There are a number of separate issues regarding the mounting of speakers on spikes and footers. Some folks may need to decouple their speakers from things like wooden floors which are bouncy.
I have seen some people report that sorbothane footers were best with wooden floors.
But to my mind and ears the more serious problem is cabinet or headphone case resonances causing audible distortion. Spikes, footers and mass can help here plus what I am more concerned with is called "constrained damping" of the enclosures, using sorbothane.
My main system sits on ceramic tiles laid on concrete and I find the spikes of some use. I also use a rigid brace fastened to the wall, which was originally put in to prevent the speakers toppling over in earthquakes (I am in SoCal.) This also helps the sound rather like the spikes.
However the biggest problem is enclosure/cabinet resonances. What I think is going on here is that energy from the drivers enters the enclosures ( Newtons third law every action has an equal and opposite reaction) plus conservation of energy states that energy
can only be transformed or transferred from one form to another. I assume that the driver created energy in the enclosure does eventually dissipate as heat or we would be hearing it for a long time, but it does not do this fast enough to take it out before it feeds back into the driver as rubbish sound . Adding mass, such as lead shot, means more material to dissipate heat plus it probably prevents the speakers from wobbling in response to the motion of the drivers impacting air. Spikes would seem to essentially couple the enclosure to a greater mass thus having somewhat the same effect as adding lead shot.
The best results by far which I have found are adding sorbothane glued to the front panel of the speakers behind the covers or to various places on headphones. Sorbothane's claim to fame is that it converts mechanical energy to heat. I worked some my techniques out for applying sorb with old Stax SRXIII pro headphones with interchangeable covers so that I could quickly swap around different types of material. Some of the results were counter-intuitive.
Firstly use the densest sorb (called duro) I use 70 duro which is the densest I have found. This was recommended by a tech advisor at Sorbothane and I can confirm that it seems to be right.
Secondly, use small pieces, on speakers I use pieces less than 1.5 inch in any dimension, for headphones .5 inch. Frankly I don't understand why but the size effect keeps coming up.
Thirdly, in spite of the small sizes, use thick sorb, 1/2 inch or more.
Fourthly the sorb has to be properly glued to the surface. My 1/2 pieces are glued using a fairly costly ($35/can) Lord glue recommended by Sorbothane. If you use thinner sorb it often comes with a 3M double stick backing which seems pretty good too.
Finally the impact of the sorb is greatly increased by "constrained damping," ie putting some kind of backing on the unglued surface. I find 4 layers of electrical tape is quite effective. I started with clamping the sorb to surfaces but generally found I had to use the least pressure and in the end just moved over to constrained damping.
I would say that the effects are far in excess on anything other tweak or upgrade I have heard. I have taken the cheapest ear phones and made them sound like high-fi with 10 cents worth of sorb glued to them.
What exactly am I hearing from these mods? Firstly increased dynamics, I recall my first experience was that I started rocking in time to music since the rhythm was far more pronounced. Secondly greater amounts of timbre, instruments started sounding more realistic. Thirdly, better channel separation (with headphones.) Evidently sound from one earphone can travel across a headband and be heard in the opposite headphone. In fact a man named, I think Mitchell measured this effect some years back for a British audio magazine. Interestingly it was obvious with headphones that the sorb was lowering the audio output of the phones somewhat necessitating turning the volume up. This I consider to be a sign that the sorb is actually removing a considerable amount of the energy in the case/enclosure which would otherwise be there adding distorted sound back into the drivers.
I doubt that the resonance in the case/enclosure is itself audible. Wooden boxes and metallic or plastic cases do not make good drivers. Rather the sound in the case/enclosure feeds back into the drivers as Mitchell found.
Plus this is a cheap way of improving sound, ignoring the cost of glue it takes dollars or less to add this to headphones, although with large speakers you may be spending a hundred or more dollars.
If you doubt me, I suggest Googling terms like "constrained damping." I have come across several references to this being used in speakers. As regards headphones, Sennheisser has been using a "polymer" damping in the headband of its TOL dynamics and I suspect something similar in its electrostatics. (BTW sorb is a polymer) Grado has its own polycarbonate that it claims works somewhat the same way.
