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In the Forum: Audio Discussions
In the Thread: Mastodont DIY Rack: Wall Mount?
Post Subject: Mass/frictionPosted by N-set on: 12/5/2013
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 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




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