If you have your speakers facing the length of the room (5.250 meters) then the Bass Null is likely to be caused by reflections off the back wall. If you cover the back wall with bookcases, wall coverings, draperies or other acoustic dampening material the Bass reflections should be corrected. Moving the speakers will not prevent the Null. It is caused by the bass wave hitting the back wall and bouncing back into the room, in effect canceling the incoming bass wave as the two meet.
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Commcat's recommendation is bang on - see my virtual system for an alternative to a bookcase. The other issue is quarter wave cancellation in the bass from the wall behind the speakers. As the article explains, the only way to eliminate this entirely is to go to a soffit mount (as they almost always do in high end studios with their main monitors). An alterntive is to add plenty of bass trapping on both walls but at these frequencies you will probably need at least 4 GIK tri-traps or something of similar size before you can begin to make a difference.
Unfortunately a TACT or a PARC or an PEQ is unable to fix nulls (unless it alters the phase - which some EQ's do and which you might not want if you have phase accurate speakers to begin with). However, since a TACT or PARC allows you to EQ down the peaks then generally the "nulls" are much less masked by the peaks - so it really helps. Another trick is to have copious bass (more than you need) and then EQ the peaks down to get a desired response by attacking ONLY the peaks.
The unfortunate thing about nulls is that you cannot boost them or fix them in any other way except for speaker/listener placement and room acoustic design/treatment (if you wish to avoid playing with the phase of your bass signal which may affect how you perceive transients). The bad news is that this takes tremendous amounts of absorption to achieve - huge huge booskhelves and other large thick absorbers that work at bass frequencies. You can achieve good results with helmholtz tuned devices (like tube traps) but ONLY if you really know what you are doing - I'd suggest you hire an acoustician like Rives in this case - broadband bass and lower mid absorbers are easy for anyone to add to room corners and can really only do good....resonators require much greater expertise, IMHO.
What you have discovered is a common problem for 90%+ of all HT and full range two channel listeners. Most people focus on ever higher end more expensive gear and simply ignore this issue altogether and contend that it does not exist. These are mostly cases of extreme wishful thinking, where WAF or room aesthetic issues have a much higher priority than good sound.
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.
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.
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).
We haven't actually established it's a null yet. Download some test tones in the 50 Hz area you mentioned, copy it to a CD, not mp3. Play that and walk around the room. If the null is as big as you think, you should notice variations in volume, without a meter. If the volume is consistent throughout the room, you got other problems.
Acoustic treatment in that frequency range is not trivial. However, I once had a hallway closet beside the living room that sucked 90Hz if it was closed. Sounds weird, I know, but I had to open that closet every time.
Thanks everybody great response so far, Ngjockey the room I am using is going to be a master bedroom one day and I built a ensuite of it, I will try leaving the door open never have!With regards to bass there is more up the front and down the back, I have contacted a person who as got all the answers like a Rives Audio, but these blocks are in Australia, I spent all morning taking photo's and drawing a mud map with as much detail as I could give them, I will post what the result are. There is know doubt that it is frustrating, you dont know where to start sometimes as it was only that I was thinking of getting a new cd player that I stumble on a thread that made me think what if it is the room, how many people sell there gear because of the room?
I had a serious bass null at 55-65Hz and experimented with two GIK 244 panels, moving them around in different configurations near the sub until I was able to significantly reduce the null (from -10db to only -2db). Placement ended up being two panels next to one another, forming a 4'x4' mini wall, right next to the subwoofer (within 8" inches) where there was no wall (it is placed near a single wall so open on three sides). Apparently this cut off the specific freq. waves at just the correct spot. It was quite a relief to be able to do this so quickly and cheaply. So it can be done.
I'd suggest a fire place with logs stacked all around it on the rear wall of the master bedroom with bookshelves around it. The other thing is to consider a large extra thick latex mattress king size bed - avoid spring mattresses as they do little in the bass - a thick Latex mattress will be an extremely effective absorber placed at one end of the room or towards one of the four corners. The other trick is soft padded furniture - love seat with big cushions. You will be suprised - you can do a lot without causing a divorce (acoustic panels in a bedroom would certainly do that...)
BTW, what frequency spectrum would be attenuated by the mattress you recommend?
Absorption is mostly about size and thickness. A mattress of this type will usually be several layers of different density latex and therefore you should get good broadband absorption. Unfortunately, a coil mattress and box-spring has much of the space filled with air - so if you can find something to stuff the box with then you'll get an amazing low end response.
K_rose - maybe I'm missing something, but if you have reflections between two walls you should have amplification on even reflections 2, 4, 6 etc (in case of 5.25m it's 33Hz 66Hz 99Hz etc) and nulling (valleys) on odd reflections 1,3,5 etc. (frequencies in-between).
You should have the same amount of bass - only points of peaks and valleys will be different. It is possible that peak at 33Hz is useless (lowest bass E=44Hz, lowest piano A=27Hz but seldom used) and 49 Hz is suppressed giving you impression of weak bass (lack of extension). Damping room at low freqeuncies is not trivial and deep pattern foams are expensive and ugly.
You will get the strongest bass with speakers and listening spot at opposite walls.
The only speakers that don't do peaks and valleys are bipolar speakers (planar, electrostats) but they have modest bass to start with.
Try to toe in speakers at almost 45 deg angle - it will even up bass a little, will probably narrow the image but it will widen sweet spot. Room mode calculators are often very complex (3d plots) and might not be very helpful. Best bet to see what is happening would be to get good sound level meter and test CD.
Last night I tryed a test cd on 50hz on repeat and walk around the room, if this makes sense the sine wave was more noticable standing up than sitting down, clearer at the front and back, Sorry Kijanki, I have been missing the piont of what I have been doing for a while know,I will admit Iam struggle reading all the tecnical stuff with regards to the room hz and so forth, I like the old days when it did'nt matter when I was in my early teens and just love to listern to music!I like the idea of using my latex bed though, I will wait to Sunday when the other half is out and I will give it ago!who knows
343/5.25/2=32.66Hz where 343m/s is the speed of sound, 5.25m is the distance between your walls, 2 is because you need two times distance to amplify (from speaker to back wall + back to front wall + front wall to speaker = 2x distance between walls). Exact multiples of 32.66Hz will be multiplied as well since their wavelength is an even fraction.
Nulling is pretty much the same with odd divisions.
Yeah, that's why I gave up on meters / analysis / test disks and just listen.
You can do about 3/4 of what a consultant can do, for a fraction of the money with only investing time and patients.
Next thing, we'll be talking about RT60 and other phenom.
A good example, kind of OT, is my TV. Yep, I could spend the Big Bucks on an ISF guy to come out and Calibrate, but I can get maybe 3/4 the benefit with the use of a DVE disk (digital Video Essentials)
For those who don't want to hire a consultant. (The majority no doubt) The trick is broadband absorption in the bass - as many innovative WAF acceptable ways to introduce absoption as you can - from a big Ottoman to a new comfy love seat to large stuffed "beannie babbies" - yeah the big ones that are like 4 feet tall - in that sense kids can be your secret weapon - buy them big stuffed toys that always end up left in the living room.
If you have a tall thin blonde Swedish wife who likes Ikea scandanavian styles with hardwood floors and modern furniture (hates Laz-y-boy stuffed seats) then in an audiophile sense you are toast. However, in this case you probably don't listen much to music anyway ;-)