Turnable Rack Vibration

Kenny: When you say the Target Pro shelf "flexed under the load" are you saying that the rack itself flexed or the shelf material (MDF shelf).

If it's the rack itself that's flexing, I'd say dump it. If it's just the shelf material you could experiment with a number of different types of shelf materials (some more expensive than others-but some very cheap) that will give you different sound qualities and definitely will not flex.

The MDF is pretty crappy and a definite weak point with the Target rack, both from a structural and sound quality standpoint.

A couple of comments:

When I think of Target wall shelves, I'm thinking of their original design with the angled side braces. This "triangulated" design is a structurally braced shape, deriving its strength from its geometry.

The current Target design(s) use a "box" frame which is only as good as the welded connections, the shear strength of the weld itself, and the metal tubing's ability to resist bending. If I had to buy the current model, I'd take it to a welding shop and have some angle bracing installed on each side. Just look at Billy Bags designs -- every damn thing (except their little 2020 amp stand) has diagonal bracing.

Steel is the ONLY material for the BEST structural rigidity. Titanium and magnesium are better in the strength vs. weight department -- not an issue in audio (usually ;--)

A truly rigid connection between dissimilar materials is impossible because it will be mechanical, not monolithic like a weld. Further, mechanical connections almost always involve a third material like glue, or screws. This eventually leads to loosening of the joint, due to movement, structural stresses, thermal expansion, and variations in humidity. Anyone who thinks they can make truly RIGID structures out of any material that can't be welded is just nuts IMO; they are depending on intuition without knowing anything about physics and math. Intuition is useless, even when you get a lucky break, because you can't go back and figure out why it worked! (Simply PLACING a wood, corian, acrylic, granite, etc. shelf on some kind of rigid steel frame is not what I'm discussing here.)

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Acoustic isolation of a TT or other object from airborne sound (pressure waves) is another fantasy. Oh yes, sonic pressure waves do indeed exist, but they exist in a gas (air) which means they propagate in all directions -- unless you are literally on top of the source. And they exert the same pressure on all surfaces of an object, if that object is fully inside the space where the sound occurs. Floors and walls are a different story. When they vibrate, it's because the pressure on the other side is different; usually lower.

So if a TT is "dancing to the music" it's because the structure that it rests upon, (along with everything ELSE one has contrived to stick under it,) is all already resting on a moving platform: the floor!
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Having faced this issue countless times, the only technique that consistently works is to fasten a 90 degree shelf bracket (which you can buy or fabricate yourself) to the top shelf at the back and to the wall. This will cure 95% of the problem.

Failing that, even wedging something between the wall and the back or the rack/shelf can help a bit, until it falls. :)

A vertically suspended turntable such as a Linn on the top shelf that is susceptible to footfalls is actually having to contend with some horizontal movement, which it cannot handle.

Hornguys, you are talking about what's generally referred to as a "tie back"; tying the top of something that's swaying, like an equipment rack or a tall panel speaker, to something that isn't swaying (we hope) like a wall! This can be a good solution. It depends.

Let's take racks first. If a rack is on a concrete slab floor and it still sways, it is a poor design. The worst offenders are the threaded rod type, because the rods themselves aren't fat enough to resist bending and there's virtually no moment resistance in the rod/shelf connection. There is a way to cross brace these racks, but for the time and money involved, sturdier racks are available for less money.

If the rack is quite sturdy like a cheap Studiotech, or a pricey Billy Bags, it can still sway left-right, or front-to-back on a joisted wood floor. L/R if the joists run perpendicular to the wall behind the rack, and F/B if they run parallel; ditto for loudspeakers.

Speakers should not be allowed to sway F/B even a millimeter (at the top) because it can ruin transient response. If a speaker sways a little bit L/R (joists running perpendicular to the wall behind) it's not a big deal performance wise.

Unfortunately, the tie-back solution is mostly effective against F/B movement. Much less L/R movement, and not at all for vertical movement. I have my MartinLogan panels on Sound Anchor stands which have triagulated bracing (see system) but before that, I had their tops tied to the back wall by a 6 foot length of 3/4 inch white PVC pipe. The improvement in soundstage and lyrics intelligibility was amazing!

My previous apartment had a cement floor, so I never had a problem with my TT on top of the rack, but I moved into my girlfriend's pre-war apartment and I got bad vibrations if anyone walked near the rack.

This past weekend I put up a Target Pro shelf and it works great- you can jump up and down and nothing happens. I recommend going back to the shelf.