GoodSoundClub - Romy the Cat's Audio Site

In the Forum: Audio Discussions
In the Thread: Mastodont DIY Rack: Wall Mount?

Post Subject: Mass loadingPosted by N-set on: 7/26/2013

Romy the Cat wrote:
I do not know if “stupid, indiscriminative mass loading is a road to nowhere”.

Let me try to explain why I think so. Adding mass (e.g. sand+shot loading of steel profiles):

1) makes the vibration amplitude smaller for a given excitation energy; that's why optical tables are
so heavy: they need nanometer stability; the same footfall or strong musical signal
will produce much smaller vibrations with half a tone rack than with an Ikea table...unless you hit a
high Q resonance; see below

2) dumps higher resonant modes (audio range), but the added mass shifts the basic, structural
ULF modes lower and makes them less damped, much more difficult to filter out.

Having no experience with "listeting to the racks" as an audio activity, nor wanting to have one,
I speculate that with electronics those ULF vibrations are much less of a concern (excpt maybe for big DHT's)
than with TT. With TT ULF vibrations may manifest themselves as an W&F.
Now, there is no one single universal damping device taking care of all the vibrations (those optical isolation legs are in
fact 2-3 devices in one), and, just like with acoustic systems, damping should be viewed holistically, as a
combination of different devices/material designed for different modes/frequencies. If my main TT isolation
element is an air bladder with 3-5Hz fres., I don't want my rack to have it's stronegst vibrating
modes in that range. This, together with shifting of fres. of the steel construction by mass loading creates a potential
conflict. In my version 1, I believe the main resonant modes because of the heavy mass loading
were pretty much in the 3-5Hz region. Now, in v2 with additional bracings, filled with much lighter perlite, there are
somewhere higher waaay better dumped and much smaller.
The main pendulum mode (the rack wobbles on the spiked points) is still at 1-2Hz and I have to live with that.

Cheers,
N-set

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