Mystery of the Disappearing Frequency

@trytone  

I would agree that you cannot do much to fix room modes. Radically moving the speakers may help but it is unlikely to entirely fix a problem at 50 Hz - as I explained the wavelength is longer than your room! Short of finding another room for your speakers you may have to simply accept that the issue is coming from the room. You may want to try room EQ software it can tame peaks on the bass which in turn helps reduce the troughs.

Do a Google search for room modes/nodes. You can find sites depicting rooms in 3-D, showing where m/n are located in the room, depending on it's dimensions. You can put in your rooms dimensions, and the 3-D display will show where the modes/nodes are in that room. To minimize peaks and valleys in bass response, situate your speakers at areas shown in the display to be out of the mode/node zones. Do the same, if possible, for your listening position. Doing so will not completely eliminate a 10dB suckout, but it may minimize it.

Multiple subs with room treatment well help too

Hello all, sorry for not replying in a while. Tried to move speakers around today. As I moved them back the dip at 50 hertz did reduce a bit but the reduction got moved into the 32 hertz range turning my relatively flat bass response(achieved by judicious low level use of a Martin Logan Grotto subwoofer) into a 7 db peak!! I have opted to return to the 10 db dip rather than have a 7 db peak at 32 hertz and a 7 db dip at 50 hertz, the new position also exacerbated a tendency for dips around 180 and 400 hertz as well. I believe the current settings give me the flattest respons, with the notable exception of the Big Dip. Anothrer reason not to move the speakers back is they would then be a little closer to the rack that sits between them. Perhaps nothing short of multiple subs, which I would have no idea how to rig, especially since because of a hum issue I am running the solo sub with speaker connections rather than standard inputs. Perhaps an expensive visit from an acoustician is in order.  Shadorne, is it truly a room node if the dip is measured everywhere? I used to ameliorate the dip with a parametric analog equalizer but since I have changed pre amps I no longer believe I have that capacity(less input/output jacks). Eric Squires, since the dip is measured everywhere I do not think that flipping the speaker/chair alignment would help, nor could I see it as along term solution what with where the dedicated outlets are and the difficulty of moving the shelf. For those that think room treatments are the answer I am curious as to which treatment and what frequency will need to be addressed to deal with this sort of issue. Bdp24, I believe that by using Omnimic I have found the best compromise in listening position and speaker placement, for those who need precision, I am listening in the long dimension(18 feet), the outer edge of speaker is 52 inches from side wall , the back of the speaker is 43 inches from the rear wall(front 55 inches), my head is about 43 inches from the wall behind me, there is a tall LP shelf just behind me almost even with my right ear, about 22 inches behind me, I hope that this asymetric situation is a plus and that I am not too close to any reflections coming off the LP shelf, with a few hours work I might be able to slide the shelf over about a foot if essential. Distance from speaker to listening position about 10 feet. As previously mentioned I did move the Omnimic all over to find the flattest response for the speakers and listening position, perhaps if one person were to be moving with the microphone and 2 others moving with the speakers,......  thanks all for the replies, I am genuinely confused that this 50 hertz situation seems to be all over the room and resists all efforts. Keep your experiences and experiments coming. Many thanks, LS

Trytone,  I am not sure I can help other than to offer my sympathy.  I have been working on my room since last October, and although I have made a good bit of progress, I have a fairly high level of frustration at this point.  I have spent some money, and done a lot of work, and am at the point where everything I try results in improvement in one area at the expense of another.  I have dealt with two different reputable companies, and they are both quite reluctant to offer anything definitive in terms of systematic diagnostics.  I've been told it is very difficult to do better than plus or minus 5 dB without having a custom built room.  After several months of work and spending about 2K in treatments, I'm still seeing plus or minus 9dB and having some ringing issues below 50 Hz.
With respect to your 50 Hz null, you eliminated speaker boundary problems in that you moved the speakers and it did not shift the frequency of the null.  I use the  REW software, and I plugged your room dimensions  and speaker and chair locations into the room simulator.  It does not predict a null at 50Hz.  You have changed most of your equipment recently, which seems to rule out your stereo system.This is weird.  Can you use your Omnimic program to send a single frequency continuous tone?  I was thinking it might be worth sending a 50 Hz tone through your system and seeing if you perceive a null from your chair.  If you hear an obvious audible reduces volume vs 40 and 60 Hz tones, you know it is real and not some sort of software measurement glitch.
One other possibility.  It is possible that something in your room has a resonant frequency at 50 Hz.  If, for instance, the flooring were selectively absorbing 50 Hz, it might well result in a general reduction of that frequency around the room.  I read somewhere that 1/2 plywood tends to absorb at 70 Hz, and I am seeing a dip in my room centered at about 68 that seems to resist my efforts to mitigate.  Perhaps others can comment who have plywood sub flooring in their rooms.