Once a year me and some friends organize a party for creative computer enthusiasts (aka demosceners). Since me and one of the other organizers also like to dj with vinyl, we use the opportunity to play a set on Fridays. However, we've had problems with feedback from the turntables two years in a row, and haven't found a way to avoid it yet. We've taken some countermeasures to reduce vibrations from the table/floor, but it seems like the problem actually is air vibration and not feedback coming from the table (we tested holding one of the turntables up in the air, but that didn't do anything). Apart from having a heavy and dampened table (we use a couple of sleeping mats, which resembles acoustic foam), what can be done about feedback? Are some cartridges/styli better than others , does slipmat thickness have any effect, and so on..?