Sub Placement - SPL vs. Flat Response

Discussion in 'Archived Threads 2001-2004' started by Doug_L, Jul 10, 2001.

  1. Doug_L

    Doug_L Stunt Coordinator

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    Question for you guys:
    I know that placing a subwoofer in a corner can give you a big boost in SPL (is it 6db, or 9db?).
    I've read about modes and nulls (peaks and valleys in SPL, essentially) and most of the articles I've read emphasize a near center-line placement on subs to flatten out the response across the room. (If I remember correctly, near center-line is about 2/5 across the room, as opposed to 1/2, 2/3, 3/4...). This assumes a rectangular room, of course, which almost none of us have but... the graphs, numbers, charts and scientific reasoning all seem accurate.
    So... how come people are always recommending that others "put their sub in the corner". Won't this result in overly boomy bass? And won't this result in vastly different levels of bass depending on where people sit? Should I assume that this means that many of us have a preference for too much bass, even though we labor with SPL meters to match all of our other speakers?
    Just looking to see if there's some iffy information being passed around of if I don't understand this well at all.
    Hope somebody can clear this up.
     
  2. Ray R

    Ray R Stunt Coordinator

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    My experience is exactly like you described, corner placement = boomy bass and wild swings in frequency response. Something else that happens when you don't have the sub in the same line as the mains and center is phase cancelation. This is huge concern in the field of profession audio and sound reinforcement, but is rarely considered in HT.
    Personally I go for flat response which for me was placing the sub along the side wall nearly even with my main speakers and facing the back wall. I feel that if corner placement is needed to avoid distortion, then your sub/amp is under sized.
     
  3. Phil A

    Phil A Producer

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    I would agree with the above post. If your goal is to get the maximum bass reinforcement without other considerations then corner placement is generally the best. If SPL is not the only thing you are considering (e.g. smooth response when listening to music) then you need to take into account the placement of what you have now in conjunction with the sub and the room and the bass management available in your preamp/receiver, e.g. whether it can be modified or turned off (and use the crossover on the sub set to match your mains), etc.
     
  4. Bob McElfresh

    Bob McElfresh Producer

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    Here is my understanding & theory why moving a sub along a wall changes things:
    A lot of the sound from a sub does not come directly from the driver, but is reflected from a nearby wall.
    A given length of wall will reflect one frequency better than others.
    And the longer the wall - the lower that one frequency will be.
    And it takes a LOT more power to produce sound the lower you go in frequency.
    So corner-placement is trying to use the maximum length of the wall to re-enforce the lowest-possible frequency. And since many sub's tend to curve off lower down, this can help bump up the lower end and make it sound linear. (But it can also over-emphasize the lower frequencies.)
    MOVING THE SUB:
    Moving the sub along the wall to the 1/3 or 2/5's position changes things. When the woofer pulses the sound, it has 2 smaller walls to reflect off of, not one. This is because the reflecting surface should be measured from the center of the woofer to each end. (Big theory point here - I could be wrong).
    So moving the sub towards the center will no-longer reflect the lowest possible frequency. It will now emphasize 2 other frequencies depending upon the distance to each end from the center of the sub.
    This CAN (but not always) reduce the lowest peaks and increase some other frequencies higher up. This produces a smoother, flatter response, but at the cost of less over-all SPL, and less SPL at the lower frequency.
    SO WHICH PLACEMENT IS BETTER:
    I think the corner placement is better for movies. You want the over-emphasis to make you queasy and drag you into the action. And to do this, the higher SPL at the lower frequency is better.
    But for music, you are listening to a blend of sounds and voices. And part of the "art" is how some sounds swell and reduce. You don't want to over-emphasize any part of the sounds because you want to appreciate how the artist chooses to do this.
    So this is my model of how moving a sub along a wall changes things. (And I know there are lots of other issues but I am focusing on the "...where along the wall" issue).
    Am I way off?
     
  5. Ned

    Ned Supporting Actor

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    Tom Nousaine has definitively said that corner placement will almost always give the smoothest and deepest response. This corner placement = boomy belief is an old myth that you guys need to forget about.
    Will a few rooms get better response with non corner placement? Yes. But the vast majority of rooms will do best with corner placement.
    ------------------
    My Home Theater Page
     
  6. Robert Fellows

    Robert Fellows Stunt Coordinator

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    Doug:
    What I have seen recommended is to place the sub where you would normally sit, and move around the room until you find the best "bass response sound" that you desire...then place the sub there.
    ------------------
    Bob
    p.s.: This advice is worth exactly what you paid for it...
     
  7. Richard Greene

    Richard Greene Stunt Coordinator

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    If your goal is the tightest frequency response variations at subwoofer bass frequencies (under 80Hz.), a room corner is usually best. Placing a subwoofer in a room corner will drive all room modes (standing waves) to the maximum. At best, you could obtain a relatively flat frequency response with three narrow sharp axial room modes at 566/room length in feet, 566/room width in feet and 566/room height in feet
    in a rectangular room.
    Let's assume these bass peaks are 5 Hz. wide and 10dB louder than other frequencies. The resulting frequency response could be described as +/- 5dB in the bass frequencies (this would be excellent in-room frequency response with no smoothing). Some bass notes will sound too loud and linger too long in the listening room.
    If you place the subwoofer at any other non-corner position,
    you will not fully excite at least oneo of the three axial modes. This will result in a frequency response trough around that frequency. So now you will have some frequency response peaks AND at least one frequency response trough too. Some bass notes will sound too loud and linger too long in the room ... and now some will sound too soft -- combining the peaks and trough(s) will greatly increase the frequency response deviations -- perhaps now you'll have
    +/- 10dB rather than +/- 5dB. Larger bass frequency response variations do not sound better to me.
    In 1994 I had two "stereo" DIY subwoofers used as left and right satellite speaker "stands". Based on measurements by fellow audio club member Tom Nousaine, I tried placing both subwoofers stacked in the same corner. At first they sounded "boomy"
    ... but after measuring them I realized why:
    The ouput in the octave centered at 30Hz. was now 9dB louder than before and my side wall was resonating! The solution was simple and inexpensive: I turned down the volume by about 9dB, moved the sub a few inches away from the side wall and placed a vertical cement patio slab between the sub and the side wall, leaning against the wall to silence the loud noises it made. Now my sub sounded better than ever when listening at my "sweet spot" location and at several other seating locations too.
    Earlier this year I replaced my two 12" corner subs with a single 15" DIY Tempest tube sub (same corner) -- it rattled the back wall so I had to use a second cement slab to silence that wall. If my floor under the sub was not made of cement, I'd probably need a third concrete slab right under the sub.
    In summary, the initial "boom" from my corner sub was from the sub playing much too loud and the side wall resonating. It is also possible to get a boom from a corner sub if it has a lot of output above 80Hz. because moving a sub that's located away from a corner into a corner tend to boost higher frequency bass even more than lower frequency bass. That's one reason corner subs sound best with 24dB/octave slope low pass filters no higher than 80Hz. -- another reason to use 24dB/octave filters is that bass output above 80Hz. is directional and can make the bass sound like it's coming from the corner when you really want the illusion that the deep bass is coming from the main front speakers.
    It's interesting that at very low frequencies below the lowest axial room mode (566/largest room dimension in feet for a rectangular room), sub location makes no difference.
    the sub will be realatively "close" to a corner no matter where it's placed (very long wavelengths at those very low frequencies).
     
  8. Wayne A. Pflughaupt

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    Richard’s experiences mirror mine exactly.
    I’ve had two subs for the past 4-5 years located to the sides of the front speakers. One of the locations did happen to be in a corner, and it was immediately obvious that it had much higher output that the other one, which was on a long, flat wall. Even after EQing out a 40Hz hump in the corner sub, it still had considerable more output than the other one, not to mention better extension. Response to 40Hz was the best the non-corner sub could do; response fell like a brick below that point
    When I finally a couple of months ago experimented with putting both subs in the corner, I was astonished at the improvement. I was able to achieve the high and effortless output at 20Hz that I had never been able to accomplish, which was difficult to realize in my cavernous 6200 cubic ft. listening space. Overall SPL level increased at least 4dB. Of course, you can always get 4dB more SPL level by adding a second sub to a corner, but this showed that with the prior left/right arrangement I was getting no more total SPL level than if I had been using one sub!
    Responding to some of the posts above:
     
  9. Ray R

    Ray R Stunt Coordinator

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    I guess what it comes down to is every room is different. I have taken measurements for several different sub locations and the flatest response was out of the corner. I also notice less lingering of bass which is important when I'm being hit in the chest with a good double peddle kick drum. [​IMG]
    Richard,
    How does not exciting one of the room modes cause a trough at that frequency? I thought modes just "add to" the flat response. I thought a trough would be due to phase cancellation. Which leads me to Wayne's comments about phase cancellation. Since the crossover point is not a brick wall, the mains and sub do share a band of frequencies and therefor could cause phase cancellation. I experienced this for one location of my sub around the 57 hz frequency.
    One last thing,
    You bastards. You put just enough doubt in my mind that now I have to spend the weekend moving my sub around and taking measurements.
    [Edited last by Ray R on July 11, 2001 at 09:02 AM]
     
  10. Doug_L

    Doug_L Stunt Coordinator

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    First, thanks to all for the informed responses.
    Second, I should say that I'm not a corner placement kind of guy, and I'm not looking to move my sub; I was looking for a more theoretical discussion, which I think we've accomplished.
    I guess we should assume that we're dealing with rectangular room. As far as exiting all three modes I agree with Richard Greene that corner placement is the way to go. I have some problems with this, though.
    My concern with intentionally exciting the three first order room modes is that the sound level becomes extremely dependent on the room location of the listener. Just looking at one axis for now, the first order mode will have peaks at either end wall (or is it in the middle for the first standing wave? I always forget.) As we go to the second, third, fourth etc. standing waves we see a distinct pattern. If the listener is sitting in the middle of the room (1/2 of length) then he's on the strongest peak, right? So far, so good.
    But what about the person sitting to his side? Lets say the side-sitter is 1/4 along the wall. He may be in the deepest trough (he may not be, though. I can't recreate the graphs to look at now). The point is that there will be a trough somewhere.
    If we accept that there has to be a trough, I'm wondering how to minimize that trough RELATIVE to the highest peak, ie: the flattest response across all listening positions (within reason - nobody sits with their head at the ceiling or along the front wall). That should have been my initial question: is corner placement appropriate for the flattest response across multiple listening positions, or is there some other variable point?
    For both movies and music I'm looking for the flattest, not strongest response. Am I right in assuming that corner placement is not appropriate in attaining this? Please explain if I am wrong.
    P.S. The resonance from the corner walls making/contributing to the boom makes a lot of sense.
     
  11. Richard Greene

    Richard Greene Stunt Coordinator

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    My word "trough" didn't work at all in my last post, so I'll try using "mountain ranges" and "valleys" this time because that's what cumulative spectral decay charts of room modes look like!
    I'll also try to differentiate between how room modes measure (interesting charts) and how they sound (more important than charts) to our ears. If our listening rooms sounded as bad as they measure with a slow sine wave sweep, we'd probably give up this hobby!
    The narrow (5Hz. wide) sharp (+10dB) room modes I described in my first post are a better description of what you'd hear in an all-concrete room -- in a normal room with flexible walls and windows the room modes tend to sound like they cover a broader range of frequencies than 5 Hz. and USUALLY don't sound +10dB loud when listening to music unless two modes overlap. Our ears smooth the frequency response somewhat and so do flexible listening room walls and windows.
    These frequency response peaks sound worst when three of them overlap (cubical room), two of them overlap (square room) or two of them are directly adjacent (near-square room). Using a corner sub to fully excite ALL the room modes in a cubical, square or near-square room seems counterproductive. Alternate non-corner positions have the potential to sound better for square or near-square rooms.
    Near field sub positions often work well in "problem" rooms, at least for those people sitting real close to the sub.
    Another problem that makes room modes more annoying is a subwoofer position that excites the length axial mode and the height axial mode, but fails to excite the width axial mode between them. If you place one sub against the front wall half way between the left and right side walls, for example, it will be in a low pressure zone for the width (side wall to side wall) axial mode -- won't energize that mode very much. On a cumulative spectral decay chart that looks like two "mountain ranges" separated by a wide "valley".
    Compare that to using a corner sub:
    the cumulative spectral display chart of a corner sub would look like three mountain ranges with two narrow valleys between them.
    Most people would assume two "mountain ranges" would sound better than three "mountain ranges". And that seems logical because on a chart three "mountain ranges" do LOOK worse than two "mountain ranges" ... but they usually SOUND better!
    Here's why:
    Two peaks and wide valley between them are not smoothed well by our ears -- both of the peaks and the valley are easy to hear. You can use an equalizer to reduce the peaks but boosting output won't do anything to fix the valley.
    It's interesting to note that using left and right subs under the main left and right speakers, as I used to do, also doesn't excite the width axial mode very much. Each subwoofer individually excites the width axial mode but when both are playing they excite the mode out of phase and "cancel" each other. So you still have a wide valley between the room length mode and room height mode.
    The corner sub with it's three peaks and two valleys is more effectively "smoothed" by our ears -- so the two relatively narrow valleys may not be audible at all. And the peaks can all be reduced with narrow band equalization.
    At even higher frequencies where there are many peaks and valleys (greater modal density) the peaks and valleys are smoothed even more by our ears and are even less audible.
    Above 300Hz. they may not be audible at all.
    Hope this explanation was easier to understand.
    I don't have the energy to re-write again!
     
  12. Ray R

    Ray R Stunt Coordinator

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    Richard,
    You're talking about the effect that corner placement has on the decay of the frequency and not just the SPL, correct? So then corner placement with an EQ on the mode frequencies would flatten the frequency response and reduce the space between frequencies where decay is not effected. It sounds like the way to go, but I always assumed that boomy bass was caused by slow decay?
    ------------------
    HT pictures
    [Edited last by Ray R on July 11, 2001 at 07:07 PM]
     
  13. Richard Greene

    Richard Greene Stunt Coordinator

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    The boom is mainly caused by a 1/2 wavelength reflection "piling on to" the next sound wave coming out of the speaker = too loud at those frequencies. You can use an equalizer as a notch filter to reduce the SPL of the standing wave but it's still there because you have not done anything to prevent the room reflections (such as using bass traps), so the slow room decay is still there too (although not as loud as before, so it's less annoying).
    Standing waves are a time-based room acoustics problem that can't be solved with with an electronic component such as an equalizer adjusting the energy coming out of a loudspeaker ... but an equalizer can sure make the problem much less noticeable. I've used equalizers with my subwoofers for two decades.
     

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