But having concrete walls, you might need to add some large dampers.

Some people use round tubes filled with absorbent stuffing, but I prefer panels. They're simple, really, basically adding framed walls inside your concrete walls. You'll make a sort of a false wall spaced several inches out and fill it with insulation. You don't want the panels to be braced, you want them to vibrate. Their size determines their resonance, which in turn determines the frequency they'll damp the most. Their effectiveness is also a function of their position, because you want them to be placed on nodes. That's where they'll damp the most. Then again, for aesthetics, it might be best to make a false wall span the entire length of each wall with panels sized to damp the frequencies that are most troublesome. You can find your room modes by measurement or with a program like CARA.

• DIY bass trap panels