[Wayne A. Pflughaupt]

what causes normally the peaks that I see at 20 and 40 hz?

I’m probably not the best candidate to be answering this one, but bass response is greatly affected by the room, modal issues that have to do with specific frequencies reacting to the room’s physical dimensions. Much of is mathematics, having to do with a combination of the frequency in question and the distance between the sub and various boundaries – the floor, ceiling, left-, right-, and across-the-room walls. Move the sub to a different location, where the distances from the boundaries is different, and you’re going to get a reading that doesn’t resemble this one much at all. Same thing if you take the sub to a different room in your house, especially one that’s drastically different.

For instance, take a look at these two graphs of four subs, taken in two vastly different rooms:

As you can see, in each room all subs exhibit peaks and troughs at the same frequencies, but notice that each room has its own peak and trough frequencies.

I generated a waterfall graph of sub+spkrs but it doesn't look right.

What’s wrong with it? It looks fine to me. The bass traps are giving you extremely low decay times all the way down to 45 Hz or so. Even from 45 Hz to ~25 Hz decay times are excellent. It’s only below 25 Hz that decay times are long, but that’s very typical, as I understand the effects of bass traps.

Regards,
Wayne