As you can see, I've used louvre doors and a ladder with cushions from old couches as foam. Quick to rig up and no cost at all to try. The response doesn't fully reveal the result:
|Green - before Red - with traps|
Two main peaks are removed, but surprisingly dips are made worse above 100 Hz. The waterfall plots tell more:
While the response is fairly smooth, you can see evidence of modal ringing.
|Bass traps added|
Notice in particular how the decay rate is faster. Not only is modal ringing reduced, but the bass stops faster.
|Overlay of both plots|
How does it sound?
The bass is tighter and leaner and cleaner. It is the kind of bass you don't normally hear indoors. It sounds like outdoor bass. Tight and punchy. More dynamic. It's a definite improvement and not a subtle one. The difference is like you might experience in upgrading from a $500 subwoofer to $3000.
This change has got me pulling out a lot of CDs I haven't heard for some time. Tracks that I could never get to sound right in the bass have fallen into place. Unlike EQ which tames a small area, the improvement is global. Reducing peaks in the response with EQ is a definite improvement, but at times it looks better on paper than the actual audible improvement. However, bass trapping improves in the time domain as well and the result is more audible than the charts might suggest.