Using this for fun and profit

There are number of commercial products on the market that can help you with all of this. Unfortunately, they tend to be relatively expensive because the manufacturers also have the cost of labor and materials to worry about, plus the cost of shipping, advertising and so on. However, the devices from companies such as RPG Diffusers and ASC do work, and are worthy of your consideration.

Meanwhile, if you feel like trying it yourself, have some reasonable carpentry skills, and like figuring these things out, you can do a lot by yourself.

Sonotube is a common building supply material that is simply cardboard tubing in any length and a variety of diameters. I usually specify two feet diameter in the right length for the frequency the client is bothered by, and get them to hang it up near the ceiling, out of the way.

Soft corner traps can work well, particular if you can rig them to run horizontally along the ceiling wall line, where they tend to be a little more out of the way.

The flexural absorber is a real space saver, particularly if you also use it as a diffuser. You have to build a curving frame, and bend the masonite around it. Pack it with fiberglass and put it on the wall.

The goal here is to reduce the reverberation time in all frequency bands to a reasonably consistent and short period, say 250 milliseconds. How do you know if you need to do this with low frequencies? How do you measure this stuff? Sweeping with a sine wave oscillator and listening for and determining low frequency resonances is one simple and not very expensive way. Calculating room modes and treating them is another. Also, do you notice, over extended listening periods in the room, particular low-frequency bands that seem to be troublesome. A couple of years back I did a piece on quick and dirty acoustical measurements you can do. Check that out article for some other measurement techniques to help you locate the low-frequency big wave you want to hunt down and domesticate.

Happy trapping!