I'm going to add something to this thread - as a sort of editorial comment - rather than as a reply to anything in particular.
(But let me start by saying that I second everything that MarcL had to say about the subject.)
The whole subject of speaker "isolation" is an interesting subject...
And, sadly, it is also an area where there is an awful lot of snake oil, and a lot of ideas and claims that are even goofier than the usual audiophile goofiness.
(Often specifically oriented around the idea that some magical feet "carry unwanted vibrations away, on a one way trip, and make them magically disappear".)
There's this little thing called "an equal and opposite reaction".
Because of this, if you have a normal subwoofer, with a single driver, when the driver moves forward, the cabinet wants to move backward, and vice versa.
If the subwoofer is sitting on a carpet this may actually cause the cabinet to move back and forth a little bit.
This can both produce unwanted and unexpected output... and may actually act to cancel out some small part of the output of the subwoofer.
If you have some sort of spikes or something similar then the subwoofer is "pinned" to the floor so it essentially cannot move relative to the floor.
Arguably, having the cabinet move back and forth can produce unwanted sound output, so avoiding that is a nice idea.
However, depending on your floor, by pinning the subwoofer to the floor, you may end up causing the entire floor to move a little bit...
This could cause unwanted sound output... from the floor...
Or it could actually contribute to getting more bass (the floor becomes a secondary sound radiating surface).
The point here is that you cannot make all of that energy magically disappear (although Sorbothane can actually do so - but mostly at higher frequencies than we're talking about.)
Now, all of that really pertained to subwoofers, and low bass, but there is a second thing: vibration... and this applies both to subs and regular speakers.
Rather than movement of the entire cabinet as a whole vibrations tend to "ricochet around the structure of the cabinet".
And, because of this, they may radiate from any or all surfaces of the cabinet...
For example you may have sound that was supposed to come from the midrange coming from the sides of the box too.
And this can have a major impact on imaging and sound stage.
With a speaker sitting on soft feet, or on carpet, there's really nothing to discourage this.
With spikes, some of this energy will be coupled to the floor, which might cause the speaker to vibrate less...
But it also may turn the floor into a secondary source for that sound (a "sounding board")...
And, again, depending on the details, this could be awful, or might actually make a positive contribution.
Now, to be quite frank, hard feet, or "spikes and plates" or "spikes and cups" are NOT going to magically make this vibration disappear.
And, while it's possible for a design to be vaguely "directional" it really doesn't work that way in real life.
A vibration is the mechanical equivalent of an AC signal... it goes back and forth... and you can't make it go in one direction.
(If you could make all the vibrations only go down, which might be called "mechanical rectification", and is a real thing, then the speaker would end up rising into the air... right?)
NOTE that "balanced subwoofers", the kind with two opposing drivers, cancel out the "equal and opposite reaction" at the source...
And virtually all speakers have some sort of mechanical dampening to absorb or otherwise prevent excessive vibration already.
SUGGESTION....
If I REALLY wanted to do the utmost to reduce a speaker from vibrating... while NOT making the floor vibrate...
I would want a relative heavy solid platform sitting on the floor - probably with spikes - with a sheet of Sorbothane between the platform and the speaker.
(Sorbothane is the only practical commercial material that actually absorbs and "destroys" vibration - by turning it into heat by way of internal friction.)
(Sorbothane is a very special sort of "squishy urethane rubber" that is designed to actually absorb vibration. Sorbothane is a product brand name - there are knockoffs.)
(An alternative would be to sit the speaker on a two inch layer of sand in a shallow box - but that would be somewhat messy and impractical.)
Either would do an excellent job of dampening the vibrations from a speaker (but would be much less effective at preventing the "equal and opposite" movement of a sub.)
Notes:
- Sorbothane, and even the "off brand equivalents", can be somewhat expensive (but not nearly as expensive as some silly "audiophile feet").
- You CAN purchase Sorbothane in sheets of various thicknesses and self-adhesive sheets are available (try eBay or Amazon).
- You can also use multiple separate "pads" or "feet" between two solid platforms. (Sorbothane comes in sheets, pads, washers, and various other forms).
- Sorbothane is somewhat sticky, and so will mar wood surfaces, so should NOT directly touch wood surfaces you care about.
- Sorbothane has specific viscosity and WEIGHT ratings. (With Sorbothane these REALLY do matter.)
(If you purchase a sorbothane sheet, or multiple small Sorbothane pads, they must be MATCHED to the load you plan to put on them, or they won't work well at all.)
(A given piece of Sorbothane will be rated for a specific weight RANGE. If you go much less than that, or much over the maximum, it doesn't work nearly as well.)
(Remember that weight is additive - so, if you have a fifty pound speaker, you want a pad rated at fifty pounds, or four 12.5 pound separate feet.)
- Sorbothane is squishy and somewhat soft... so you need to pay careful attention that your overall design doesn't turn out "tippy" or "wobbly"
(This could be an issue if you were to make a small platform - with the top and bottom separated by several relatively thick "pads" or large "hemisphere feet".)
- If you Google Sorbothane you'll find all sorts of detailed manufacturer's application notes...
