Listening possition kills 60hz!

Discussion in 'Archived Threads 2001-2004' started by Dan_Gtp, Jan 8, 2003.

  1. Dan_Gtp

    Dan_Gtp Extra

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    I have built my 240L vented Tempest (17hz) and been enjoying it for over a month, despite the terribly unflat in-room response. I finally got the time and energy to take it apart and inspect the inside, as I was giving up to the point of deciding it must be a design/construction error. What I mean by terrible in-room response is this: Picked an arbitrary volume, 85db and started playing test tones from 120 down. Highest db :97db @ 25hz, lowest db 64db @60hz!! Wow, not quite in the +/- 3db some of you shoot for : ) 33db diff!!

    I put it back together, retested it and got roughly the same. Shaking my head I put the SPL meter down on the ground a foot or two away. Checking this way resulted in a much better +/- 8db from 120hz-20hz. So the enclosure seems to be performing appropriately, but listening possition is just murdering 60hz somehow. I have moved the sub slightly, but until I get a longer length of speaker wire, I wont be able to do anything drastic.

    The listening area is only about 13' wide, 7' high, but 24' deep and opens up to my room and laundry room. As a whole the basement is 24'x26'x7', sp worst case somewhere around 4000 sq ft. Alot of space to fill with bass, but volume isnt the problem, this thing is plenty loud. The sub is on the left wall halfway between the TV and couch. Not a whole lot of room for the HT setup and you're quite close to the sub. Weird thing is, the loudest area is on the opposite wall of the sub, at the opening into my room.

    Basically I was wondering if anyone has started out with such a severe problem as this? Anyone put one of these behind their couch/listening possition? Half my basement is open behind it. Still plenty of options of moving it around when I get more wire, and I'm definately hoping this problem is solvable.

    Thanx for reading, and ideas or suggestions are welcome.

    Dan
     
  2. Frank Carter

    Frank Carter Screenwriter

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    In my old room, I had a 30db dip around 56hz. Moving the subs about a foot and the listening position about 1 ft took care of the problem.
     
  3. Stephen Dodds

    Stephen Dodds Second Unit

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    I'd try corner loading the sub and then using a Behringer Feedback Destroyer to tame any peaks.

    Steve
     
  4. You could have my room, and instead of a huge null at 60hz, have a huge peak at 60hz!
     
  5. Chris Tsutsui

    Chris Tsutsui Screenwriter

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    It takes a wall to wall distance of like 9-10 feet to create a 60hz null between them. I wonder if what you're measuring is a result of the standing wave between the 13 foot length, or perhaps the height of the room?
    If you can't play around with the sub and listener placement, then the only other option that can reduce the null is to build bass traps for the corners.
    First I'd try corner loading the subwoofer and see if the null becomes less noticeable.
    You probably won't get the response to be perfect so I'd just get it close enough to where it doesn't become a bother. Honestly, does the 60hz null ever bother you during movies? [​IMG]
     
  6. Jeremy Stockwell

    Jeremy Stockwell Supporting Actor

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    Dan,

    I've got a similar problem at my listening position. I'm running a 280L Tempest tuned at 17.5Hz that is +/- 3dB from 80Hz-21Hz measuring right at the bottom of the sub. Measuring at my current listening position is a different story though. My null is between 50 & 55Hz with the bottom being 53Hz. I've only had the beast running for about a week and I haven't been able to experiment much with different placement "options." The proverbial 800-pound gorilla apparantly didn't have a wife, or he wouldn't be able to sit "wherever he wants" either!

    JKS
     
  7. Richard Greene

    Richard Greene Stunt Coordinator

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    Using a very slow sine wave sweep, the frequency response in a typical room measured from the listening position is roughly +/- 10 to 20dB. Your listening room is typical. (Using 1/6 octave sine waves for measurements will tend to reduce the measured deviations by few dB).

    You're lucky if the bass is +/-10dB.
    Of roughly one dozen rooms I've measured, the best
    non-equalized bass frequency response I've ever measured
    from 20 to 100Hz. was +/-6dB using a slow sine wave sweep at the listening position, which was only six feet away from the speakers (near field speaker locations reduce the effects of the listening room, and so do dipole bass speakers).

    In a home listening room almost every possible seating position will have at least one bass frequency response peak and at least one bass frequency response null due to standing waves. The smaller the room, and the more solid the walls (such as cement), the stronger the room modes will be = large frequency response changes as you move about the room.

    Move your ears and the microphone a foot or two or three in any direction to get away from the 60Hz. null you're in.

    To determine which direction to move your ears without a lot of trial and error, you may want to play a 60Hz. test tone while moving the microphone from side to side, and then from front to back and finally from floor to ceiling.

    If a side-wall-to-side-wall room mode is causing the 60Hz. problem, for one example that may apply to your room, the SPL at 60Hz. will vary tremendously as you walk across the room from side to side holding a sound meter while measuring the SPL of a 60Hz. sine wave. Moving the microphone from front to back or from low to high will have no effect at 60Hz. ... unless there is another 60Hz. mode between two other opposing surfaces (might happen in a square or cubical listening room).

    With a side-wall-to-side-wall room mode, you'll need to move your ears to the left or to the right to get out of the null. (A front-wall-to-back-wall room mode is affected by moving your ears forwards or backwards ... but a floor-to-ceiling-room mode is more of a problem, as you'd have to move your ears higher or lower, which is often not feasible).

    Moving the subwoofer to a corner may provide a better distribution of bass in the room (frequency response variations measured at the listening position tend to be larger with mid-wall sub locations) ... but you'll still be sitting in a 60Hz. null (corner sub locations fully excite ALL room modes ... which means the 60Hz. mode will be fully excited ... just like it is now).

    The location of the 60Hz. null is not affected by subwoofer positioning ... and it can't be fixed with equalization. A null is caused by out-of-phase bass reflections ... only bass traps which reduce the SPL of the reflections will reduce the null. Of course it's usually easy to move your ears -- in some well shaped rooms, it is theoretically possible to find a seating position that will place your ears away from all bass peaks and nulls ... but even if you could locate such a seating position, that's not likely to be a great seating location for listening to the other speakers = the bass is smoother ... but higher frequencies sound worse !
     
  8. Lee Carbray

    Lee Carbray Second Unit

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    Richard is correct boosting frequencies in a null will not get rid of the null. That is if the null is caused by a standing wave. If the speaker is just not putting out that certain frequency then it could help. Look at it like this. Say you have a peak from two waves coming together in phase. Let us take an arbitrary amplitude of 50. The two waves come together in phase and create an peak of 100. To lower the peak you can lower the amplitude of that frequency. Lets bring it down to 25. Now when the waves are in phase the total amplitude is 50, and it is even with the rest of the frequencies. Now lets take the same wave out of phase. The two waves cancel out and become 0, because we have one wave at 50 and one out of phase at -50. If we boost the frequency, lets say by 50. We get a wave of 100 but also a wave of -100(because the out of phase wave gets the same boost) and the overall amplitude is still 0. The only thing you can do is move the null( by moving the sub) or fix the room( bass traps etc).
     
  9. Dan_Gtp

    Dan_Gtp Extra

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    Lots of information here, thanks guys. Wow Frank, it was just that simple? Now that's sensitive, move your listening position and sub by a foot each and you solve the 30db prob, nice.

    Hehe Anthony, wouldn't a peak be easier to fix than a null usually? Well I guess a 60hz peak would be pretty overwhelming, esp if you're referring to your dual Blueprint 15's. That's gotta be alot of output from a sealed enclosure.

    Chris and Richard, from the little I've checked, it does seem to be a side-wall to side-wall standing wave problem. I will check more thoroughly. I now have some more speaker wire so will be able to move the enclosure around more. I guess I will just live with the best of the worst that I can find since it sounds like you cant just EQ it out. Corner loading isn't an option since one corner is where the plate amp is tucked, right beside the rack with receiver and such, other corner is depressed side entrance to the sauna that is behind the wall the TV/rack is on. Back corners are either a stairway upstairs or taken as well, on top of being 10+ feet away from the back of the couch.

    I have thought of looking into bass traps but was never sure what they were or what exact situations called for them. I will have to look into them Lee, as well as hopefully moving the null away from listening position.

    In regards to whether the 60hz null is annoying during movies, well knowing I'm missing 30db in that range is a bummer first of all. 2nd, I'm not sure what hz range is covered in Sauron's explosion scene in the beginning of LOTR, but it is not impressive at all until it dips quite low with my setup, very disappointing, and was hoping to add that to my demo selections. My sealed 10's in my car do it much better. Yes, cabin gain is no fair, but I'm still bummed.
     

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