Should I really set sub crossover to the highest setting (120 Hz)??

Discussion in 'Archived Threads 2001-2004' started by Scott Stephens, Jan 4, 2002.

  1. Scott Stephens

    Scott Stephens Stunt Coordinator

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    I know the topic of bass management comes up quite often, but I was reading a thread on bi-wiring yesterday and HTF member Nick G provided a link to a thread in the AVS forum concerning bass management:
    http://www.avsforum.com/ubb/Forum3/HTML/003524.html
    It is a very informative and enlightening read on the proper subwoofer use, setup, and management, but something that was stated in that thread (and has been stated in this forum as well) concerns the subwoofer's crossover level.
    I realize most receivers have a crossover set at somewhere around 80 Hz, which (correct me if I'm wrong) normally will send all above-80 Hz material to the speakers and below-80 Hz material to the sub.
    The AVS forum stated that since Dolby Digital and DTS 5.1 encoded DVDs have a dedicated LFE channel, then you should set the sub's crossover to MAXIMUM. The rationale behind this is that in the LFE channel, there is bass mixed at above 80 Hz levels meant to be sent to your subwoofer as it is a dedicated LFE channel. In theory, then, if you set the sub's crossover at 80 Hz (or whatever level below maximum you set), you will lose low frequencies (above the crossover level you have selected) that were intended for the LFE channel.
    I'm sure this is true, but if the receiver's crossover is still fixed at 80 Hz, shouldn't your mains still be getting frequencies down to 80 Hz? And if your mains can handle these frequencies, why wouldn't you want THEM, rather than the sub, to produce these frequencies. It would seem, in theory, to be a better balance if the mains were able to handle some of the bass. I guess the flip side of this theory is that, since the sub is designed to handle strictly LFE, why shouldn't it produce ALL of the LFE?
    I have tried setting my sub at the higher crossover level (120 Hz) and the bass just feels TOO MUCH. Not too deep, but just too much. It doesn't sound as balanced as it did before (when I had the crossover set at around 90 Hz). I love bass, don't get me wrong, and I like it loud, but with the crossover set to max it seems like there is an omnipresent level of bass that, to me, makes the soundtrack sound a bit unnatural. The AVS forum expert (Brian Florian, I believe) would say that I need a better sub if this is the case. I have a Paradigm PW-2200, and, while not the best sub, is quite a capable performer. So I don't think I need a new sub....yet...[​IMG]
    I know that many people will say, "whatever sounds best to you, then do it," which I agree with. But I just was wondering if it is inherently UNACCURATE (with respect to the filmmakers'intent and the DVD sound mastering intent) to NOT set the crossover to a maximum level.
    Just curious...
     
  2. Denward

    Denward Supporting Actor

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    Basically, you don't want two crossovers determining what goes to your sub. By setting the sub xover to max, you are essentially turning it off.

    Remember that the xover frequency (say 80) does not mean that 81Hz goes entirely to your mains while 79Hz goes entirely to your sub. It means your main's loudness starts tapering off at say, 85-90 Hz, and at 80 Hz, your mains are x dbs softer than the average db over the rest of its rated frequency range. Conversely, your sub's dbs start ramping up somewhere around 85-90 Hz.

    So let's say your receiver's xover is doing its job. It sends a signal to your sub starting at about 90 Hz and it reaches a certain db level at 80 Hz. If you set your sub xover at 120Hz, its circuitry will already be passing 100% of the signal at 80 Hz. If you set your sub's xover to 80Hz, it's still going to be further reducing the input signal at 80Hz so neither your mains nor your sub will reproduce those frequencies at adequate levels.
     
  3. Scott Stephens

    Scott Stephens Stunt Coordinator

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    Denward..thanks for the response. But I'm a little confused (still haven't gotten this crossover thing down...). You said:

     
  4. Vin

    Vin Supporting Actor

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  5. JohnDG

    JohnDG Stunt Coordinator

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    "I have tried setting my sub at the higher crossover level (120 Hz) and the bass just feels TOO MUCH. Not too deep, but just too much. It doesn't sound as balanced as it did before (when I had the crossover set at around 90 Hz)."

    My experience is that different crossover level on a sub (PDR-10) can affect the frequency response curve of the sub. Whether or not this is true for the 2200...?

    IMHO measure the frequency response curve for the sub at various crossovers. You might be giving up maximum db's, but you might also get a flatter curve. AFAIK setting the crossover at 100Hz should 1) not interfer too badly with the receiver's 80Hz crossover, and 2) not lose too much LFE signal, which is 120Hz at its highest, and should roll-off before that.

    jdg
     
  6. Dustin B

    Dustin B Producer

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    Yes you should crank your sub's crossover setting as high as it can go. For the LFE track info mentioned above, and for what Denward said. The only time you won't, is if you are setting your sub to no on the receiver, and running the front left and right channels through your sub, then you use it's crossover since you aren't useing the receivers crossover.

    To elaborate on what Denward said 80hz is the point where change starts to happen. Depending on the receiver you will likely have either a 2nd order possibly a 3rd or 4th order crossover. A 2nd order crossover will attainuate the sound at 12 dB/octave, a 3rd order at 18dB/octave and a 4th order at 24dB/octave. An octave is a doubling or halving of frequency. I'm not sure on this, but I think that the frequency which is reported as the crossover point is where the signal is down 3dB. So lets say a 90dB signal being sent to a receiver that has a 2nd crossover at 80hz. The mains will receive 87dB at 80hz and at 40hz it will be down to 75dB and at 20hz it will be down to 63dB. The sub will be at 87dB at 80hz, at 160hz it will be at 75dB and at 320hz it will be at 63dB. The idea is that the over lapping signals will sum to 90dB. You have two gradual slopes as opposed to a brick wall. The slopes are designed to sum to a flat response.

    If your receiver is attenuating the sound, and you set the subs own crossover into a range that will compete with that crossover, you will have two crossovers cascading with each other and really mucking up the sound. Big no no. If you set the subs crossover as high as it will go (usually somewhere between 120hz and 180hz) the signal is so far attenuated by the receiver, that the subs crossover will effectively do nothing, which is what you want to happen.

    The final point is I do believe some high end receivers and processors do allow you to redirect the part of the LFE channel that is above the receivers crossover to the the mains or all channels. I don't think it's a common feature though.
     
  7. Scott Stephens

    Scott Stephens Stunt Coordinator

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    Vin,

    Thanks for the help. I understand the fact that the LFE track contains frequencies as high as 120 Hz. I guess what I'm trying to ask is if those frequencies in the LFE track above 80 Hz (receiver's xover) will be ALL sent to the sub, or if they will also be sent to the main speakers?

    In other words, I realize the LFE is a separate audio track, but are those frequencies duplicated in the decoding of the signal for the L/R front channels, since most speakers (well, maybe not most, but some) can handle frequencies of that magnitude??

    Again, my problem was that the bass just seemed too much and unbalanced. Perhaps its just that I'm not used to that setting. Or perhaps my sub level is set too high. I guess a recalibrating would be in order.

    Sorry if I got any info wrong. Thanks for the help.
     
  8. Dustin B

    Dustin B Producer

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    Not sure I you were typing while I was or not, but I'll elaborate on my understanding of the LFE part.

    No, most receivers do not duplicate the LFE info above the recievers crossover point into any of the other channels. However, some high end receivers and processors do allow you to do this. What I don't know is if these receivers or processors just duplicate the info, or pass it through the crossover as well. If they don't pass it through the crossover though I would think the portion of the LFE track that was sent to the mains would become a lot louder than it was supposed to.
     
  9. Jason Wolters

    Jason Wolters Stunt Coordinator

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  10. BruceD

    BruceD Screenwriter

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    Scott,

    The 5.1 in the DD and DTS means 5 separate full range channels (20Hz-20kHz) and the LFE or .1 channel sub (20Hz-120Hz).

    The LFE always comes out of the .1 channel and never gets re-directed to the main left and right channels unless you select NO SUB in the receiver or prepro menus.

    If you use full range speakers on the 5 channels, there would be no need for bass management i.e. no need to re-direct bass frequencies from these other 5 channels to the .1 channel sub.

    In your case you have selected to re-direct some bass from the other 5 channels to the sub i.e. you are giving the sub additional duties beyond it's normal LFE role.

    So even though the sub is now doing multiple jobs, the LFE still goes only to the .1 channel, no matter what the frequency between 20Hz-120Hz.

    The reason you want the sub xover set as high as possible (if you can't turn it off) is so that double crossovers don't interfere with each other. This means calibrating the sub with the sub's crossover set to 120Hz. Also pay attention to the phase switch if you have one.

    BruceD
     
  11. ThomasL

    ThomasL Supporting Actor

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    Scott, I'm not an expert on this but if you get Avia, you can use the low frequency sweeps for all channels including LFE to see exactly where the sound is going, so to speak. THe LFE channel should, in all cases that I know of unless your receiver has some special options, be going to the subwoofer out jack on the receiver. You can verify this by playing the Avia low frequency LFE sweep which goes from 100 - 20 Hz in increments of .5 I believe.

    --tom
     
  12. Scott Stephens

    Scott Stephens Stunt Coordinator

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  13. BruceD

    BruceD Screenwriter

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    The easiset answer to the phase question is to first calibrate your sub with the sub's xover at 120Hz, and the receiver's crossover at 80Hz, using the VE DVD and SPL meter.

    Then listen to some bass (from a movie for both main and sub together) in your listening chair (best done with a helper) and have the helper turn the phase control slowly. Repeat this a number of times and select the position of the phase control that produces the loudest bass (either by ear or with the SPL meter).

    Then you may need to go back and recalibrate the SPL level of the sub to match with your mains.

    Your VE DVD actually has a separate LFE sub-out only test level signal for setting SPL to match mains.

    BruceD
     
  14. Dustin B

    Dustin B Producer

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    I suppose this should have been asked quite a while ago. Do you have all 5 channels set to small or large? I'm in the camp that all 5 should be set to small (even if they are close to full range towers) if you are using a seperate subwoofer. If all the channels are set to small then it doesn't matter what mode you are in (DD, DTS, Stereo, Prologic II, 5 ChStereo) any frequencies below 80hz will be redirected to the subwoofer. So set all the channels to small, hook the sub up, set the subwoofer's crossover as high as it will go and forget about it. You won't ever have to change the settings again unless you get new equipment or want to listen to stereo without the sub (in which case you just set the sub to no and the mains to large).

    With the phase. Do you have a continously variable (a dial) or a just a 0-180 switch. The switch is easy, just try both and which ever sounds better to you is right. Same can be done with the continously variable one, it will just require someone else to slowly turn the dial while you listen.

    If you want to be more precise about it though you'll have to get a SPL meter and Avia. Then do a search for Avia and phase on the forum to get directions on how to adjust it this way.

    And Jason, a crossover is NOT a brick wall. If you haven't please read or reread my first post in this thread and or do a search in the forum about crossovers to learn more. But you are right. If the subwoofers built in crossover is used and set to 80hz then any thing over 80hz in the LFE track will be attenuated by it. However, this will cause all sorts of problems with any of the redirected bass from the other channels. Which again, is why you don't use the sub's built in crossover if the sub is set to yes in the receiver. Only if the sub is set to no in the receiver and you are running your mains through the subs crossover will you use the subwoofer's crossover.
     
  15. Jason Wolters

    Jason Wolters Stunt Coordinator

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  16. BruceD

    BruceD Screenwriter

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    Jason,

    The 80Hz 4th order crossover typically used in receivers for the low-pass portion of the signal (the LFE and re-directed bass portion) has a slope of 24dB per octave. This is not a brickwall and the sub will still be producing plenty of signal output at 81Hz.

    24dB per octave means the signal will be down by 24dB only at 160Hz. The signal dropoff between 80Hz and 160Hz is a steeper slope than say a 12dB per octave filter but it's still a slope and not a brickwall.

    BruceD
     
  17. Dustin B

    Dustin B Producer

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    Here is a link to the article you are talking about Jason:
    http://www.hometheaterhifi.com/volum...pril-2000.html
    Here is the appropriate quote from the article:
     

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