It’s caused by hard surfaces – concrete, glass, hardwoods, etc. - and parallel walls (i.e., a rectangular room). Not much you can do about the latter, but for the former all you need to do is apply acoustic treatments – like carpet, for an extreme and unattractive example. Draperies, tapestries, etc. are other options.
Thanks for the help! I think it's due (mostly) to the rectangular-ish shape of my room, but adding softer surfaces has helped a bit. I just didn't know if I needed one of those diffuser do-hickies!