How would you sort this awful room!

Discussion in 'Home Cinema Buying & Building' started by PH001, Jun 23, 2015.

  1. PH001

    PH001
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    Having major issues with my room with big resonances at 50hz and other harmonics.

    Not helped in that the wall behind the speakers is virtually all glass (patio doors) and all walls are solid brick. Floor is wood laminate with Supercell foam insulation beneath.

    I do have a sub in the opposite corner which helps a bit, but what treatments should I apply to kill some of these monster peaks and dips? Note I do not have access to any corners due to the patio doors at the front and a door into my garage at the back!

    I am thinking limp mass bass absorbers on the back wall, but am I correct in thinking I need to address the 25Hz primarily and all the others should reduce as well? Of course 25Hz is not that 'audible' but is there any point in trying to address the 50hz without doing the 25 first?
    room response.jpg
     
  2. KelvinS1965

    KelvinS1965
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    I can't help with the acoustic treatment stuff, but even allowing for that you've got a massive null around 35Hz like I used to have. The only 'cure' for it was to put one sub in the rear right corner to match the other in the front left corner (try adding them in REW sim above and you'll get near enough my set up as your room is similar, though slightly narrower/longer than mine).

    If you sort that null out then add any recommended treatment to help, though it sounds (like most of us) that you are limited as to where you can add treatment. I would recommend trying a miniDSP DDRC-22A if you are only interested in 2.1 music. This will remove those peaks and generally help give you a better blend of subs and speakers. I use the 7.1 channel version myself which is the DDRC-88A and even without any room treatment currently (I do plan to try adding some when I redecorate/revamp the room next year) it sounds much better than ever before.

    Another option might be a Lyngdorf TDA digital amp which has 'room perfect' and is designed for 2.2 use (though they generally recommend putting the subs in the front corners, which might not sort out your null as it wouldn't with mine).
     
  3. PH001

    PH001
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    Thanks for the reply. Am I right in thinking the huge null at 35Hz is a direct result of the room nodes at 25 and 49Hz etc. So if I kill those, the null will disappear also? Otherwise i don't understand how you would compensate for that null - there is no point putting more bass energy into that frequency as it will just cancel out in the same way.
     
  4. KelvinS1965

    KelvinS1965
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    I don't know enough about room acoustics to answer your question, but I suspect that you won't be able to treat the room enough to get rid of the null. If you play with the 'surface absorptions' settings in the REW room sim and increase their values, then you will see the peaks (and nulls) flatten out. But only once you get to very high (ie not practical in a living room) values. Bare in mind that you will have to achieve that high value over the entire area of that wall/ceiling/floor to achieve what it will simulate.

    If you add a subwoofer in the right rear corner and one in the front left corner than I suspect that null will disappear (or at least get very much narrower). This is due to the different distance the sub is from the MLP. I agree that just putting huge subs where your speakers currently are won't make a jot of difference because of the combination of distance from the MLP and the room. (ie putting more bass energy into the room at that point).

    Using REW room sim I soon worked out that there was no point in my buying one single very expensive sub to put in the front of my room as I always had a null like yours. It was only once I added a second sub behind that the null was 'filled' in. It is much easier to use two identical subs for this as there is enough set up issues using two subs as it is without making it harder by mixing sub types/models. In fact the optimum is four subs, one in each corner, so I'm planning on that next year along with some room treatment, but I will mostly rely on Dirac for room eq.
     
  5. PH001

    PH001
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    Cheers. I did some more measuring last night and got all the distances really accurate. I confirmed all the resonances and nulls with a sine sweep and REW gets it pretty much bang on. The first pic below is my actual setup with the sub active. The second shows what it would be like with a second sub in the opposing corner. Still got a horrible boom around 47Hz and 72Hz...


    withsub_accurate.jpg
    with2subs_accurate.jpg
     
    Last edited: Jun 24, 2015
  6. PH001

    PH001
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    Going back to a single sub..this is the result of a 50% absorption on the back wall. Looks better - but what frequency should I be targeting for the bass trap - the 25Hz, the 47Hz or the 72Hz?

    withsub_accurate_bass trap.jpg
     
    Last edited: Jun 24, 2015
  7. KelvinS1965

    KelvinS1965
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    Are you going to be able to cover all of the back wall such that it will absorb 50% though? If you can't then you'll still have some peaks to deal with as you can't have 100% absorption material (since it doesn't exist) covering 50% of the wall. You've also got quite a dip around 60Hz in the last chart too.

    I would also try ticking the 'align subs individually' as you ought to be doing this since they aren't the same distance from the MLP. It may make further differences to the two sub version.

    I've found REW room sim was quite accurate for my room, when I measured afterwards I got very similar results, so it's worth persevering with it rather than dragging subs around the room. Also try subs in the middle of the front and back walls (whether this is practical or not is another matter, but just for comparison's sake). I tried every possible position, then compared to what I could practically implement (just so I knew what I was missing out on) and my compromise wasn't too bad as it turned out. Even XT32 could correct the bass peaks, though Dirac does it better these days.
     
  8. PH001

    PH001
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    The 'align subs individually' doesn't make much difference at all in this case. I have considered using some good room eq hardware such as the DDRC-22A or perhaps the Dspeaker Anti-mode 2.0 dual core but it is difficult to integrate into my system as all my music comes from my PC via hdmi (I also run a projector and a Yamaha AV).

    I don't seem to be able to get anything even approaching OK no matter where I place the subs. Unfortunately the MPL is pretty fixed too.
     
  9. KelvinS1965

    KelvinS1965
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    Just the usual AV fan's compromises to deal with I suppose...

    I'm sure that some room treatment will help, just that I don't think it will achieve as much as you'd hope, so I think room eq is inevitable at some point. I'm just coming from the opposite direction in that I've used room eq for years (antimode, XT32 now Dirac live), but have the opportunity to add some treatment when I rebuild my room.

    I'm surprised that no acoustic guys have chimed in as I've been reading a few of their posts to gain some knowledge for my own room.

    Since you use a PC then I believe there is a PC version of Dirac Live. Probably worth talking to @mattkhan about PC based options as that's what he uses; probably cheaper than buying the hardware too. It may be more technical than just connecting up a box with rca leads though, so what you save in money, you may have to put back in terms of time and effort.

    Another option is the HDMI version of the miniDSP room eq, called a nanoAVR DL. However it requires LPCM input (as it can't decode, so your PC would need to be able to output in LPCM). This also means that you won't be able to use this device if you plan to use Atmos in the future for AV listening (probably not an issue as it seems more a stereo based set up you have). Details below:

    MiniDSP nanoAVR DL (Dirac Live) | Kalibrate Limited | Home Cinema products, Calibration, DVD Players, 3D Glasses | Audio Systems | Cinema Systems
     
    Last edited: Jun 24, 2015
  10. mattkhan

    mattkhan
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    what player software do you use?
     
  11. PH001

    PH001
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    Great, thanks. I am very interested in any PC based software that can do room eq as that may be a great solution for me. It's a slight shame the MiniDSP nanoAVR DL downsamples everything to 48Khz as I generally play high res and FLAC which it will essentially convert to CD quality. Otherwise it looks like a nice solution.

    I will contact mattkhan.
     
  12. PH001

    PH001
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    Typically Media Monkey for FLAC and iTunes for general MP3 & M4a
     
  13. mattkhan

    mattkhan
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  14. PH001

    PH001
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    Ooooh, great - thanks! Will give it a go and let you know how I get on.

    One thing I don't quite understand with REW is if I take a very simple setup with just a stereo pair of speakers and no sub and then set the lower freq response of the speakers to something really high like 150Hz - I still get a big room node at 25Hz??? Is this true in reality - i.e. can the room really 'create' lower harmonics of the freq being output?
     
  15. mattkhan

    mattkhan
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    yes absolutely, that is exactly what a standing wave is
     
  16. PH001

    PH001
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    I must not be understanding this correctly - surely to resonate at 25Hz, you would have to subject it to 25Hz to begin with?
     
  17. mattkhan

    mattkhan
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    sorry, I misread your post about "creating" lower harmonics.

    A high pass filter is not a brick wall. If you set it to rolloff at 150Hz then it means to gradually rolloff (attenuate) the output below that point.

    For example, say you have a typical sealed speaker rolloff of 12dB/octave from 150Hz and play back at 75dB. As a 1st approximation this takes you down 1 octave to 75Hz another octave to 37.5Hz and ~2/3rd of an octave more down to 25Hz, i.e. you're down about 32dB at 25Hz.

    Now lets say your 1st axial length mode is at 25Hz (corresponds to a room that is nearly 7m long) and it adds a 20dB peak, you're now only 12dB down from the 75dB baseline at 25Hz.
     
  18. PH001

    PH001
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    Yep, I'm with you. I guess what REW doesn't specify is what rolloff it assumes. Of course, in practice a bookshelf speaker with a 4" driver is going to rolloff a hell of a lot more then 12db/octave when it gets down below 40Hz or so. There is no way the drivers are going to give any output energy to speak of at 25Hz or so.

    The room sim is only a guide at best anyway as it assumes a perfectly rectangular room - does a pretty impressive job overall.

    Room EQ aside, I still don't think I ever got a clear answer on bass traps, i.e. if I have axial room nodes at 25Hz, 47Hz and 72Hz - do I need to simply address the lowest frequency and the others will disappear? Not that 25Hz is an easy frequency to address!
     
  19. mattkhan

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    if it's a sealed speaker then there's no reason why it would (start to roll off faster)

    I would think it depends on how you address them. I think addressing frequencies that low tends to be done via active cancellation (e.g. multiple subs placed appropriately). You can try using some sort of passive tuned device but I think it's pretty tricky to do. AFAIK absorption is not effective that low due to the wavelength frequencies involved.
     
  20. PH001

    PH001
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    Is there perhaps some confusion here between cross-over networks and general mechanical properties of a driver and it's associated enclosure. A typical 2nd order crossover does indeed roll off at 12db/octave but there are many other different types of filters that roll off at 6db/octave (1st order), 18db/octave (3rd order) or 24db/octave (4th order). It's not clear what REW assumes.

    If you take the crossover away, you are left with the properties of the driver / enclosure - the roll off of which can be pretty much whatever the design dictates. Certainly not guaranteed to be 12db/octave and also not linear.

    Anyway, we digress a little. I agree that 25Hz is going to be hard to attenuate meaningfully with a bass trap. Certainly a limp mass bass absorber appears to be more efficient at these frequencies then any kind of foam (which would have to be many feet thick). Maybe hitting all the three primary harmonics would be better than a larger one dedicated to 25Hz? I really don't have much option in terms of multiple subs and I couldn't get a good result out of REW on them anyway at the MLP.
     

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