Vibration Analysis: BL404 rack, Empire 598 turntable

[master_fo]

Here, I have conducted a series of experiments using an iPhone App called 'Vibration Analysis'.

Here is the pic of setup and vibration spectrum graphs observed at the floor, 3rd shelf, top shelf loaded and unloaded, and adding another same MDF shelf on top, making a sandwich with two layers of Dynamat between them:

 

[master_fo]

Now those graphs are all done with sandwich configuration above, from the top shelf, Empire plinth and platter:

 

[master_fo]

Same graphs as above, with Sorbothane added between the rack metal and the top shelf sandwich:

 

[master_fo]

Same setup with Sorbothane under the shelf, but instead of stock (well, generic hard rubber) Empire feet, I have used thin blocks of Ipe a.k.a Ironwood (same as under the rack on setup photo):

 

[master_fo]

Same setup with Ipe wood as above, but without Sorbothane between rack base and the shelf:

 

[master_fo]

Another set of experiment similar to previous two above - but instead of Sorbothane, silicone hemisphere bumpers were used between rack base and the shelf:

 

[master_fo]

Conclusions so far:
- MDF sandwich works better than a single (with dynamat on one side) MDF shelf
- Empire likes more rigid feet - will try ebony spikes next. I also did try silicone hemispheres on it directly before - did not measure and they seemed to sound fine, but would exaggerate footfalls.
- Silicone hemispheres work better than (this particular amount/shape of) Sorbothane under the shelf at those frequencies.
- I need to brace my rack to the wall to eliminate horizontal vibrations. Hopefully that would mostly get rid of 20Hz peak - suspension seems to do good job eliminating its horizontal components (that are much more audible) - but apparently, that comes with the cost of amplifying vertical vibration.

 

[master_fo]

BTW - I did not really expect that, but the system sound really changed already - I am getting much more articulate bass now with the silicone hemispheres and ipe block 'legs' ...
To experts - Is there usually any difference putting hemispheres flat side up or down ?

 

[Akustic]

Empiricism pays dividends ....

 

[dcgibby]

I've got the BL404 as well and I have a couple questions. Is your wood floor suspended or on a slab? Did you fill the legs with sand or the like? I'm not sure I fully grasp what's happening here, but it seems the rack is vibrating as expected. The addition of vibration damping materials isn't eliminating the vibration, but seems to be reducing the amplitude of the vibrations. Am I reading these graphs right?

 

[master_fo]

I've got the BL404 as well and I have a couple questions. Is your wood floor suspended or on a slab? Did you fill the legs with sand or the like? I'm not sure I fully grasp what's happening here, but it seems the rack is vibrating as expected. The addition of vibration damping materials isn't eliminating the vibration, but seems to be reducing the amplitude of the vibrations. Am I reading these graphs right?

Yes I filled the legs with sand. The floor is suspended. Yes the rack is vibrating as expected + adding horizontal vibrations because of lever effect. Some damping materials do help eliminating them. Sorbothane is rather tricky to get right for below 50Hz - for 40lb weight and 0.19" thickness/50 Duro, calculator says 0.2"x0.2" for 6 pieces, and they get squished 300%. Just a bit larger and it would amplify 5-20 Hz instead of damping.
For 0,5" thickness it's about 0.3"x0.3", I may experiment more with that, but silicone hemisphere feet seem easier to work with. It's proven better to add damping under the top shelf and keep TT coupled rigidly to the shelf.

 

[master_fo]

Also: the isolation factor in fact looks better with rubber TT legs than the ipe blocks - however, it does sound better with ipe (more rigid coupling, while still very good damping factor of 0.5). Apparently there are 2 things happening at once in reverse directions with structural-borne floor vibrations and TT's own vibrations while playing music. Or maybe a slight positive feedback at ~30Hz makes bass sound fuller. I will do some experiments playing sweeps on the turntable to check that out.

 

[master_fo]

Bolted the rack to the wall. No more problems with footfalls. None.

 

[master_fo]

This is how the platter vibration looks now.

 

[master_fo]

Another update: most efficient way to deal with feedback:
https://audiokarma.org/forums/index...-suspension-vibration-feedback-update.985777/