Dual Sub Placement

Discussion in 'Home Theater Projects' started by TimForman, Jun 26, 2003.

  1. TimForman

    TimForman Supporting Actor

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    As a result of a slightly off topic discussion I was told about this white paper which researches multiple sub placement. My question is directed to those that run multiple subs. Where do you have yours? The white paper concludes that the best placement is in the middle of the front and back walls for two subs. They also play around with 4 and even 5000 subs but real room testing concluded two was the most practical. My Tempest has not moved far from the front left corner for some time now. The Shiva has moved from behind the second row of seats to the back wall and now on the right corner of the front wall. Every time I move it I have to change the EQ. I like where it is now but I never actually took measurements with it on the back wall so I'll probably try it again. You elitists with dedicated rooms can chime in but I'm most interested in those, like me, who've had to put up with non-rectangular rooms.
     
  2. Wayne A. Pflughaupt

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    Tim,

    The link to the white paper bounced, but I’m going to hazard a guess that the writer did not post any frequency response charts to support his hypothesis – right?

    If so, this is typical. As far as I can tell, Tom Nousaine is the only reviewer who has tested and measured subs in numerous real-life situations (read residences, not a lab). His conclusion is that in most instances:
    • Corner loading gives the best results – the smoothest response (before equalizing), the highest output, and the deepest extension.
    • A second sub added in a separate corner will generate a 3dB increase in overall low frequency levels. However, if the second sub is placed in the same corner as the first, a 6dB increased in SPL level is typically realized. Thus you get the best results when both subs are placed in the same corner.
    My own experiences mirror Nousaine’s. (My system is set up in the living room, which is open to other areas – does this qualify me has having a “non-rectangular” room? [​IMG]) I had my two subs separated for a number of years, and I never could get the one situated along the wall to perform as good as the one in the corner. The former had significantly less output and extension, and when I tried to use equalization to “force” it to perform as good as the one in the corner, it would frequently bottom out.

    When I finally moved the second sub to the same corner as the other, I was amazed to measure a 6dB increase in SPL level. That’s right – all that time the second sub had been contributing absolutely nothing to the “bottom line.” I may have well been using only one sub.

    Some folks will tell you that corner placement will give you a huge spike in response and boomy, “one-note” sub. Depending on the room, there is some truth to this. This is why the same people will recommend the alternate placements you have been using. While alternate placements may result in performance that “sounds” better, that “improvement” comes at a price: Both extension and output will be reduced. Furthermore, (for reasons I won’t get into here) their response is most likely very ragged; the sub merely “sounds” better because they’ve minimized the peak they got in the corner.

    Having hung around this and other forums for a number of years now, I’ve noticed a trend. Without fail, I’ve only seen people who don’t use an equalizer recommend alternate, out-of-corner sub placement. And I’ve never seen any of them post charts or numbers to factually show us what “impressive” frequency response they’ve obtained with alternate placement. Never.

    Since you have an equalizer, Tim, you can place both your subs in the same corner and never look back.

    However, if you’re going to equalize, you need to do it right. Since you’ve never taken any measurements, you’ve been “flying by the seat of your pants.” You’ll never get optimal results without taking measurements and equalizing with a parametric equalizer.

    Regards,
    Wayne A. Pflughaupt
     
  3. TimForman

    TimForman Supporting Actor

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  4. TimForman

    TimForman Supporting Actor

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    Don't assume too much Wayne. First off, now that I've created the link correctly you may want to check out that white paper. Secondly, I didn't say I've never taken any measurements. I said I didn't take measurements with the Shiva on the back wall. (BTW, an open wall qualifies you to be part of the non-rectangular club, in my mind anyway.) I started off with my Tempest in the corner, took readings and discovered peaks at 40 and 70 and a huge dip at 56. This was a challenge to overcome but I got it flattened pretty good. I added the Shiva but didn't take any readings until I put it on the front wall (flying by the seat of my pants as you say). I still had similar peaks though not as dramatic and the dip at 56 was gone. I found this much easier to EQ out than I did with the single sub. I haven't done full SPL readings for the sake of SPL alone yet, however, I did hit 114 db with both subs with headroom. That's probably close to max during "normal" listening levels anyway. My starting measurements were taken at the center of the listening area while the EQ was bypassed. The findings that having both subs in the same corner produces higher SPL is interesting and I've heard that before. That finding seems to contradict the one in the white paper. I see I forgot to mention I'm using the BFD parametric equalizer.
     
  5. Wayne A. Pflughaupt

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    Tim,

    I’ve seen this paper before. As with all papers of this nature, the results aren’t worth much to us, the end user, unless we have a room the exact dimensions as the one in the test – that being 24’ x 20’ in this case. And – note that the primary focus of the test was to get the best response at the maximum number of seating locations (I don’t think many of us entertain 16 guests at a time), not to get the best performance from one or two subs in a room. Bottom line, this paper has little to do with the day-to-day situations the majority of us live with.

    Beyond that, take a look at the conflicting conclusions drawn at the bottom of the paper. Slide #74 says “ Four subwoofers are enough to get the best results of any configuration tried. Two subwoofers is nearly as good and has very good low frequency support as well.”

    Yet, the text right below that says “One subwoofer at each wall midpoint [i.e., two subwoofers] is the best in terms of Std. Max-ave and Max-min but does not support low frequencies well (emphasis added). One subwoofer in each corner... has good low frequency support, but does not perform as well... in terms of Std. Max-ave and Max-min.

    With the second quote, they say exactly the same thing I said in my post above. Namely, that out-of-corner placement gives less extension, while in-corner gives better extension but more extremes in response deviation.

     
  6. TimForman

    TimForman Supporting Actor

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    I found the conclusions less than conclusive too. Another point that created a question mark in my mind was the authors attempt at finding practical real-life solutions and then he puts the sub in the middle of the front wall. I'm pretty sure most of us put our video display devices there. [​IMG] Another wp on the same site was "Getting the Bass Right" I thought was interesting and gave me a better understanding of room modes. It explains very well why I have a 56 Hz dip with one sub. I only have six seats in my HT and the bass sounds very equal at all of them. I have one odd corner where the bass sounds like crap, so I put the cat scratcher there. (I made it out of surplus Sonotube so technically it's a Sono-scratcher). Thanks for your input. It's to my benefit to have pros like you on this forum. Anybody else?
     
  7. TimForman

    TimForman Supporting Actor

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    I have another question. The "both subs in one corner" setup results in higher SPL but does it help overcome nulls? It seems that with a second sub in the adjacent corner I've reduced the effects of a ~50 Hz null in the center of my room, which coincidently is the listening area.
     
  8. Justin Ward

    Justin Ward Supporting Actor

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    When you say both subs in the same corner, what do you mean?

    1) Both subs are stacked
    2) The two subs are equally close to the 2 walls which meet to make a corner. Ex, a sub in the corner and then another sub that touches the corner of the sub not against the wall.
     
  9. David Giesbrecht

    David Giesbrecht Second Unit

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    I seem to get really good results using two subs in my HT. My room is 21'wide 12'deep and 7'high each of my sealed tempest subs are placed about 1.6 meters from each side wall which is pretty far out of the corner and I get a flat response +0/-2 db response at any seeting position in my room actually as long as your close to the back wall where the seetig is it doesnt matter where you are. Thats a 21 foot wide sweet spot!
     
  10. Wayne A. Pflughaupt

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    Tim,

     
  11. Dean Mar

    Dean Mar Stunt Coordinator

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    Great information here everyone. I only have one sub (its a forward firing sealed Tumult) and it is corner loaded. The question that I have is, does it matter which way I position the driver to fire ? At present, it fires out away from the wall. Is there any benefit (more spl ?) to having it fire at the wall ? It is presently sitting about 3 inches from each wall in the corner and firing more or less at the seating positions.

    Thanks for any light that you can shed on this and if you can't, then maybe I will give it a try and do some measurements.
     
  12. Wayne A. Pflughaupt

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    Dean,

    The way you presently have it positioned is fine.
     
  13. RichardHOS

    RichardHOS Second Unit

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    Unless my memory has gone whacky, a null in the middle of a room is typical of the first axial mode for each direction.

    From all the reading I've done, this is my conclusion: placing a sub in the corner excites all room modes, which is good if your room has many modes relatively evenly spaced, and bad if your room has fewer modes bunched into groups. I'm not sure there is really any generalization that can be made, other than to say "put it where it sounds best in your room."

    I do, however, find the logic of "maximizing output" instead of "optimizing response" with placement a bit suspect. If you place the sub simply with regards to maximum output, you'll likely have that max output at a few peaks which you will EQ out. If you place the sub with regards to an even frequency response, then you don't have the maximum output at a few frequencies you otherwise would have had, but then you don't have to remove those peaks (to as great a degree at least) with EQ. In the end, the output should be roughly the same. Also, when you activate all room modes that includes nulls at certain locations, which may be impossible to get rid of with just EQ.

    If I had the measuring gear and free placement of a sub, I'd move it around until there were as few nulls with as with as small a dip as possible in the widest listening area I could, and wouldn't worry about output or extension. I'd then use EQ to level out peaks and low frequency extension (or use EQ and a transform to the same effect).
     
  14. Dan Wesnor

    Dan Wesnor Second Unit

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    HSU has consistently recommended that the sub be placed as close to the listener as possible. I agree.

    Aside from that, the best way to deal with two subs is to stack them on top of each other. Trying to position the subs and listener for optimal interaction with room modes is a crap shoot at best. The ideal placement is usually only a few inches wide with severe changes in response if you're not exactly where you need to be. (Voice of experience.)
     
  15. Chris Tsutsui

    Chris Tsutsui Screenwriter

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    This White paper has taught me a lot about subwoofer placement.

    I spent a week with my two tempests in my rectangular room, and took calculations, and SPL measurements at many locations. Pretty much all of my records supported that Harman white paper which is why I chose to position my dual subs the way they describe.

    I did not like how the sub sounded when it was against a side wall, nor did I like how it sounded too close to me. I ended up with my 2 subs placed in the front two corners because I had to use them as two stands for my bass traps.

    Anyways, I'm a firm believer in placing stereo subs to cancel out standing waves and help tune the bass response. If you follow that paper closely, it'll teach you how to position both your mains, subwoofer, and listening position to achieve greay results before using a BFD.
     
  16. JoeFish

    JoeFish Stunt Coordinator

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  17. TimForman

    TimForman Supporting Actor

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    Yep, 'round these parts when the term "corner loaded" is used in reference to sub placement it means the low frequency reinforcement given by the three surfaces near the sub.
     
  18. Drew Eckhardt

    Drew Eckhardt Stunt Coordinator

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    No.

    When the space a transducer operating into is cut in half, you get 6dB more on-axis output. IOW, a sub-woofer in the corner of an infinitely large room plays 18dB louder than the same woofer floating in the middle of the room. A sub-woofer in the middle of a parking lot plays 6dB louder than one in an anechoic chamber. Etc.

    In reasonable sized listening rooms, the boundaries are close enough that you still get a reasonable contribution to SPL from the off-axis output of a sub-woofer. This means that the gain achieved is less pronounced (IIRC, I lost about 3dB when I temporarily moved my sub from the front left corner to somewhere along the side-wall for construction), but still there at all frequencies in the sub woofer's range.

    In practice, this also has the effect of creating the fewest nulls which can't be equalized.
     
  19. Chris Tsutsui

    Chris Tsutsui Screenwriter

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    "So, how can a forward firing sub be corner loaded if it's facing away from the wall?"

    I have been wondering the same thing.. I think physics has the answer though..

    The subwoofer radiates sound in a sphere pattern radiating pressure from the subwoofer driver. Don't think it as a tweeter which will shoot sound more like a sprinkler or garden hose.

    Perhaps think of it as a person passing gas in a room and how everyone around you can smell it even though your rear end isn't facing them. lol bad example...

    Anyways, you know that bass must be able to travel through bending tubes like ports or pipes, and that bass isn't going straight through the MDF box and hitting your ear when the driver isn't facing you... So you are basically hearing the sound from the driver simply because air particles are moving around the box and to your ear.

    Put the sub in a corner, and the walls block air from passing in that direction and this gives the blast a direction like a horn.
     
  20. Brett DiMichele

    Brett DiMichele Producer

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    "think of it as a person passing gas in a room and how everyone around you can smell it even though your rear end isn't facing them"


    OKAY! I vote Chris as the MVP Of The Year for that interesting
    Analogy! [​IMG] [​IMG]
     

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