rear sub a bad idea???

Discussion in 'Speakers & Subwoofers' started by Travis_R, Nov 21, 2004.

  1. Travis_R

    Travis_R Stunt Coordinator

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    I was thinking about putting a dual sub setup in my theater, one in the front left and one in the rear behind last row of seating on my riser "there will be two rows of seating" Is this a bad Idea, I was talking to one guy and he said it would throw things off because the bass was supposed to come from the front of the room. but a friend of mine has a Bose system, DONT WORRY I DONT WANT BOSE, anyway his sub is right next to his couch about 10 feet back from the tv and it sound great, I cant tell where the bass is coming from.
     
  2. Lewis Besze

    Lewis Besze Producer

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    Yes, if you use multiple subs but fed from the same signal[LFE] then a single position is prefferable.It doesn't have to be the front of the room though,it really depends on a lot of things so experimentation is the key.
     
  3. Cees Alons

    Cees Alons Moderator
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    For frequencies below 100 Hz (which is usually the case for subwoofers) there is hardly room for a full wave in your room, so the effect of a sub will be to higher and lower the total pressure inside the room - much like a pump.
    That's why standing waves are almost impossible at these frequencies and the place of your sub is hardly detectable by your ears.

    That said: it's not wise to place two or more subs too far from each other (and DO make sure they're in phase - or else one pump will have the effect to more or less cancel the other one), because some phase shifts may still occur.

    The best place for (all) subs can be determined by the old reverse-trick: put your subs in your preferred seating, walk around the room to hear where the total bass sounds strongest: that's the ideal place for your subs.


    Cees
     
  4. gene c

    gene c Producer

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    I'm currently using two Dayton 10"ers, one on each side of the couch under end tables. I don't know about phase shifts, air pumps or same signal lfe experimentations (not that there's anything wrong with them!) all I know is it sounds, and feels, great! With one sub at low listening levels, since there isn't enough volume to fill the room, you can tell where the bass is coming from. With two subs, one on each side of the room, the bass still sounds as if it's coming from everywhere, even at very low levels. And at higher volumes? Well lets just say I don't need any of those "bass shacker" things! But I would need to send letters of apology to my neighbors! But two subs, front and back? That I'm not to sure of. Maybe learning about phase shifting, air pumps and same signal lfe experimentation isn't such a bad idea. I've always felt bass should come from the rear but obviously I'm not as knowledgable about this stuff as most others. I just know what I like. And I'll take two please!
     
  5. Brian L

    Brian L Cinematographer

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    While I am not sure I can site a actual scientific reason why frequencies below "X" are not locatable, I think I can say with some certainty based on my readings here and elsewhere that standing waves surely can develope in the bass region in a normal sized room.

    In fact, the only place they won't develope is outdoors, which is why many serious sub reviewers will take initial FR plots outdoors. And if you have a really big room (like a commercial theater) they are said to develope, but in higher frequencies, above the bass region.

    Peaks and nulls are exactly that, standing waves where certain frequencies stack up. When they are additive, you get a peak, when they are opposite in phase, you get a null.

    There are good many users on the HTF that are using Behringer Feedback Destroyers (basically a parametric EQ) to combat and correct for those very occurences.

    Now, as for the question at hand, there are some pretty sharp people (Russ Herscehlman, Dr. Toole at Infinity) that advocate properly placed and adjusted multiple subs. But, it is not something that can really be done properly by ear; you must measure to have any hope of getting the phase, crossover, and level right. Many equally smart and serious guys advocate a single properly placed sub for most users.

    I am going back and forth between one and two at the moment; a HSU 1225 behind my couch and a smaller, traditional Klipsch in the front corner. No verdict yet. I have had great sound with one and great sound with both.

    I will say this; parking a sub behind a couch such that the prime listening spot is in the near field does wonderful things for ones tactile sensations. Bass shakers my butt...just park a capable sub behind the couch, and be very ready when the depth charges in U571 start going off!

    BGL
     
  6. Travis_R

    Travis_R Stunt Coordinator

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    OHHHHHH THE DEPTH CHARGES IN U571!!!!!!!! THAT is going to be one of the first movies I watch when I get this theater done.... the I CANT WAIT factor is unbearable, I think I am going to go ahead and make an extra run of flex tube and an outlet to where I would have a rear sub so if I want I can put one there and if I dont like it no harm done, I do plan to add bass shakers regardless, thanks alot guys
     
  7. Cees Alons

    Cees Alons Moderator
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    The standing wave would have the length of ... well its wavelength. If your room is smaller than that, it's impossible. Many so-called experts forget about that.

    The reason you cannot easily locate a bass frequency with your ears is because the wavelength is so much bigger than your head. So the airpressure is almost the same on both sides of your head, where your two ears are located.


    Cees
     
  8. Mary M S

    Mary M S Screenwriter

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    I barely shoehorned one SVS PC-Ultra in my room. But I have always been intrigued by some posted arguments favoring dual subs, and would like to attempt that config. In my short amateur career learning how to set up my AV, it has been my experience that if your sub is integrated correctly for your room acoustics and in level with all speakers, it should NEVER be locatable. If you can locate it, something is wrong and your system needs tweaking. During explosive type sequences in movies, this non-locatable LFE, is amazing effective in my room. If the explosion is recorded in the left rear track, as if it were suddenly occurring behind me during a movie, you would swear the sound IS all behind you, (If you did not know better) During a lower freq Knocking on Door recently, enforced by my left surround (HILL HOUSE movie, a guest got up to open my door ...to the left of the room!) Just that tiny psychoacoustical indicator of the higher freq. portion of explosions etc. recorded in the BL surround channel track, running the Ultra causes us to almost feel the heat of the flames on our backs. (you feel the slam). If its CD and I am listening in stereo and hear the bottom octave hit on a grand piano placed stage right during a recorded live performance. I hear/feel that note thrumming from my left (stage right to the artist) when seated facing my fronts.


    One SVS is completely adequate for my room but just 3 days ago I actually pulled out a Stanley and was measuring (when my husband was not looking) to see if I could squeeze an SVS (box style) 2nd unit in, (by replacing my end table for it!)
    No room for playing with optimizing dual subs. … IF I was ever crazy enough (must be that Edward rubbing off!) to try to fit a second one in, there is no choice for me in location. I could only adjust via an EQ, correct phase, and hope to have the sheer dumb luck, that the only locations available would work in this room. Would be love-ah-ly to try it. [​IMG][/size][/font]
     
  9. Kevin C Brown

    Kevin C Brown Producer

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    Cees- OK, you're saying what I think you're saying but in a different way than I'm used to. Standing waves can't develop for freqs longer than a room dimension.

    I know that if you have a dimension of a room L, that the 1st standing wave develops at:

    the speed of sound
    ------------------- = Freq
    2 * L

    (I use 1130 ft/sec for the speed of sound.) So for me, L = 17.1 ft, so the 1st node is 33 Hz. Then, you just multiply that node by n to get the nth node. 33, 66, 99 Hz, etc. For comparison, the freq at a particular wavelength is the same eq without the 2. So the freq at a wavelength of 17.1 ft is ... 66 Hz. So you can have standing waves develop for freqs with wavelengths up to *twice* a room dimension.

    I think the 2 is in there because a wave gets reflected at a wall backto the room. (Got the eq from SGHT and those columns by Russell Herschelman. I did spell that wrong. [​IMG] )

    My height nodes are H=7.85ft so 72, 144, 216 Hz. Similarly, the freq at a wavelength length of 7.85 ft is 144 Hz.

    So I get no nodes below 33 Hz. This where it gets neat: I have plotted my freq response to a 1/10th octave spacing or smaller, and I get flat response +/- 3 dB from 14 Hz to about 40 Hz. So I'm good to slightly higher than what theory predicts. (I do have a small dip at 33 Hz btw!)

    By the way (ADVERTISEMENT FOR SVS SUBWOOFERS HERE), if I open up the limits to +/-5, no eq btw, I get flat freq response from 11 Hz to 40 Hz. (I have an SVS PB12-Ultra/2.) The wavelength of an 11 Hz signal is ... 103 ft. Which seem impossible that my room can generate. But it does. My last dimension is 29 ft. (My HT is 1/2 of the big room that includes the family room and kitchen.) I did read an article once about how that occurs, but can't remember the details. Maybe due to reflections again.
     
  10. Jon W.

    Jon W. Stunt Coordinator

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    Well I have 2 Klipsch KSW-15's and the 10's in the bottom of my NHT VT-2.4 towers are driven with an NHT SA-3 amp all hooked to the LFE out. Going from just 1 KSW-15 to this setup was like OMG WOW! There is a 15 and a 10 in the front left and front right corners and I eq'd the 10's via one channel of a BFD and the 15's via the other channel and produced fairly good results. I plan to ditch the Klipsch subs after xmas due to their lack of output below 25 hz but I used the BFD to boost their 20 hz output and they are impressive. Say what you want about dual subs and not co-locating them but it worked well for me.[​IMG]
     
  11. Jeff Gatie

    Jeff Gatie Lead Actor

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    A wave does not need to complete an entire wavelength in order to be "heard" (or felt), this is a myth. The wave only has to propagate through a point source (your ear, body, a microphone, etc) in order to be heard or recorded. You can very easily hear a 20Hz wave from a headphone capable of producing it and that distance (source to ear) is much less than the 20Hz wavelength. The problem that comes when the wavelength is longer than the room dimensions is reflections that cancel out the wave before it propagates. This is what causes cancellation, which is very different from the saying that a wavelength can't be generated in a room smaller than it's wavelength. Cancellation is effected by many different factors besides room length (geometry, sound dampening, etc.) and there is really no surprise that a 11Hz signal is detectable in your room.
     
  12. Cees Alons

    Cees Alons Moderator
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    Kevin,

    What Jeff says.

    You do not need a standing wave (which is bad, BTW) to hear the sound. In a way (I try to go on saying it in a different way than usual [​IMG] ), your sub "slowly" (slowly, because it's a low freq.) increases the air pressure in your room and lowers it again, and that will move your eardrums. There may indeed still be resonances at specific frequencies, or canceling out of some others as a result of other properties of (in) the room.

    But here we're talking about the effect of more than 1 sub in a room. And they can only cancel each other if the air pressure stemming from one sub at a certain point is cancelled by the effect of the other sub. That may happen if they're not in phase, but if they are, not likely by wave path differences. The lengths of those very low-freq waves are simply too big, so the phase differences inside a typical room are minimal.

    Of course, it's not totally zero! Just minor, so they still better be close to each other. [​IMG]


    Cees
     
  13. Jeff Gatie

    Jeff Gatie Lead Actor

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    That's good advice, with colocation being the best type of placement. Otherwise it sometimes is very difficult to address bass nulls due to cancellation, requiring many trial and error placement and/or equalizer settings.
     
  14. Kevin C Brown

    Kevin C Brown Producer

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    Jeff- That's the exact explanation that I couldn't remember. [​IMG] I just went through about a 3 month period trying to decide whether to do 2 subs (or up to 4) vs just 1 big one. I obviously ended up with one big one. [​IMG]

    I did get the impression that it is possible to do a multiple sub setup very well, but an RTA is necessary to do it right. Exactly because of the potential of much worse peaks and valleys due to the constructive and deconstructive interference effects of each sub with the room, and the subs with each other. And I even have ETF5 software and have extensively characterized my room with one sub. But I knew I just wasn't up for the effort with 2 or more.

    But I also wonder if 4 might have actually worked better (or "easier") than 2. I can't do the recommended spots that the Harman International paper suggests (mid points along all 4 walls), but I would have been able to do 2 corners, and the mid points along each of the 2 longest walls. (A setup they unfortunately didn't look at.) Someday maybe.

    http://www.harmaninternational.com/w...articleId=1003

    But a wise man once said, if you like what your system sounds like (no matter how many subs you have and where they are located), then theory be damned! [​IMG]
     

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