for well-designed audio equipment, I can offer a few thoughts about vibration isolation.

In my business (testing of missile inertial guidance systems) for some tests vibration must be minimized. Our test stations, where the GS is mounted, are set on top of 3000 pound granite slabs, which are resting on pilings that are driven about 60 feet into the ground. These "piers" are isolated from the building.

Over the years we have learned that this type of construction is almost always effective, but occasionally is not. There is no way to tell before you build the pier. You build it, and then you test it. At our facility we have a room with five of these piers in a row. Four of them are fine, but one has so much vibration that sensitive optical measurements can't be done. I'm told that this type of pier can pick up ground vibration from trains passing a mile away.

Accordingly, I surmise that while a concrete slab is usually vibration-free, this cannot be guaranteed.
eldartford
4,262 posts
 
NZ has earthquakes that are of low intensity but more frequent than anywhere else on earth. How near to Rotorua (SP?) are you?
eldartford
4,262 posts
 
Unhalfbricking...As you may have guessed, I spent some time in NZ as a child, and still have the right to reactivate NZ citizenship. You have a great country.

I think that the worst NZ quake some years ago was near you in Dunedin. Seismic activity at a very low level is frequent which is fortunate because it relieves stresses and minimizes occurrence of big damaging quakes. Much earthquake research is done in NZ, which surprises most people because well publicized big ones are so rare.

If you have any of that super fine pumice sand near you, try that as a damping medium in hollow tubular legs of you stand.
eldartford
4,262 posts
 
Just an odd thought, but has anyone thought of hermetically sealing hollow tube stand components, and pressurizing them? This is a not-uncommon way of making such a tube more stucturally rigid.
eldartford
4,262 posts
 
impep_whatever...Yes, the pressure will stiffen up the tubes by prestressing the steel. I think that high performance racing bicycles have pressurized frames. It permits the use of lighter tubing, and for those bikes every ounce counts.

By the way, use nitrogen. Nitrogen is a byproduct of oxygen production (they distil liquid air) so it is inherently a completely dry gas, and quite inexpensive because you get lots of it when you make liquid oxygen. Co2 would work also. The main thing is don't use air because the oxygen could cause corrosion.
eldartford
4,262 posts
 
inpepinnovations@aol.com..You are right about the CO2, (a careless afterthought on my part) but wrong about internal pressurization stiffening up a structure. There are many examples. I picked the bike example because it somewhat resembles the tubes we are talking about. Blimps, Atlas ICBMs, and children's inflatable toys are others that come to mind. Heliocopter rotor blades are pressurized, but this is mostly done so that the pressure can be monitored for evidence of cracks.
eldartford
4,262 posts
 
inpepinnovations@aol.com...Sure, chopper rotor blades flex: that's why cracks are such a concern.

The Atlas ICBM structure is aluminum, not vinyl. It is kept pressurized at all times.

Yes, depending on the wall thickness of the tube, fairly high pressure would be necessary. But the stiffening effect will come into play long before risk of tube explosion.

Actually, I am not a big fan of super rigid TT mounts. I prefer isolation. If you are lucky enough to have a completely non-vibrating floor, I guess that coupling would work just as well. (But who is that lucky?)
eldartford
4,262 posts
 
inpepinnovations@aol.com...I guess your system is like a rigid rack except that you are coupled to the ceiling instead of the floor. If you are in a one floor building, this will avoid problems with people walking around your listening room. If you lived in an apartment house you would be at the mercy of the folks upstairs. Isolation would be provided if you replaced the wires with bungee cords, but that would introduce many new and interesting problems!

It was always said that Atlas pressurization was for structural reasons. The design is about 50 years old so I can't imagine that there is anything classified about it.
eldartford
4,262 posts
 
Stehno...Cables to the ceiling provide a rigid path to structure: it's just that the structure is rafters instead of joists. Either one could be stable or vibrating.

Airborne vibration, which I find to be my only significant problem, is absorbed by the mass of an isolated turntable/base. If the mass is high enough it won't be moved by the vibration. The isolation alternative is more practical IMHO because it does not require a floor (or ceiling) with no vibration.
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