Herschelmann's dual sub remarks

Discussion in 'Speakers' started by scott>sau, Jul 16, 2003.

  1. scott>sau

    scott>sau Stunt Coordinator

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    Russ Herschelmann gave a good CEDIA speech on multiple subs from his "Home Theater: Essential Elements".
    "As a general rule use twice as many subs as you think you need."
    "Multiple subwoofers placed asymmetrically to yield smoothest bass response." I can send you a scanned pic if you want of the placement . Basically he has a sub front left corner and the second sub mid-to-front right side (staggered, lack of balance).
    Dual subs in proper positions reduce standing waves.

    Doug Osbourne of M&K Pro suggest two enclosures stacked in the front corner. Avoid center of the room placement. Place subs in the first half of the room.
    Hope this helps
     
  2. TimForman

    TimForman Supporting Actor

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    Wow, I "accidentally" did it like Herschelmann suggests. Whaddya know?
     
  3. Bhagi Katbamna

    Bhagi Katbamna Supporting Actor

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    Herschleman also recommends placing the subs asymmetrically wrt the height distances from the floor.
     
  4. Kevin C Brown

    Kevin C Brown Producer

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  5. Wayne A. Pflughaupt

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  6. Terry Montlick

    Terry Montlick Stunt Coordinator

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    I agree with Kevin on having a look at Todd Welti's paper. He did some really top-quality research, which he presented at the 2002 AES Convention in Munich. I know this paper has been passionately debated in another thread, but I think it's worth bringing up again.

    Welti, an acoustician at Harmon, did extensive simulations of multiple subwoofer placement along the perimeter of the room. He used a few different criteria to rate the uniformity of subwoofer sound heard over a modest seating area. His simulations were well confirmed by measurements in a real room.

    He found that the best placement of 2 subwoofers was front center and rear center. Adding extra subwoofers above 2 didn't help much, but slightly better were 4 subwoofers placed in the center of each wall. Above 4, there was no significant improvement; at least not until you got to on the order of 50, placed randomly throughout the room to equally positively and negatively excite the room modes! A huge number (5000) randomly placed subwoofers essentially canceled all room modes, as you would expect.

    He modeled this assuming that the the single subwoofer channel would be equalized to be flat. Now, most home theater owners do not own a parametric equalizer with an arbitrary number of filters. He reran the simulations without this assumption, using the absolute, non-EQed flatness of the response. He got the same results! Furthermore, these results were the same for different room dimension ratios.

    People have strong opinions about multiple subwoofer placement. But this paper is pretty hard to ignore - it was the first exhaustive scientific study of its kind.

    Regards,
    Terry
     
  7. Wayne A. Pflughaupt

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    Personally I have a few problems with Mr. Welti’s work.

    First, when I read the conclusions that Mr. Welti draws from his research, I draw a different conclusion.

    He states near the bottom of the paper,” 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 first part of the quote, Mr. Welti confirms the same thing Tom Noussain’s research has consistently found, that out-of-corner placement gives less extension. Many users (in as many rooms) do indeed find true what Mr. Welti noted in the second part of the quote, that in-corner placement gives better extension but more extremes in response deviation. However, the deviations are usually uniform enough to allow for effective equalization (as is evident in Mr. Welti’s charts).

    Second, Mr. Welti’s work was done in a nearly-square room – 20’ x 24’. It is commonly recognized that square rooms are the worst, from an acoustical standpoint.

    Third, 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. Indeed, Mr. Welti states near the top of the paper, “ A large percentage of listening rooms are rectangular [again, calling this room rectangular is a stretch], but for those that are not, conclusions reached here are not valid.”

    Further limiting practical application (as Tim noted in an earlier thread discussing this paper) Mr. Welti recommends locating one of the subs in the middle of the front wall – which is exactly where we put our display devices!

    Regards,
    Wayne A. Pflughaupt
     
  8. Terry Montlick

    Terry Montlick Stunt Coordinator

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    Hi Wayne,

    You raise some good points. Here are my answers to them.

    1. Yes, you do lose efficiency in terms of low frequency support. I've always thought that this is a case of not being able to maximize both power transfer and smooth response. You either excite the modes maximally or excite them neutrally. Welti's data consistently shows a negative correlation of these two. You can't have your cake and eat it too, with regard to subwoofer power efficiency and smoothness.

    2. A 20 by 24 room is squarish, but there is a 20% difference in these two numbers. This is much greater than the 5% or less difference which indicates too-similar dimensions, according to Robert Walker's work. Much above the 5% number, the room modes from the different dimensions don't bunch up and overlay each other, causing problems.

    3. Welti repeats his experiment for 4 other room ratios, including 1:2.22:2.89, a commonly recommended ratio. The results were the same in each case - no different from the test room. So it appears that this work is valid regardless of room dimensions.

    As for practical application, you are absolutely right. The recommended 2-subwoofer placement, putting subwoofers on the floor at the midpoint of a wall, is not practical for everyone. You need a projection screen or wall-mounted display. Then you can put your center channel directly below or above (or for front projection, behind) your display, and one subwoofer on the floor right below this.

    Again, thanks for your comments!

    Regards,
    Terry
     
  9. Kevin C Brown

    Kevin C Brown Producer

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    Hee, hee. I've been toying with the idea of getting another sub for about a year. But, ... I have a TV smack dab in the middle of the one wall. [​IMG] (Stacked in the corner is what I'd probably do.)

    Also, the two-sub/mid-point placement seems to be the best spot for a "compromise" solution between low freq extension *and* flat freq response.

    The largest issue is that for the 20x24' dimensions, the two sides are not equal, and hence, does not reinforce the same nodes within the room. So is applicable to most people's setups. And, ... you cannot argue with real measurements. Most of the Hershel... dude's arguements are simply based on calculations. (Yup, read all his SGHT articles.)
     

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