Dual subwoofers....will this work, or will the cancel each other. One front one rear

Discussion in 'Speakers & Subwoofers' started by Todd smith, Oct 12, 2005.

  1. Todd smith

    Todd smith Supporting Actor

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    In my 12x17 HT I have a null in the 40-60 range with the sub in either of the front corners of the room. If I put the sub in the rear corner the 40-60 null is gone but I have a null in the 15-25 range. I have tried both ways for a few months each and overall prefer the sub in the front of the room. Either way I have a null, but in my particular room I can tell the sub is in the rear when a big explosion happens or something like that and it detracts from the effect. Explosion happens in front of you and boom is heard over my shoulder. I cant tell most of the time that the sub is in the rear, but it happens enough that it bothers me. In the front the sub blends beautifully with the rest of the system and never draws attention to itself which is the way it should be. Everything is calibrated with Avia including phase to optimize the sub in either location. Having the sub in the rear just is unatural to me.

    Ideally what I would like to do is to get another pb12/2 ultra for the rear (or pb12 ultra) so I can have the 40-60 range back at my primary listening spot. Would putting one sub in the front left corner and one in the rear right corner work together well or would they cancel each other out and not solve my problem? Like I said I have a pb12/2 ultra, 80hz cross, 12x17 fully enclosed room, and everything calibrated with avia. I cant move the couch either. Would dual subs not co-located help or worsen my situation? If having a second sub in the back would solve my null I could deal with having the second sub in the back especially since I will have one in front.
     
  2. John S

    John S Producer

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    Dual subs is a crap shoot. I think you can get another one, yet return it if it does not work out.


    Cancellation can and does happen, but near as often as many make it out to happen. You will probably be fine.


    I run dual 15" velo subs not co-located, they are on each side of the sofa as corner tables sort of. I sure don't see any cancellation effects on the meter and my sub'age seems even less localized than when I only had one.
     
  3. John Garcia

    John Garcia Executive Producer

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    Have you considered an EQ such as the BFD for the sub? Small rooms can actually be more of a problem than larger ones, because of standing waves. Maybe you should look at dual cylinder Ultras? Have you talked to SVS? They're usually very good about helping with this kind of stuff.
     
  4. Chris Tsutsui

    Chris Tsutsui Screenwriter

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    You're going to get peaks and nulls no matter what the dimensions your room is.

    A BFD will be able to get rid of peaks just fine, but it won't and is not recommended that equalization be used to get rid of nulls. Think about it... If two waves are canceling eachother out, by increasing both waves, they are still going to cancel eachother out...

    2 subs setup in a stereo left/right setup will actually eliminate one of the nulls. This null will be one of the left/right standing wave if you put the sub where the null is supposed to be. I used to be very familiar with bass equalization, but this was a couple years ago so you'll have to research and figure out where to place your subwoofers, and what specific nulls you'll be cancelling out. There are 1st and 2nd order standing waves that are most prominent or something...

    However, the drawback by using two subs is you will get rid of SOME of the nulls/peaks, but you won't be able to get rid of all of them. This means if you get rid of one null, and there's another one... You will make the other null more noticeable.

    It's basically a balance of subwoofer placement and BFD equalization (used only to reduce levels not increase) that will result in a good bass frequency response. Remember a key factor in bass equalization in regards to nulls and peaks is LISTENER position as well as subwoofer placement. I've been in a room where the bass was horrible where the couch was, but then when I moved 3 feet behind the couch... the bass sounded great. You know you are becoming "picky" when you can tell that one of the bass notes sounds bloated because of a room mode... This is generally a bad thing... hehe

    This is why I think I'm a dipole fan...

    Generally you don't want subwoofers behind the listener. My ideal location in my room that's like 13x10 is about 2-3 feet from the corners of the room. The close the sub gets to the corner, the louder the bass gets, but this made the peaks and nulls more severe.

    Then if you become real picky about the way your bass sounds and this still isn't good enough. You can construct your own bass traps and place them in the corners of the room. I did this, and the result was an INCREASE in 2-3db where all my nulls were... There are four 4ft high bass traps made of fiberglass.

    Remember that nulls are harder to notice than peaks, and if I had to choose a null... I'd probably choose the 15hz one because that frequency isn't used in too many movies. 40hz is the "chesty" bass region that is a LOT more commonly used.
     
  5. John A. Casler

    John A. Casler Second Unit

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    I have 4 LARGER VMPS subs, 2 in front and two rear.

    I run the rear "out of phase"

    This makes a push pull.

    It seems to work very well and the also seems to reduce room interaction.

    As mentioned above, each room is different, but it is an option to try (reversing the phase in the rear sub)
     
  6. drobbins

    drobbins Screenwriter

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    My room is about 14x23 and I am using a 2 sub setup. I have a Sony 12" forward facing in the front center & a JBL 10" down facing in the back center. They were both gifts. By having 2 subs, it helped even out the SPL in the different seats through out the room. I still had a 20 Db difference between frequencies so I used a BFD to flatten it out. The end result smooth bass in every seating location. Now I just need a sub that goes down further.
    I learned alot of info form this web site If you have time to read the 30 pages it is quite informative and coming from Harman, I figure they might know what they are talking about.

    Dave
     

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