Mystery of the Disappearing Frequency


I have only been involved in the pursuit for about 3 years(since my retirement) but have tried to read and research extensively. In this work I have not yet found a satisfactory answer to my dilemma. I have a 10 Db dip at about 50 Hz!!  This alone of course is not that unusual, what seems to be unusual is no matter where I take the measuring microphone(calibrated Omnimic) the dip never goes away!!  I have walked all around my somewhat small listening room(15 by 18 by 8) and the dip never get significantly lower. I have certainly read about dips but always thought they were specific to certain areas due to nulls and cancellation effects. My room is a little unique but I do not believe the following factors account for this dip. The long sides of the room have a 10 foot long bank of windows covered by thick drapes. It has a wood floor on a pier and beam foundation and the wall behind the speakers is the wall where the main house met the attached slab garage before the garage was converted to a few small rooms. The small amount of wall material is not sheetrock but rather a very thick, ribbed solid wood paneling. I followed expert advice as far as acoustic treatments(room tunes pillows at the ceiling corners,panels on the ceiling to reduce echo near the speakers, a rug with thick under pad to cover about 80% of the wood floor. I have corner bass traps on the speaker wall. The room is overflowing with records and CDs. This phenomenon was present with my previous speakers(NHT 3.3s) as well as my current BBC monitor style speakers. I have tried a variety of experiments to see if I can change the issue. The room has 2 doors but opening doors,windows and/or opening the drapes did not change the situation. Also tried removing the bass traps to no avail. Have run out of theories and experiments at this point, outside of wondering just what materials there may be in the room that absorb this frequency. My equipment has also mostly changed since I first measured a few months back, have mostly Allnic gear at this point. Your theories and suggestions are kindly solicited. Many thanks, LS
trytone
have you checked the calibration of the microphone?
Problem caused by speaker placement?
http://arqen.com/acoustics-101/speaker-placement-boundary-interference/
50Hz has a wavelength of more than 20 feet (longer than your room). You are hearing room mode bass response. Since the effect is everywhere then it is also likely you have quarter wave cancellation. 

http://www.mh-audio.nl/CancellationFreq.asp

The only only thing you can do is move the speakers. 

Quarter wave cancellation occurs around 50 Hz when the speakers are 5 to 6 feet out from the back wall behind them. Try placing the monitors right up against the back wall or try placing the speakers far out into the room in a nearfield listening setup.
Thanks for replies. Microphone was calibrated upon receipt and the phenomenon shows up on an Ipad app as well. Old speakers and current speakers have been in about 6 different positions(the NHT's are designed to be only a few inches from back wall)-the front edge of the current monitors are about 4 feet from back wall and as much as I hope that moving the speakers might do the trick I think that past history would rule it out but I can always check.
see if you can rent some tube traps
I think your problem in part may be your measurements.

Are you using a steady state signal? Try Room EQ Wizard or OmniMic instead.

Next, if this is real, you'll need bass traps. Tube traps are good, but ASC is pricey, GIK Acoustics Soffit Traps are as good and better value.

Best,

E
Am using Omnimic, think it was mentioned in original post, if not my apologies, have floor to ceiling corner bass traps from GIK in corners on wall behind speakers. Most would probably think my room has too much rather than too,little room treatment, am curious as to the theory that adding even more absorptive treatment will cancelthis phenomenon but am willing to listen. Thanks all so far for the reasoned if ineffective suggestions, please keep them coming. Even though the speaker position is probably not a factor will try to move them around soon.
So very sorry! So the usual fix is to reverse the speaker/listening position while you find a better place.

That is, a speaker at your listening chair, and move the mic around until the holes fill out.

Best,


E
@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.