These are great for its price and if you cannot afford anything else more expensive.
I use these to separate my monitor speakers from my wood table.
However, it seems as if some bass frequencies (between 55 Hz to 120 Hz; I am no audio expert, so do NOT take my word for it) still penetrates through the foam pads and I can feel some very slight vibration.
This vibration is not annoying at all, and it is not like your hands are shaking or something. However, you might lose some quality or color from your bass because of this.
So, if you want to enjoy your bass more you should invest in something that separates your speakers more from where you have your speakers (from the wood table as in my case).
Otherwise, I recommend these cheap foam pads which are a must if you want to put any kind of speakers on hard surfaces.