Soundproofing/Covering HVAC vent

I have 5 total vents on one of my walls and am trying to come up with a way to cover/seal them while I am recording. The high frequencies are not leaking too badly, but I am getting a decent amount of bass leak. Any ideas? Thanks.

If you actually want to temporarily close them off, maybe an adaptation of this idea - http://www.johnlsayers.com/phpBB2/viewtopic.php?t=272 Otherwise, a complete re-do of the ductwork would be necessary; larger cross-section for lower air noise, Johns Manville "superduct" for ducts, at least two 90 degree bends per run, each run originating at the air handler and ending at room, no two rooms served by the same run, etc... Steve

you could get some really strong magnets and build vent covers around them.

That could work; with bass, you need mass and good seals whatever you do... Steve