How to fix a lack of bass Null

From your post it would appear that you have established a fixed listening seat. If that is so, then you might eliminate the null by simply moving the listening chair back or forward somewhere between +/- a foot.

Get a test disc, a SPL meter, some graph paper (for easy visualization) and start carefully measuring actual results of speaker/listening chair positions. Programs for speaker placement, etc, are nothing more than suggestions for good starting points. There are far too many things that can change the speakers response that can't be factored into a program.

One of the benefits of the meter/test disc assessment is that you will soon learn (or if you are dumb like me, slowly learn) what nulls are inherent in the rooms dimensions which will defy correction by most any means, and nulls which are location related. The only problem with the former is you really can't, as a practical matter, change them. The only problem with the latter is that often you can change the frequency peak/null by moving stuff, but often you just move the peak/null around a bit. No matter what you do you will have to accept some form of compromise.

A personal experience. I spent some years trying to improve my frequency response, close to flat as it were except for a 6db suck out at 40 and 50hz. Never did it - I even tried using a sub with a very low cross over point (40hz) with mains running full range and when I tried to eliminate the null the sound became boomy and muddy. Wana sub? :-) I just concluded that this null was barely audible (speakers are flat at 32hz and down 3 at 25hz) something I could live with.

Shadrone's post is an excellent summary I think, except that I'm not at all sure about Commcat's comments about the surface or furnishings on the back wall having much effect on deep bass nulls. But that is just my opinion.

O

Another remedy is to place your speakers facing the longest wall. That is against the long wall (5.250 meters) and facing the opposite long wall so that you have a distance of 4.00 meters. Toe-in the speakers so that the sweet spot is approximately 10 feet from the speakers (or less than 10 feet if space is restricted). I use a flashlight placed on the top of each speaker to find an accurate sweetspot (where the light beams cross is the sweetspot). The sweetspot should be approximately 18 inches from the back wall. Cover the back wall with bookcases, canvas paintings, drapes, etc. This short throw may eliminate the bass null or cause it to occur behind the sweetspot, allowing you to hear the full effect of the bass without distortion.

1. Google room mode calculator. Enter your room dimensions
2. It isn't just front to back or side to side reflections.
There are models which include 2 wall, as above, or 3 or 4 wall reflections. each diminish in importance as wall reflections are added.
3. Sub crawl isn't a bad idea. A little tough sometimes, especially if you have some HUGE sub.
4. Multiple subs. Located asymetrically, they compliment each others peak / null to produce flatter response over a larger area.
5. Experiment.
6. Is your sub out of phase? This can produce this 'suck out' effect you note.

Start changing stuff until you get a result. Even if you KNOW you can't leave it like that, just move things around.
Experiment with speaker placement. READ UP ON THIS.

What Newbee adds is quite correct - both frequencies appear to be due to room length mode. The 31 Hz null will dominate as it is one mode with a high signal at each end of your room and a complete null right in the middle (and you probably sit somewhere near the middle). See this - just enter your room dimensions.

Big Book shelves and lots of thick bass absorbers at both ends of room (treat all corners) are your best bet as moving the speakers will NOT help (at least not at 31.5 Hz)

Another option is to run a sub up to 80 Hz and place it up against the side wall between one quarter and one third of the room length. This will strengthen your width axial modal response and may cause a new issue to begin to apppear at 43 Hz - but it will certainly help you hear 31.5 Hz and 63 Hz at your listening position - if you can get the balance right between mains and sub. The idea is to use the mains and sub to balance competing room modes for a more even response. Placing a sub there would be a horrible place to put a speaker but could work a little better particularly for 31.5 Hz mode (which is probably a total suckout).