Making the subwoofer less disturbing to the neighbours?

I have an older subwoofer which I've had connected to my receiver for a while but I've never really used it because of my neighbours. I live in a small flat where the neighbours always knock through the ceiling when I use the sub, even on a low volume.

I've heard that you could place the sub on bricks to make it "vibrate" less through the floor (and walls). Is this right, or are there also other ways?

 

I had a similar problem with a flat below, so I tried placing my sub between two paving stones. This resulted in a slight reduction in vibration around the room.

However, putting some pieces of rubber between the sub and the bottom paving stone made a huge difference (at least my fireplace and windows don't rattle so much).

I'm not sure how much difference it makes to the neighbours but my sub sure sounds better

A decent thick rug on the floor/walls also helps.