Stereo Subwoofers

Discussion in 'Speakers & Subwoofers' started by Robert AG, Oct 17, 2003.

  1. Robert AG

    Robert AG Stunt Coordinator

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  2. Hartwig Hanser

    Hartwig Hanser Second Unit

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    very informative, thanks for your suggestions. I was thinking about the same thing. The problem I see is with SACD/DVD-Audio, since many players, like mine, do no good job at bass manegment. My Pioneer just drops the .1 info, if I tell him "no subwoofer". So what to do about those music discs, that contain a .1 channel? Or is the .1 channel just duplicating the bass info of the main channels for those that do not have large front speakters?
     
  3. Rick_Brown

    Rick_Brown Second Unit

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    I've read that one potential problem with stereo subs is that the bass frequency response becomes poorer due to phase and cancellation problems between them?
     
  4. ColinM

    ColinM Cinematographer

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  5. Robert AG

    Robert AG Stunt Coordinator

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  6. Dan Stone

    Dan Stone Stunt Coordinator

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    Tom Nousaine has an interesting take on stereo bass that was posted at AVS when he was a special guest in December 1999 (http://www.avsforum.com/avs-vb/showt...0&pagenumber=2):

    "Stereo Bass: Very few recordings actually have any out-of-phase information between channels at low frequencies. Think about the recording process and acoustics. If you try to record a 20 Hz note (50 foot wavelength) how do your get 'stereo' if your microphones are 20 feet apart? Those that do are often mistakes or special effects. In summary stereo bass is generally not an issue. I once performed an experiment where listeners were given mono vs stereo presentations using a pair of subwoofers as stands for satellite speakers. No one was able to tell them apart with the best bass programs I could lay myhands on....even with a acoustic jazz recording where all the bass was recorded in the right channel."
     
  7. Robert AG

    Robert AG Stunt Coordinator

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  8. Doug BW

    Doug BW Stunt Coordinator

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    Robert, one concern I have with using your suggested arrangement for 5.1 is that it could potentially put a huge load on the front left and right speakers (and corresponding amps).

    At least, here's why I think this is true:

    A typical 5.1 setup has all 5 main speakers set to "small" and the sub is driven off the sub/LFE out of the pre/pro or receiver. In this configuration, the left and right front speakers receive only what's in the left or right channel, high-passed at the crossover frequency. The maximum strength of the signal to each speaker is mandated to be no larger than what will produce 105 dB at the listening position. So the worst case scenario for the L and R speakers is to see about 105 dB of bass (or less) around the crossover frequency.

    However, in your suggested configuration, the bass from all 5.1 channels ends up getting high passed to the left and right speakers! (That is, the receiver/pre/pro sums the LFE and the low-pass of the center and surround channels, and "splits" this bass between the L and R channels. Each subwoofer thus sees "half" of the total bass, which it then high-passes back to its corresponding main speaker.)

    This means that the bass from the LFE channel near and above the crossover (plus a bit of the bass from center and surround channels) now needs to be reproduced by the main speakers. Since the LFE can be 10 dB louder than any single main channel, the excess load placed on the front speakers could be pretty nasty. I think the worst-case scenario has each front speaker trying to put out about 116 dB of bass!

    For all the people who include a subwoofer or two in their system because their main speakers are unable to cleanly handle the bass load of a 5.1 soundtrack, this alternate method of hooking up the subs may be counterproductive, and should only be done with caution.
     
  9. Robert AG

    Robert AG Stunt Coordinator

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  10. Richard Greene

    Richard Greene Stunt Coordinator

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    "I have used my subwoofers in "stereo" for years."
    RG
    I used "stereo" subwoofers from 1980 to 1994, and a mono subwoofer since 1994. If one sharply limits output above 70-80Hz. a subwoofer becomes sonically invisible almost 100% of the time while listening to music. When a subwoofer is sonically invisible, it can not create a stereo image.
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    "Now say you have someone playing a bass drum or string bass on the extreme left of the stage. The sound of that instrument will reach the left microphone earlier than the right microphone."

    RG:
    This rarely applies to popular music which is recorded in a studio. The bass may be panned left or right but the bass coming from the left and right satellite speakers will place the bass instrument properly on the soundstage if the subwoofer's output is minimal above 70-80Hz.
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    "Considering the frequency of the instruments will be be around 30 Hz in the case of the bass drum and 40 Hz in the case of the bass"

    RG:
    The fundamental frequencies of most kick drums is in the 60 to 80Hz. range -- even unusually large drums are rarely below 50Hz. while the lowest open string on a four-string bass guitar is 41Hz., that note is rarely heard in real music.
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    "If you were to play this with a single subwoofer, or two subs with the bass between the channels summed by the crossover as it is when you use the "sub/LFE" output on your pre/pro, this acoustic delay would simply cause peaks and dips in the response of the bass from that instrument."

    RG:
    That's called comb filtering and I'd prefer to avoid it.
    When you listen to a live acoustic bass in a jazz band,
    the sound has no comb filtering caused by using two microphones different distances from the instrument!
    The sound from the instrument is mono!
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    "As an example of another real-world benefit of stereo subs located next to the main left and right speakers, consider the following example. You have your single subwoofer connected to the subwoofer/LFE output of your pre/pro, and the sub is located next to your couch, and you have it crossed over at 80Hz (in other words, a pretty conventional arrangement). The distance from your listening position to the mains is 10 feet (the sub is 10 feet from your mains). You play a studio recording that has a bass player coming from the right speaker. He plays an open "E" string. Guess what? The fundamental frequency of that string (about 40Hz) will come from the subwoofer next to you. The second harmonic at around 80Hz will come from both your sub and your main speakers. The 3rd harmonic (and above) of the bass will come exclusively from your mains."

    RG:
    One mono subwoofer located near one of the main speakers, and the same distance from your ears as the main speakers, solves that hypothetical problem .. that has nothing to do with the stereo vs. mono sub debate. Some people put two subwoofers next to their couches.
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    "I notice an increase in sense of "air" and "realism" on almost all recordings that have live performers, even studio recordings. While low bass cannot be heard as "directional" in the traditional sense, the way the bass interacts as it mixes acoustically in the listening room certainly can be sensed and felt as added realism, and the filling in of that dimension that you are in the space where the recording was made."

    RG:
    This tends to be most often heard with classical music recorded "live" in large venues and not heard at all with studio recordings where there is no out-of-phase bass information in about 99% of popular music recorded in a studio.
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    Remainder of my speech for those not yet asleeep:
    - The main advantage to using a mono subwoofer located in the vicinity of one of the main speakers (or two stacked mono subwoofers on one side of the room) is the ability to excite the first-order side-wall-to-side-wall room mode (565/width of room in feet = approx. center frequency of that room mode in Hz.).

    Left and right subwoofers will be out of polarity for that room mode so can't excite it. This actually reduces overall bass output by 1 to 3dB., if anyone is interested.

    Since the primary listener is probably sitting half way between the side walls, his ears will be in or near a null for that room mode. If you use "stereo subwoofers" and also sit near in/near the null, those bass frequencies will be extremely weak. So you are usually better off strongly exciting that room mode, because the results in a better bass frequency response for "middle-of-the-room" listeners.

    While using two subwoofers to cancel odd-order room modes between two opposing surfaces can be good news, but cancelling the first-order room mode between the side walls is usually bad news for bass frequency response measured at a "middle-of-the-room" listening position.

    It is true that left and right subwoofers will integrate better with left and right satellite speakers. With a mono subwoofer, output above 70-80Hz. must be sharply restricted
    for good integration. That's not so critical with left and right subwoofers which form a phantom center image for the bass frequencies.

    In summary, while two subwoofers are usually better than one because they produce lower harmonic distortion at a given SPL, there's no guanantee that "stereo" subs will sound better than the same two subs stacked on one side of the room playing "mono" bass. This varies with room acoustics, listening position and the program material played on your system.
     
  11. Robert AG

    Robert AG Stunt Coordinator

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  12. Doug BW

    Doug BW Stunt Coordinator

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  13. Robert AG

    Robert AG Stunt Coordinator

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  14. Doug BW

    Doug BW Stunt Coordinator

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    Actually, with companies like SVS, Hsu, Adire, it doesn't take a lot of wherewithal to own 2 subs. And many who do have gone the bookshelf/mini-monitor route for main speakers.

    There are many people who have reported getting much improved performance from their mains (in the mids and highs) by offloading bass from the main speakers/amps to a sub.

    Even if the excessive bass doesn't damage the speaker, it doesn't mean that the bass will be produced with low distortion or without compression.

    Small main speakers just can't do a good job of producing loud, clean bass. The summing of the signal to the sub and the main to achieve a 0dB power response is merely theoretical if the speaker can't produce the sound at that volume.

    Yes, soundtracks can have strong bass in the main channel. With a theoritcal limit of 105 dB to each speaker. The theoretical limit with your proposed setup is something like 116 dB. That requires more than 10 times the amp power and 10 times the power handling capability by the speaker.
     
  15. Robert AG

    Robert AG Stunt Coordinator

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  16. Phil A

    Phil A Producer

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    I've had stereo subs for yrs. as well. For the majority who are more into HT vs. music it may not be an issue. Properly set-up (yes I am a lunatic with an RTA and other tools) they can make a nice improvement.
     
  17. Bill Polley

    Bill Polley Second Unit

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    Soundhound, if only I had the money...err...the wife's approval to add a second SVS. Getting the new Axioms was more of her way of rewarding me for the work I've done on the house remodel as of late. And to think that at this point I have a brand new Epic 80 system and nowhere to play it!

    [​IMG]

    Willscary
     
  18. dave alan

    dave alan Second Unit

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    The first point to address is the crossover. A fourth order, dual cascaded Butterworth, or 24 dB/octave Linkwitz-Riley crossover is the universal BM crossover. This crossover's strong point is that it's a 'constant voltage' crossover, meaning that the sum of high pass and low pass frequencies is unity in the entire crossover region.

    The HP filters used in BM are 2nd order (12 dB/octave), designed to be used with sealed satellites which have a 2nd order natural roll off. In combination, the resulting HP slope is 4th order (12 dB/octave roll off + 12 dB/octave filter = 24 dB/octave slope). This mates as designed with the 24 dB/octave slope of the LP filter applied to the subwoofer.

    The problem is that the vast majority use vented sats which have a natural roll off of 24 dB/octave. The resulting slope from applying a 12 dB/octave HP filter is 36 dB/octave.

    The simple addition of a 6th order LP slope selection would solve this problem...IF it were available. Otherwise, 'seamless' integration of sub and vented mains, using standard BM is impossible.

    With RAG's configuration (which calls for an outboard crossover), you only need a crossover with selectable slopes to notice improvement.

    Two subs, placed equidistant, either physically, or by using delay, is better than one. And, if they are in a stereo configuration, so much the better when it comes to reproduction of stereo recorded bass. Period.

    The problem with all MC audio to date is the summing of the LFE (.1 channel) with redirected bass. Introduction of a mono bass signal inserted equally into each of the stereo subs collapses the stereo effect. Also, though introducing redirected bass + originally intended bass + LFE bass into the FL/FR sats may not stress them if they are capable speakers, it certainly muddies the reproduction and introduces intermod distortion.

    A discrete LFE sub system that is full range capable (via a 2-way crossover that allows for an LFE sat) is the answer to complete any MC audio setup.

    I've tested this theory, switching from A (RB + LFE summed into a sub) to B (one sub for RB and another sub for LFE) and got 100% response from people who don't own nor understand MC systems in the least, for over a year. 'Clearer', 'More defined', 'Less bloated', 'Tighter', etc. are typical comments.

    Regardless of from whence the .1 channel was born, it's a very cool addition to the artist/producer tool bag. It just shouldn't be summed with the RB sub during production or playback of MC audio.

    Rant aside, thanks RAG, for the very excellent post. It's detailed, well thought out and, at the very least, offers a great option for experimentation that many may not have thought to try.

    Can you suggest some software to use in the stereo subs scheme that will help to better appreciate your suggestion? I will gladly try the setup and report what I find.
     
  19. Robert AG

    Robert AG Stunt Coordinator

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  20. dave alan

    dave alan Second Unit

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    RAG,

    I appreciate the points you bring up.

    I've played electric bass since '66, when, at age 13, I had my first studio gig as session bassist. I've written and arranged music since '72. So, I've been in studios of all kinds and have heard various philosophies on recording and processing bass over the years.

    The possibility afforded by the .1 channel and subsequent advances in the ability of subwoofers to reproduce the channel are exciting to me, and in their infancy.

    No MC audio recording I know of has used the .1 channel for anything other than a bass boost channel, much less have low frequency effects specifically written for it. Using a mono LFE signal to add 'oomph' (seems to be the word producers prefer) definitely collapses the 'air' and 'realism' you describe when mixed into the subs. It's not the same as panning electric bass into the center.

    For example: If I use 3 tracks for the same electric bass line, 1 is panned left and 1 is panned right and the 3rd is sent to the .1 channel, then they are all summed and sent to a subwoofer, then I agree that this (A)is unnecessary and (B) will still appear to be centered in the soundfield.

    But, if you had done the exact same thing in your pipe organ recording, the LFE signal would collapse the stereo effect if summed into the 2 subwoofers.

    This is easy enough to verify using the A/B playback comparison I describe above and the right software example.

    If I want to incorporate a charging rhinocerous or earthquake or ICBM attack in a musical piece, who says I should limit my choices to orchestral instruments? To say that there are no dinosaur footfalls in music baffles me. It's the artist's job to create the music and the engineer/producer's to help him or her realize the creation by any means available.

    I, like you, have met similar resistance when suggesting a seperate subwoofer system for a discrete LFE playback system.

    Lexicon's MC12 has stereo RB plus discrete LFE outputs. What is so hard about that I/O config? Use it, or don't use it, but at least it's there. (and, it's the only correct MC LF output configuration I know of)

    I assume the organ music you refer to is live to 2 track? If so, there isn't a way to compare it with the LFE summed into the stereo subs vs. a 3rd sub for discrete playback of the LFE, which is what I would really like to set up to compare.

    It's apparent that you have extensive experience in the state of the LFE and LF art from a production standpoint by reading your posts, and I appreciate your information.

    As I said above, I plan to experiment with your outlined scheme and will report what I find.

    BTW, I'll guess here that you ain't usin' a cheesy plate amp/crossover. What crossover are you using in your setup?
     

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