“Our band recently built a drum riser to use on gigs. Now it seems like the drums feedback in the low end a lot more, especially the bass drum. What can we do?”
The drum riser is sympathetically vibrating (resonating) with the drums and/or PA system. Either way it is feeding these resonances back to the drums, causing the heads to ring a little more, which is being picked up by the mics, etc. Or it’s causing the mic stands to vibrate, which is picked up by the mics. The best way to stop this action – things resonating – is to add mass, which either changes the resonant frequency or dampens it altogether, or decouple the resonant items.
You can add mass to the drum heads themselves, but this will radically change the sound of the drums. The best way to fix it is to add mass to the riser. This means using really heavy woods and/or putting extensive bracing inside of it. There is a reason why high quality speakers are usually made out of thick, heavy woods, or sophisticated composite materials – to prevent or dampen unwanted resonances. Sand bags, bricks, scrap rubber from tires are all things that can be added to a drum riser to dampen it from vibrations in addition to heavy woods. Most good risers are made of steel, with a very heavy wood top. Similar to isolating a studio space there is no getting around mass to dampen vibrations. Short of that you at least have to fill up the air space underneath so you can dampen the air movement. A band I worked with years ago had custom cases made that doubled as a riser. Once they got most of their equipment set up they filled these cases with all the extra cables, soft gig bags, and other stuff, which made a big difference.
Isolation is another method that may prove helpful. Get good shock mounts for your microphones at the very least. Isolating the drums from the riser can help a lot, but it’s difficult to do. Rubber exercise mats, like what people use under treadmills, can help, but drum pedals will eventually eat them up. You can also isolate the drum riser from the rest of the stage with similar methods, but that’s a bit more hit or miss in terms of results.