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

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12-04-2013

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N-set
Gdansk, Poland
Posts 609
Joined on 01-07-2006

Post #:	51
Post ID:	20341
Reply to:	20340

Ringing

Rings=poorely damped, due to its crystaline structure. Take a granite tile with two fingers, put your ear close to it and knuckle. Observe the sound.
Repeat with other stones to compare. For that reason it's neither used in nanolabs anymore (see e.g. the Newport link)
nor in, say, billiard tables, not ot mention TT plinths. But looks good on graveyards Smile

Cheers,
Jarek
STACORE

12-04-2013

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N-set
Gdansk, Poland
Posts 609
Joined on 01-07-2006

Post #:	52
Post ID:	20342
Reply to:	20332

Vibrational jungle

N-set wrote:

A very approximate calculation shows that in my case I should have the profile fundamentals around 20Hz, so
perhaps what I take as subsonic vibrations are actually 20Hz, poorely damped due to the added sand mass.
I've ordered a dirt-cheap guitar piezo and try checking that.

It has arived today. What a little, freaky gadget! Bloody sensitive to EVERYTHING, I had to sit a complete silence
and still, attached to the frame with a masking tape it was catching the slightests sounds, like my laptops vent...
So what it catches is a mix between vibrations of the frame and airborne vibrations.

Spending some charming 2hrs positioning the piezo and endlessly hitting the rack and the floor with the base of my palm,
I got some glance on the vibrational frequencies:

the top of the rack, hitting the bottom: 2-3 (?) , 5-7(?), 15, 30, 50, 75, 150Hz

The most pronouced (with an unknown piezo response curve of course) were 30 and 50 Hz depending on how I hit the rack.
The lowest two I'm quite unsure of (the piezo does have an output below 20Hz, it reacted happily to
some idiot testing his motor outside, showing 10-15Hz broom). Might be the vertical and the horizontal modes of the airsprings.
As to the rest I don't know to what extent they represent the frame frequencies and to what the airborne vibrations (the dull
sound of my palm's base hitting the rack).

here are the floor freqs alone: 12, 25, 30-50, 100-200Hz (the last two are whole regions, shown with 256k FFT!)

If all that is true, than avoiding floor resonances is like avoiding raindrops biking in a heavy rain. Of course
the rack is heavily coupled to the floor via spikes hence the "rack vibrations" come from
the rack-floor-air intercation.

Next I'll try putting the TT and see how it reacts to footfalls.

Chhers,
N-set

Cheers,
Jarek
STACORE

12-04-2013

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Paul S
San Diego, California, USA
Posts 2,577
Joined on 10-12-2006

Post #:	53
Post ID:	20343
Reply to:	20342

Harmonics

12(.5), 25, 50, 100, 200... Now you just need to track down the dominant mode. 25 sounds about right for a floor mode, but a long, loaded joist span might be way down there. And we still don't know how principal modes are "translated" by your rack. A "grouping" around 20/30 might be span + cavity resonances. I wonder how much hitting the rack feeds the floor/cavity(cavities).

Best regards,
Paul S

12-04-2013

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Romy the Cat

Boston, MA
Posts 10,052
Joined on 05-28-2004

Post #:	54
Post ID:	20344
Reply to:	20341

Observe the sound… with your brain or ears?

N-set wrote:
Rings=poorely damped, due to its crystaline structure. Take a granite tile with two fingers, put your ear close to it and knuckle. Observe the sound.
Repeat with other stones to compare. For that reason it's neither used in nanolabs anymore (see e.g. the Newport link)
nor in, say, billiard tables, not ot mention TT plinths. But looks good on graveyards

N-set and others…

Even I do not disagree with all that you writhe here generally and on the subject in particularly but I would like to remind you that all your comment about racks, suspension, desolation and everything else do not have direct correlation with sound. It might have but it also might not have. I certainly do not want to diminish the value and the efforts you spend on your rack and it is very good project but the relation between the auditable benefits and purely intellectual investment you made into your rack is not so simple. Your complain about granite is a good example of it.

I am with you and I do not like granite. I do not think that it rings, come on get real. It in my experience adds some “coldness” to sound but only in some cases, in other cases it does not. I spent a lot of time listening shelves, equipment, chassis’, isolators, platforms, suspenders and anything you only can only imagine with stethoscope and heard how different type of vibrations get propagated, absorbed, dissipated or consume by different surfaces. However, during all my experiments I was not able to observe the obvious correlation between the quietest chassis and better sound. It was always case by case and in some circumstances it were absolutely absurd results. If I tell you that I know one DAC with which I was able to put the cone under and by moving the cone with 1-2 inch I was able to attenuate HF response within 2dB then you will tell me that I am a fool of it. Well, I demonstrated it to one famous industry person and it in fact very true story.

So, what I am saying is that all that we do and discuss about the subject has only intellectual stimulation. What we say/do on the subject is not wrong but it is not sufficient to be right. The rightness comes from the actual Sound we accomplish and it is not always Copley with your best intellectual objective and methods. Sometimes, live, ringy and shaky platform sound the best in this or that applications and all theories need to be set aside. I do not know the answer but I think not to have precompiled answers is the best approach.

"I wish I could score everything for horns." - Richard Wagner. "Our writing equipment takes part in the forming of our thoughts." - Friedrich Nietzsche

12-04-2013

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N-set
Gdansk, Poland
Posts 609
Joined on 01-07-2006

Post #:	55
Post ID:	20345
Reply to:	20344

I could not agree more...

...with Romy. So far it has been all abstract, following some known good solutions (like Vibraplane)
...and it will largely stay like that for some time unfortunately.
My actual space is too small to build a reasonable acoustical system, I use headphones, so the rack will not see the
the real acoustical field soon, at this moment it's goal is to deal with the floor.
Anyway, I do what I can given the circumstances. Perhaps foolishly, perhaps wisely...I have no idea.
I hope to have a good base which I would only tune when it starts working embedded in an acoustical field.

Cheers,
N-set

Cheers,
Jarek
STACORE

12-04-2013

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N-set
Gdansk, Poland
Posts 609
Joined on 01-07-2006

Post #:	56
Post ID:	20346
Reply to:	20343

Floor feedback

Paul S wrote:
I wonder how much hitting the rack feeds the floor/cavity(cavities).

Paul, standing barefoot right next to the spikes and hitting the rack with my palm's base moderately
(like if I wanted to push a bit blocked door), I can feel some faint floor vibrations, so they do communicate
through the spikes.

Cheers, N-set

Cheers,
Jarek
STACORE

12-04-2013

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Paul S
San Diego, California, USA
Posts 2,577
Joined on 10-12-2006

Post #:	57
Post ID:	20347
Reply to:	20344

Brain or Ears

Romy, I'll bet everyone has been thinking like you all along (I know I have), but N-set has already dismissed simple "results-oriented" solutions as "ad hoc" and indicated that he is determined to work with his rack as it is and understand each action/reaction as it regards his original design, and then move on from that knowledge base. N-set, at this point I have assumed that this is simply because you want to understand what is going on, in the spirit of scientific inquiry. And I certainly appreciate the desire for repeatable results.

Best regards,
Paul S

12-05-2013

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N-set
Gdansk, Poland
Posts 609
Joined on 01-07-2006

Post #:	58
Post ID:	20348
Reply to:	20347

Yes and no

Paul, I did not dismiss result-oriented solution. I'm fighting hard to get one actually.
Honestly, I initially hoped that brutal mass loading will be enough to deal with what I have,
and as a bonus I'll have a +/- universal solution I could move when I change the location. Well, the live is not so simple
and having invested quite a lot in this mastodont I'm a bit slow to trash it. That's why I dismissed wall fixing as ad hoc in this context.
As for repeatable results and understanding,
you are right, I wanted to have some base, at least to know what is shaking and how to move on with sound tests.
And this is what I'm about to begin.

Cheers,
N-set

Cheers,
Jarek
STACORE

12-05-2013

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

Post #:	59
Post ID:	20349
Reply to:	20347

The science

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. Once decoupled, the high mass would be damping, rather than acting as a reservoir of kinetic energy for the transmitted vibrations, especially if when decoupled the perturbing forces are non-stationary, as I imagine will be the case (random footsteps, something falling nextdoor, etc). The situation seems to me analogous to sound proofing, where what works best is sandwiching a high mass layer between two decoupling layers, no?

12-05-2013

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N-set
Gdansk, Poland
Posts 609
Joined on 01-07-2006

Post #:	60
Post ID:	20350
Reply to:	20349

Mass decoupling

Decoud, if you look carefully at what I'm doing you will see how and where the table is decoupled.
The logic behind the high mass I've already explained. I was unaware though how tricky it is to implement-this is my 1st construction.
The logic behind decoupling where I have it is to decouple also from the support structure vibrations--the closer to the
TT the better, I think it's clear. The spikes I've already discussed -- I'll try wide feet instead of pointed spikes.

Cheers,
N-set

Cheers,
Jarek
STACORE

12-05-2013

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N-set
Gdansk, Poland
Posts 609
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 609
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 2,577
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 609
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 609
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 609
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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