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In the Forum: Audio Discussions
In the Thread: Mastodont DIY Rack: Wall Mount?
Post Subject: Vibrational junglePosted by N-set on: 12/4/2013
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 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

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