LFE and bass below 80hz

Discussion in 'Speakers' started by SteveCallas, Jan 3, 2004.

  1. SteveCallas

    SteveCallas Second Unit

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    When setting a crossover to 80hz, and then using the sub for the LFE channel, are you missing the bass that would be going to the speakers that is below 80hz, or does the LFE channel include this?

    If this bass is being missed, would it be logical to use two subs, maybe one in the front left corner for the below 80hz material, and then one in the back right for the LFE?

    I know dual subs should usually be placed together, but if they are next to each other and one is doing the LFE and one is doing the below 80hz stuff, wouldn't it sound muddy?

    Also, wouldn't making the preamp send a signal to the sub that combines the LFE and below 80hz material make the sound coming from the subwoofer seem not very defined?

    Or is the below 80hz material that is being sent to the speakers not very important?
     
  2. Edward J M

    Edward J M Cinematographer

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  3. WayneO

    WayneO Supporting Actor

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    Though the LFE and Front L/R low frequency material does differ many times, usually when you enable a crossover, all the under 80hz material should then be redirected to the LFE, but it can depend on what recevier or pre/proc. you have and would require reading the manual. This doesn't mean it'll be muddy, it may actually sound better with a good sub, letting your amp have cleaner/more power for your mids and highs. Material below 80hz is important and a clean output is just as desireable as any other frequencies.
     
  4. ChrisWiggles

    ChrisWiggles Producer

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    First, read those great articles that Ed linked you to.




    Take special note, that the output is almost always labeled "Subwoofer out," and NOT "LFE out" or some such.

    The whole point of bass management is to route bass to different places. By setting your speakers to small, and using, say an 80hz crossover in your receiver, all the bass in the main channels is added to the LFE, and sent to the sub. BM can be used in more complicated ways as well, depending on what features are available in your processor. The "misunderstoon LFE" article linked explains this better.

    It is misleading to call the LFE (.1) channel a "subwoofer channel," since the signal your subwoofer is getting is usually that, PLUS all the bass rerouted from the main channels. Unless you set things incorrectly, you should not lose ANY bass at all, and should have improved performance.
     
  5. SteveCallas

    SteveCallas Second Unit

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    Thanks for the explainations, I was unaware that the frontal bass was added to the LFE.
     
  6. Lewis Besze

    Lewis Besze Producer

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    Not just the front, but any speaker's selected as "small".
     
  7. dave alan

    dave alan Second Unit

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    It's interesting to me how logical thinking:

    ["Also, wouldn't making the preamp send a signal to the sub that combines the LFE and below 80 Hz material make the sound coming from the subwoofer seem not very defined?"]

    Is wiped away by pointing to an article that, though it contains some useful facts, is rather dated and is by no means the definitive 5.1 audio resource.

    The answer to your question is absolutely yes. The single, summed digital bass signal renders impossible the task of properly reintegrating redirected bass into the soundfield from which it's derived, lops off a potentially full range .1 channel at whatever crossover setting is selected and asks an impossible task of a single (or whatever number, if they are all playing the same signal) subwoofer.

    This has led to two things. The endless 'music vs movie' sub debate and the design and production of impratically larger and more powerful subs.

    The 'musical' sub and the boom channel sub have diverged down 2 seperate paths, each designer claiming that their sub is good at both, yet no one, to date, has seen the logic of combining the 2 in a single system.

    EQ should be the last option employed, yet it's become the first option. ('Place the sub in the corner and EQ it flat. Better yet, convert the signal to digital, EQ it to taste for a given format, save settings and store as many preset curves as need be to satisfy all MC sources, convert back to analog and bingo, the ideal, technical acheivement award winning solution')

    If that were really the case, I think you'd see plenty of single-driver, 1-way satellite speakers out there getting rave reviews.
     
  8. ChrisWiggles

    ChrisWiggles Producer

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    I disagree STRONGLY.

    First, I am *very* hesitant to differentiate between movie and music reproduction. The similarities may be lesser in terms of bass, since the impact and SPLs needed for HT are usually much more significant than music. Also, music can sound ok with boomy(and inaccurate) bass, this is what most people are used to. Not so with movies.

    A flat response, clean sub with great extension should be ideal for both realms. The only thing that you might want to tweak, is perhaps not so much extension for music.

    Unless you have a SERIOUS sub on each channel, routing bass to a speaker that is actually *CAPABLE* of recreating that bass is the best option.

    I agree that the ideal would be 7 identical, full-range speakers, which would be VERY expensive, and quite large. However, real-world budgetary concerns cannot be ignored. You can get better performance with cheaper, superior monitor-type speakers, and a dedicated (high-quality)subwoofer that is well placed.
     
  9. Zack_R

    Zack_R Stunt Coordinator

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    IMO, the validity of the article depends on what receiver / pre-pro you are using.

    For example, with my Sony 4ES receiver, the crossover is totally independent of the .1 channel. I can set my speakers crossover at 40 hz. and set the LFE cut at 90 hz and I lose no LFE. LFE is not lopped off nor are the mains speakers now crossed at 90 hz.

    I verified this through Avia's LFE 100-20 hz frequency sweep. I set my crossover on all speakers to 40 hz and the LFE at 90 hz. When the Avia tone starts at 100, the LFE from the sub is muted around 10 dbs so and then it kicks in fully at 90 and takes it all the way to 20. I also experimented by putting the center crossover at 120 hz and the surrounds at 150 and left the mains at 40. I then set the LFE cut to 80 hz, played the Avia signal and got the same result. The sub kicks in at 80hz regardless of the crossover setting for the other speakers.

    I can't adequately measure the phase effect of using the different crossovers when playing strictly re-directed music bass and what the summing effect constitutes. The only phase issues I may have would be between my mains and my sub if I cross my mains at 80 and my center at 120. But since I don't listen to multi-channel music, the potential phase problem is not a big issue.

    For movies, I'd much rather have my less bass capable surrounds and center speaker crossed higher and not subject them to boosted non-LFE bass and live with any potential bass phase issues between 80 and 120 hz.

    I took this phase part of the article as being theoretically slanted and may not apply equally in different room / listener settings. In other words, I think you'd have to measure this occurence in a room before determining that there is actually a problem.
     
  10. SteveCallas

    SteveCallas Second Unit

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    Dave, would you then agree that two subs, one in front for the redirected bass, and one somewhere else, say in a back corner, would be good for the LFE?

    If redirected bass and LFE are sent to a sub, no matter how good or bad a sub it is, the sub would have to try and reproduce two distninct sounds at one time.

    Without extended knowledge of how drivers recreate sounds besides them creating pressurized air waves, I wonder if the following example would be true?

    If there is an explosion going on on the screen, and the LFE track calls for a frequency that is in the 20-28Hz range for a couple seconds, but the redirected bass is calling for a frequency in the 55-65Hz range at those same couple of seconds, how can the single driver of the sub output both frequencies at the same time without it sounding kind of blah?

    Wouldn't this be similar to having one of your speakers recieve two seperate inputs at once? Can drivers reproduce this?
     
  11. Zack_R

    Zack_R Stunt Coordinator

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    To my knowledge a sub or any speaker for that matter has to reproduce multiple sounds of varying frequency at the same time. Music can have a bass guitar, double bass drums and an organ playing at the same time. I don't think a sub cares where the sound comes from, whether LFE or re-directed bass, it has to re-produce the signals it is sent.

    For the most part, an LFE blast, such as an explosion, will most likely drown out any other bass. LFE can go upwards of 10 dbs more than the non-LFE.

    A speaker with only two drivers has to play all the frequencies. I myself don't know how speakers can reproduce multiple sounds but they do. The problem you will likely run into with two subs non-colocated is that through-out their operating range there is likely to be areas where you have bass cancellation.

    As an example, my mains are large speakers with 15 inch woofs. They are capable of hitting 20 hz, though down many dbs. When the mains are set to large and the subwoofer is on, the mains and sub are phased almost perfectly from 100 to 40 hz. Below 40 hz, it's a real disaster with as much as 12 dbs lost below 30 hz ( due to cancellation) when compared to the mains set to small and the sub crossed over at 80. As you can see this would be a terrible setting for movies with high impact sounds. If there was any non-LFE bass recorded below 40 hz.

    If you have two subs it certainly doesn't hurt to experiment with different connections and placement but I would certainly measure the results.
     
  12. Edward J M

    Edward J M Cinematographer

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    A few pre/pro's have the capability of setting the xo for the LFE channel independent of the high pass xo for the surround speakers. I just don't see this as a terribly useful feature.

    While the LFE channel in theory goes to 120 Hz where it is brick walled, in practice it has very little content above 80 Hz. But it should not be set lower than 80 Hz or you risk lopping off channel content.

    Proponents of a lower xo for the surround channels claim they can still localize bass at 80 Hz, so they opt to use say 60 Hz (or whatever). If the goal is to have non-localized bass, then ostensibly you must also set the xo for the LFE channel to 60 Hz (or w/e), otherwise you would be defeating the purpose!

    It is for this reason I see no usefulness (other than purely academic like Zack's experiment) in setting the xo for the LFE channel any differently than the xo for the surround channels, or any lower than 80 Hz.

    One could argue that if they have bass capable speakers, they want to make the best of them, but the more sources of low bass there are in a room, the potentially more difficult it becomes to avoid undesirable room modes. Furthermore, very few speakers are truly full range, and the deeper you ask them to play, the more AVR power/current they require, and the more IM and TH distortion they generate.
     
  13. ChrisWiggles

    ChrisWiggles Producer

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    Yes. It's called music.

    Music is just an endlessly changing HUGE pile of sine waves stacked on each other.

    If you had a zillion speakers to reproduce, say the sound of a piano note, for all the different sine waves being reproduced, constantly changing, you only have ONE diaphragm in each ear, and all these waves interact with each other, creating the same result as if you had one driver doing it all.
     
  14. SteveCallas

    SteveCallas Second Unit

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    Fair enough.
     
  15. dave alan

    dave alan Second Unit

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    Steve,

    You are a very astute person.

    Your question very basically describes intermodulation distortion, which occurs anytime 2 inharmonic tones are sent to the same speaker simultaneously.

    If the 28 Hz tone were left to the LFE sub and the 50 Hz tone were routed to the RB sub, intermod distortion would be reduced as opposed to a single sub receiving both.

    Zack,

    Your experiment is valid, except where SACD/DVD-A MC audio is concerned, which the article fails to address in any way. The sub most definitely 'cares' what signals it's receiving and reponds in kind. If you leave the soundtrack (as you point out, with enough problems of it's own for a sub) to a RB sub and shift the responsibilities of the LFE+10 explosions to a discrete sub designed for that purpose well, let's just say it will be better. There is enough evidence that Steve's questions shouldn't be dismissed out-of-hand.

    Edward,

    Same comment. The .1 channel is only limited by Dolby (and even then, not brick walled at 120 Hz.). You would immediately find, if you ever decide to try a dual sub discrete LFE/RB setup, that the sub is anything but localizeable at 60 or 80 Hz.

    Also, I agree that multiple full range sats is not desireable. Not because they would be harder to place (placement difficulty having been blown way out of proportion), but because the sats have NO placement options and must be placed, ideally, precisely where they must be. This is the best argument for having a redirected bass sub.

    Chris,

    You are correct, but I think intermod at the ear is a very different situation than intermod from the sub.

    Steve,

    To quote Sundance..."You just keep thinkin' Butch, that's (apparently) what you're good at".
     
  16. Richard_M

    Richard_M Second Unit

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    In theory how would these subs be connected i.e. would the LFE sub be on the sub out channel of the amp and xover bypassed on the sub, and the RB sub wired via front main speaker cables, mains set to large so RB from all speakers gets sent to mains, and RB sub xover set at -3dB point of mains?


    BTW this is a great thread, thanks for the info guys.

    Richard
     
  17. dave alan

    dave alan Second Unit

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    Richard,

    I'm currently alpha testing an analog LFE preamp/LP filter/selectable slope/adjustable phase piece of hardware.

    The routing scheme requires a player with 6 analog outputs, and a pre/pro or receiver that has analog BM or the Outlaw ICBM outboard unit.

    You simply connect the player's SW output into this piece directly.

    You set the player's speaker settings to all sats 'large' and subwoofer to 'yes'.

    Your LFE sub gets only LFE signal directly from the source.

    In the pre/pro menu you set all sats to 'small' and select your crossover particulars as the pre/pro or ICBM and individual system/taste may be.

    You connect the RB sub to the pre/pro's (or ICBM's) SW output, which only outputs bass derived from the sats and no LFE, since there was never any LFE input in the first place.

    That's it. you can manually adjust LFE output at the LFE preamp or the sub amp on-the-fly.

    You can use any preamp and the LFE sub's crossover/phase if you like. If your sub has HP output, you can have a 6th full range channel for Hi-Rez formats (if anyone ever grows the b***s to buck the conventional 'wisdom' and experiment).

    Since every player I've investigated outputs LFE both through the analog AND the digital outputs at the same time, you can switch the LFE pre off and listen to the conventional single mono summed bass into 1 sub, using the pre/pro's DACS and digital BM to compare the setups and see what you think, with DVD movies and music videos.

    Placement, phase adjustment then volume level and I have flat (+/-3 dB) in-room response in a not so ideal 3000 cubes.

    The trick is to have an LFE sub that's optimal to the task and a RB sub that's designed for that duty as well. There are many more particulars, but that's for another venue.

    I only wanted to encourage Steve to explore his thoughts and ideas as no one person or Company has the definitive answer on LF reproduction in a MC audio setup...yet. YMMV
     
  18. SteveCallas

    SteveCallas Second Unit

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    Thanks for the support Dave.

    Well don't certain preamp processors (Anthem AVM 20 for example) have two subwoofer outs, and each can be configured as to what it will be outputting?

    I would assume then that a good candidate for a RB sub would be sealed with probably 2, 8" drivers. Also, although I know that usually the best placement for a sub is in a corner, wouldn't it make some sense to place a sealed RB sub(s) near the screen and the front three speakers, like below or to the side(s) of the center speaker?

    One last idea, if cost were not an issue. Send the speaker wire for each speaker from the amp to 5 or 7 independent, sealed RB subs that are positioned below each speaker, considering the subs themselves have a filter on them that can be set to only recieve signals from 80Hz or 100Hz and below, and then send the output from that to each speaker. The bass might be locatable, but it is supposed to be in this setup. Sure, that is a super overkill, but that would allow you to hear the movie EXACTLY as it was meant to be heard.
     
  19. Edward J M

    Edward J M Cinematographer

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    I'm not sure what you are getting at here, Dave?

    Dolby does indeed brick wall the LFE channel at 120 Hz, but there is very little content above 80 Hz.

    I never claimed a sub was localizeable at 80 Hz. Personally I don't think it is, and people who can pick out the sub with an 80 Hz xo are probably hearing harmonic distortion being generated by it, which is much easier to locate due to its higher frequencies.

    I did state that proponents of a lower xo for the surround channels must also lower the xo in the LFE channel (assuming they have that capability) if they want to practice what they preach. However in doing so, they risk losing channel content. I do know of at least one HTF member who does indeed use a 60 Hz xo for surround and LFE and takes his chances on losing some LFE channel content.

    I like your ideas on that analog BM piece of equipment. None of my surround channels are full range (the F3 varies from 42-50 Hz), and I don't have room for more than one sub, so I got the best one I could afford. My SACD & DVD-A player (Denon 2900) has a standard digital BM circuit (12/24 at 80) and it seems to work very well in my application anyway. Like you said, YMMV.

    Finally, IM distortion is of more consequence (and much easier to hear) when a quasi full-range two-way speaker (like say the Rocket RS550 which has an internal xo of 3.8 kHz on the dual mid-range drivers) is asked to simultaneously produce a loud 40 Hz tone (low E on a bass guitar) and a bunch of signal content in the 1,500-3,000 Hz region. The high excursion required by the midrange driver to produce a 40 Hz signal will significantly modulate those upper frequencies to a greater and more audible extent than will a subwoofer producing a 28 Hz and a 50 Hz signal at the same time.

    In this case, I think the lesser of two evils is definitely to send the sub both the LFE and the low passed surround bass rather than sending all your surround speakers a full range signal (if you only have one sub). I know what you are proposing requires more than one sub but many people just don't have the room for more than one. If they do though, your piece of equipment could be very useful indeed.
     
  20. Richard_M

    Richard_M Second Unit

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    Dave



    Sounds like a great device!

    I take from your set up details that the LFE Sub only works from the DVD Player, and the RB sub works with all other material i.e. phono, tuner etc, as well as DVD. I also take it that you don't use digital for connection of the DVD player to the pre/pro, just the 5x analogue connections.

    Keep us informed how the device performs.

    Richard
     

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