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Home » Audio Discussions » Mastodont DIY Rack: Wall Mount? (69 posts, 4 pages)

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12-05-2013

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N-set
Gdansk, Poland
Posts 617
Joined on 01-07-2006

Post #:	61
Post ID:	20351
Reply to:	20349

Spike logic?

decoud wrote:
It seems reasonable to begin with the science -- as a preliminary to listening -- but I do not follow the logic here. The source of the vibration is principally the floor: the table should therefore not be coupled with spikes but decoupled from the floor.

Since you evoke some logic here, albeit post factum when I already found out and stated several times that the spikes
couple and even how they couple, let me ask you something out of pure curiosity. Floor is always one of the major sources of vibrations,
why is then every single rack, including Romy's custom high mass RixRax, spiked?
I've been publicly wondering about that and I don't know the answer. Maybe you do?

Actually I plan to float the whole electronics cage on the airsprings, if the ones under the TT prove to work well, rather than spiking it.
I did not float the whole TT stand at the floor level to avoid a double compliance system with it's potential to beat. Somewhat similarly to e.g.
Romy's style power supplies: RCLC, rather than LCLC, with the last C close to the decoupled stage.

Cheers,
N-set

Cheers,
Jarek
STACORE

12-05-2013

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decoud
United Kingdom
Posts 247
Joined on 03-01-2008

Post #:	62
Post ID:	20352
Reply to:	20351

More logic

N-Set, I do not claim to add to the facts you have already comprehensively given, I am merely asking about the logic, and I do not suggest to have a definitive answer, hence my ending with a question. I can see three reasons why other people use spikes: 1) for stability against rocking on the base 2) for where there are spatially distributed differences in the amplitude of the floor vibration so that one might thus find a local minimum and, most of all, 3) where vibration of items *on the rack* needs to be damped by coupling to a less vibrating floor. None of these seems to apply in your case because you have a properly levelled installation, directly on concrete and the vibration to be damped is mostly external. But in any event what people do is no guide to the science, which is what we are talking about here, and as you say you do not see optical tables with spikes.
I see the issue with double compliance, but this is the point about the nature of the vibration: for resonance to become an issue here the vibration has to be more or less stationary, no? And the kind of vibration you need to deal with here is mostly non-stationary (unless your neighbour's washing machine gets stuck in a spin cycle). So what is more important is increasing the damping, which can only be a matter of energy absorption. Mass damping stores it, friction damping dissipates it: so better to increase the friction component than the mass, no?

12-05-2013

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N-set
Gdansk, Poland
Posts 617
Joined on 01-07-2006

Post #:	63
Post ID:	20353
Reply to:	20352

Mass/friction

decoud wrote:
None of these seems to apply in your case because you have a properly levelled installation, directly on concrete

If I had it on a concrete I'd be very happy! On wood it is, mafrend, on wood.

decoud wrote:
I see the issue with double compliance, but this is the point about the nature of the vibration: for resonance to become an issue here the vibration has to be more or less stationary, no?

No, single excitation also excites vibrations. I hit the rack with my palm, this is a single excitation. I do not do it repeatedly.
Same in electronics: send to a resonant circuit a single impulse and it will excite the circuit (provided the impulse is fast enough, i.e. when artificially repeated will have the fres in its spectrum)

decoud wrote:
So what is more important is increasing the damping, which can only be a matter of energy absorption. Mass damping stores it, friction damping dissipates it: so better to increase the friction component than the mass, no?

This is complicated.
Sand is a very good friction damper, but it adds mass, so its both in your language.
Added mass in turn lowers the fres and makes it less damped as the sand does not
work on ULF, to the contrary: it can solidify and bridge the vibrations across the filled beam. So while covering most of the spectrum, sand
can create troubles at ULF. That's why I took the pains to cut my intitial rack and add 10x8cm double horizontal braces, reinforced with 12mm steel triangles in the corners. Only structural stiffness works against ULF. The resulting frame is still alive and I tried to understand why, but perhaps
it's not important sound-wise and I'm doing a typical intellectual masturbation. Very possible.

BTW, in the upper frame I tried perlite: ultraligt and very porous material, meaning very good friction at low mass.
Unfortunately it does not damp the whoe spectrum: there is a faint high pitch ringing left in the profiles. How that would be important
sound-wise when embedded in the acoustical field I don't know.
But given the impossible-to-avoid floor resonances I decided not to put perlite or its ore in the rest of the frame and test it as it is.

Cheers,
N-set

Cheers,
Jarek
STACORE

12-05-2013

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Paul S
San Diego, California, USA
Posts 2,658
Joined on 10-12-2006

Post #:	64
Post ID:	20354
Reply to:	20353

Full Circle

Well, I'm not so sure that the "friction from perlite or sand" inside the frame tubes is the answer. Yet, the way I picture "wall fastening", it is not to suspend the frame off the floor but it is simply friction or frictions critically applied to the frame, once the frame is better isolated from the suspended floor. And to me the beauty of it is that it might be done without knowing much about anything other than results. For instance, one might "strap" the frame to the (masonry) wall against a "pad". At this point I would think anything to get down to tuning the air bladders would be welcome. You can always revisit the spikes if/when you ever set up on a slab. As for "everyone else always using floor spikes"... come on...

Best regards,
Paul S

12-05-2013

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decoud
United Kingdom
Posts 247
Joined on 03-01-2008

Post #:	65
Post ID:	20355
Reply to:	20353

Surface vs depth

But is the wood not sitting on concrete, through which the vibrations from elsewhere in the building are conducted? If so I suppose the wood and the concrete will make differential contributions, and you might want to couple more proximally to one but not the other.
My point about the resonance is that it will accumulate only where the signal is stationary: the fres is always less than the single impulse, no? So as long as you are dissipating energy you cannot get an enhancement of the vibration you are putting in, only a dispersion in its spectrum.
For sand to work here in friction does it not have to be load bearing itself: i.e. inside a steel cage that propagates the vibration through the walls there will not be much friction, no? What you want is a sandcastle.....

12-05-2013

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N-set
Gdansk, Poland
Posts 617
Joined on 01-07-2006

Post #:	66
Post ID:	20356
Reply to:	20355

Sand damping

decoud wrote:

For sand to work here in friction does it not have to be load bearing itself: i.e. inside a steel cage that propagates the vibration through the walls there will not be much friction, no? What you want is a sandcastle.....

Vibrating steel walls set the sand grains in motion. They dissipate the energy rubbing one against the other, down to some frequency when the sand
sarts to behave like a solid. Ever run on a beach?

I start to get Paul's idea I think. I could couple the rack to the wall through some kind of elastomer/oil dampers, used e.g. in motor bikes...interesting!

I'd like to end theoretizing at this point, change the oil in my TT, sit down and listen. Will report how the rack works in practice in few days.

Cheers,
N-set

Cheers,
Jarek
STACORE

12-05-2013

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Jorge
Austin TX
Posts 141
Joined on 10-17-2010

Post #:	67
Post ID:	20357
Reply to:	20356

Confined sand

I used to have a rack where sand was placed inside the structure of the rack (square pipe) and then I would insert a rugged construction rod inside it so it would be "suspended" by friction inside the sand. It didnt work, there was no advantage, vibration actually gets the sand more compact and the isolating effect dissapears.

For sand to do its job it should be in as free space as possible, hence the sand box idea.

About the Sorbotane, I am using the type they use to "glue" windshields in cars. I got it by the roll and just cut a bunch of stripes and place them under the wood, I left the waxed paper on it so it wouldnt stick to the rack shelve. It holds it shape nicely and works good for vibration.

12-19-2013

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N-set
Gdansk, Poland
Posts 617
Joined on 01-07-2006

Post #:	68
Post ID:	20385
Reply to:	20356

First sounds

Ok, it took me longer than I thought (fucking servicing an EMT is a nightmare for me),
but I've finally put the TT on the rack:

It's all still a bit provisorical as the inner cage is not fnished yet (the suspension is missing), but I could test the
pneumatic table. It works! I can jump on the floor, kick the rack, hit it, etc, no audible feedback.
Good. Those tons of steel and sand do seem to work, at lest in the no-acoutic-field condition.
The spectrum analyzer sees an increase in the few Hz-100Hz region during a heavy footfall, but it's inaudible.
Soundwise I was not expecting anything but as a little present
I got a quieter background, the sounds seems "cleaner" (paradoxically at MF/HF), better articulated.
I've also changed the plinth from plywood to slate (the one in the picture).
The change is subtle (I'm also too tired to make a decent comaprison),
but the plywood seems more "wooly", slate more "sparkling" although I think
it transfers a bit more rumble. Not to say I prefer rumble, but the sparkling nature of the slate
appeals to me so far.

Next: finishing the insert (I mount a pneumatic suspsension under it too)
and checking cones vs large feet on my floor. I also have the ball bearings already
machined but so far little motivation to try them.

Cheers,
N-set

Cheers,
Jarek
STACORE

02-05-2014

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N-set
Gdansk, Poland
Posts 617
Joined on 01-07-2006

Post #:	69
Post ID:	20539
Reply to:	20385

Finished

It's finally ass-embled! All in all I'm quite pleased, but I have to live with this mastodont in my room for a while to get used to it.
Now I move to listening and repeat some measurements.

The insert is on it's own independent pneumatic suspension:

and the spikes were changed to 120mm dia feet as it feels moe stable like that (less of an inverted pendulum) and preliminay measurements
seem to confirm that.

There is also a provision fo grounding those tons of steel, let's see if it changes anything.
I hope I'm now +/- done with the vibration control (apart from trying nice steel feet under the
TT plinth). Next comes a dedicated power to the rack!

Cheers,
N-set

Cheers,
Jarek
STACORE

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